View Full Version : The Essentials of a Precision AR-15
The Essentials of a Precision AR-15
http://www.box.net/shared/static/mla3p170ls.jpg
Without making this overly complicated, you need three basic components for a semi-automatic AR-15 to produce its best mechanical accuracy (technically, precision): a match-grade barrel, a quality free-float handguard and match-grade ammunition, preferably hand-loads tuned for your barrel. (The free-float handguard doesn’t add to the accuracy of the barrel per se, it simply prevents any outside influence from detracting from the accuracy of the barrel.) Anything after that will not immensely improve the mechanical accuracy of the semi-automatic AR-15, but several things can help you, the shooter, shoot to the level of the intrinsic accuracy of your semi-automatic AR-15.
http://www.box.net/shared/static/psau3igm35.jpg
While there are certainly gifted individuals among us that can do amazing things with iron sights, most of us will benefit from using a quality, high-power scope to achieve the highest level of accuracy from a precision AR-15. In order to hit the exact same spot on the target every time, you have to be able to see that you are holding on the exact same spot every time. It’s also important for the scope to be mounted at the proper height and at the correct eye-relief for the particular scope. One of the most common errors I see with scopes mounted on AR-15s is the scope not being mounted far enough forward for its eye-relief.
In order to maintain that exact hold on the target throughout the trigger pull, it helps not to be fighting with a heavy, gritty, stock trigger. There are a variety of aftermarket triggers now on the market for both standard size trigger pins and the larger Colt trigger pins. Personal preference will definitely play a role in trigger selection. Among the two-stage triggers, I’ve found the Geissele triggers to be the smoothest, lightest, most consistent and most reliable. For single-stage triggers, it’s hard to beat the JP Enterprise Fire Control System. Keep in mind that the JP trigger does require fitting.
Following the scope and trigger selection, some shooters will find that items like aftermarket grips and stocks will help them achieve a better “fit” with their AR-15. (Shooters don’t all come in the exact same size and shape.) Once you’ve put your precision rig together, you have to find a match-grade factory load that your barrel “likes” or better yet, develop a match-grade handload for it.
A semi-automatic AR-15 is not going to shoot as accurately as a precision bolt-gun, but today’s precision AR-15s are capable of a level of accuracy that is truly outstanding for a semi-automatic rifle. The 10-shot group pictured below was fired from one of my Krieger barreled semi-automatic AR-15s from a distance of 100 yards. I used Sierra 55 grain BlitzKings that were hand-loaded on a Dillon XL-650 progressive press. The load was developed using my Accuracy Node Detection Technique. The group has an extreme spread of 0.474”.
https://app.box.com/shared/static/c3nj830tjxcph7zq8vm7.jpg
http://www.box.net/shared/static/4zc0he19fq.jpg
….
All fact, with supporting data. As usual, great post. Thank you for putting the time, money, and effort into these threads. It's truly appreciated by many.
It's also a great reminder, seeing a post like this, both here and in life, to stick with the facts, and not speculate. I find myself guilty sometimes.
Also, great shooting. That is fantastic precision.
Printer Repairman
06-29-15, 00:33
About bolts and carriers:
I’ve been wondering how much a head spaced bolt affects accuracy. Most high quality barrels are matched at factory, but for those of us who didn’t get the memo and ended up without a matching bolt, I was wondering about the following:
1.- A Mil-Spec 5.56 bolt runs between 1.4646+.002 (5.56 Go) to1.4748 (Colt field). MSTN specs say they headspace at 1.4646+.002 at factory. This leaves a potential gap up to.0062 that affects case expansion, if one were to throw in a Mil Spec bolt that is on the lower end of the headspace scale on an SS barrel. Would anyone have real world data on how detrimental this gap can be? What would the minimum headspace required be for a half minute gun?
2.- Even if the headspace issue was solved, wouldn’t the bolt have to be perfectly perpendicular to the barrel? Following the logic that all components should be parallel and perpendicular, (given the case for billet uppers and the existence of Brownel’s lapping tool) would there be a case for using a Young Manufacturing bolt (match or mil-spec) that has increased contact surfaces and trued bolt lugs? Anyone have real world experience with incrementing accuracy by using these BCG’s?
3.- The Military stopped using chrome carriers and bolts in the 60’s. Larue currently uses a chrome bolt. Is there accelerated wear on the barrel extension/upper receiver contact surfaces due to the hardness of chrome lining or is this a myth?
The textbook answer from the internet to these questions seems to be “these variables are all minimal if not negligible, shut up and throw in a BCM carrier”. Wishing to improve upon that, would anyone have any technical insights? Because getting a box of headspace gauges and a box of bolts to try and match 1.4646 sounds expensive and not fun at all.
About bolts and carriers:
I’ve been wondering how much a head spaced bolt affects accuracy. Most high quality barrels are matched at factory, but for those of us who didn’t get the memo and ended up without a matching bolt, I was wondering about the following:
1.- A Mil-Spec 5.56 bolt runs between 1.4646+.002 (5.56 Go) to1.4748 (Colt field). MSTN specs say they headspace at 1.4646+.002 at factory. This leaves a potential gap up to.0062 that affects case expansion, if one were to throw in a Mil Spec bolt that is on the lower end of the headspace scale on an SS barrel. Would anyone have real world data on how detrimental this gap can be? What would the minimum headspace required be for a half minute gun?
2.- Even if the headspace issue was solved, wouldn’t the bolt have to be perfectly perpendicular to the barrel? Following the logic that all components should be parallel and perpendicular, (given the case for billet uppers and the existence of Brownel’s lapping tool) would there be a case for using a Young Manufacturing bolt (match or mil-spec) that has increased contact surfaces and trued bolt lugs? Anyone have real world experience with incrementing accuracy by using these BCG’s?
3.- The Military stopped using chrome carriers and bolts in the 60’s. Larue currently uses a chrome bolt. Is there accelerated wear on the barrel extension/upper receiver contact surfaces due to the hardness of chrome lining or is this a myth?
The textbook answer from the internet to these questions seems to be “these variables are all minimal if not negligible, shut up and throw in a BCM carrier”. Wishing to improve upon that, would anyone have any technical insights? Because getting a box of headspace gauges and a box of bolts to try and match 1.4646 sounds expensive and not fun at all.
My MSTN Upper has an LMT Bolt thats spec'd to 1.4646 + or - .001" and it normally shoots between .5" to .8" 5shot Groups via Mk262 Mod 0/1
This is with a Young N/M Hard Chrome Bolt Carrier where the whole BCG has been IonBond DLC'd, and is all inside a Vltor MUR-1a Upper Receiver. Oh and almost forgot, it's the Noveske 16" Intermediate-Length Gas 3-Groove 1:8 Twist Stainless BBL.
Printer Repairman
07-03-15, 00:45
HD1911,
That sounds like an awesome rifle. About your carrier: How deep is the N/M’s grove wear in the MUR? Was difficult closing the bolt until it was broken in? Also, are you running that N/M with an H buffer or HD spring with the intermediate tube? Does the Mid-length system have a propensity for bolt bounce or high carrier velocity able to set back the ogive on the 262 Mod 0?
I’m emulating one of the older MSTN rig specs: Garden variety Noveske Recon 1-7, Larue upper & 11” hand guard. Lookin at the N/M +H2+red spring. I’m doing the homework on this thing as I graduated into precision from old-school carbines with classic FSBs.
Appreciate the advice
I've seen posts on here that claim UR - LR fit isn't important to accuracy.
Personal experience tells me otherwise. A loose fitting UR will wiggle around enough to disrupt your POA, this becomes more important at distance.
I'm not advocating using some gimmick like an accu-wedge. Select your components to fit together tightly to minimize the play in UR / LR mating surfaces.
Printer Repairman
07-03-15, 11:11
HKGuns,
How much POI shift or group size growth are we talking about? And at what distance is this noticeable?
Apparently the goal of this game is to keep a bullet as concentric as possible to its neck, case walls, flash hole, and primer, inside a barrel chamber that is perfectly concentric to it’s crown and outer profile. This barrel must then be as concentric as possible with the inside diameter of the upper, the bolt face, bolt lugs and the carrier group, while maintaining exact perpendicularity with the upper rail to be parallel to the optic.
I agree that after all that tolerance work, it would be a waste to slap that group on a wobbly lower. It makes sense to apply the same high tolerance logic to the relationship between the URG and the lower. However, I noticed that after hammering in the rear lug of a tight URG in into a tight a lower, several hundred rounds of firing makes it settle and a slight play can be felt that could require additional tightening. The only way I see to correct it is to run epoxy bedding like the National match dudes.
Anyone here epoxy bedded a lower before and achieve noticeable results?
How much POI shift or group size growth are we talking about? And at what distance is this noticeable?
The math shows that a .1 degree amount of slop (up/down or side-side) equates to a shift of ~ 2 inches at 100 yards and gets worse as you increase the distance, obviously.
Real world could be slightly different in that the slop or play may not be consistent from shot to shot given you and I are the largest variable.
Anyone here epoxy bedded a lower before and achieve noticeable results?
I have not.
HD1911,
That sounds like an awesome rifle. About your carrier: How deep is the N/M’s grove wear in the MUR? Was difficult closing the bolt until it was broken in? Also, are you running that N/M with an H buffer or HD spring with the intermediate tube? Does the Mid-length system have a propensity for bolt bounce or high carrier velocity able to set back the ogive on the 262 Mod 0?
I’m emulating one of the older MSTN rig specs: Garden variety Noveske Recon 1-7, Larue upper & 11” hand guard. Lookin at the N/M +H2+red spring. I’m doing the homework on this thing as I graduated into precision from old-school carbines with classic FSBs.
Appreciate the advice
You know, I haven't really even noticed any Groove Wear. I could try and get some pics of the inside of the upper receiver and show you.
She was stiff at first, but always Returned to Battery, no problems. Now she feels like she's riding on Ball Bearings... the only noise and feel that's apparent is the Sprinco Green spring moving in the Vltor A5. She's ran Flawlessly with A5H1 & A5H2 Buffers... no issues at all.
Nothing wrong with the older MSTN Upper you're mentioning.
Oh, and I'm also actually blown away at how tight the Upper to Lower fit is on this thing... the Vltor MUR-1a is mated to a Noveske Gen2, and there is literally not one single bit of play. Yet, it isn't a workout nor does it take a special song and dance to get the pins pushed back in.
I would like to add that getting a very comfortable cheek weld which will help the shooter see consistently through the center of your scope every time. For me I like to rest my cheek bone on top of my rear stock. I read a lot of guys mount their scope as low as possible. I tried the Nikon P-SERIES mounts and found that if I got rings that was about 1/8" higher put my cheek weld perfect for me. Mine are Warne Quick Detach Ultra High Aluminum rings.
http://www.sportsmansguide.com/product/index/nikon-p-series-mounting-system?a=993875
http://www.amazon.com/Warne-Scope-Mounts-Detach-1-Inch/dp/B0002ZPS1O/ref=sr_1_7?ie=UTF8&qid=1436420383&sr=8-7&keywords=warne+quick+detach+rings
http://i.imgur.com/3szdlxK.jpg
The scope actually lines up with the end of the charging handle. Picture is at a bad angle.
More important, a "precision shooter" behind the rifle.
Little Creek
07-09-15, 14:29
What do you think of a DDM4V11 S2W with a NF SHV 4-14x56 MOAR reticle in a NF base as a precision shooter for a beginner? Preliminary shooting indicates it likes 77gr. Nosler bullets. It is new, have not fired it much.
I found this post by MTSN on Sniper's Hide. I found it to be very informative.
http://forum.snipershide.info/showthread.php?t=239418
The term "SPR style" will mean different things to different people. To some, it will mean a correct SPR length barrel format upper, or maybe even a reasonably accurate SPR clone of Mod 0 or Mod 1 format. To others, it will mean a reasonably lightweight, tactical, precision AR' build with a barrel 16" to 18" in length.
No matter what you do to an AR' upper, you can never make it shoot better than its barrel is capable of doing. But, you can take the world's best AR' barrel and screw it up easily with less than optimum supporting components.
An upper with a shorter barrel of a given quality and mass will generally outshoot one with a longer barrel of the same quality and mass. The velocity loss with shorter barrels is less than you'd think. And worth it for mounted and/or urban ops.
Why put more weight and bulk into a marginally lethal caliber weapon than the absolute minimum required? It's customary to think of the receiver as the foundation of the weapon, but with an AR', the foundation is literally the barrel.
After the barrel, what's important? I'm thinking that we want things to be properly aligned, rigid, everything straight, and therefore repeatable in terms of performance. In no particular order of importance, since any one of these factors can ruin your best efforts in all others:
1. Optimum bolt fit, with a top quality bolt. A thou' off SAAMI minimum is what I like for accuracy with match grade ammo. In the middle of the headspace spectrum for optimum reliability plus accuracy in a hard-use gun. I'm not talking go & no-go gauges, but headspace gauges in 0.001" increments. And enough spare bolts to have a good selection. The builders who don't use them will I guarantee tell you that's not important.
2. You need the best bolt carrier you can afford, ideally manufactured to the strictest standards with a hard, slick, no-stick coating that will hold up well. It's one of the few "moving parts". "Price is one measure of value" applies.
3. The primary support system for the BCG is the upper receiver. We're so fortunate to have so many superlative choices today. And so much crap to challenge us. Drop a good steel straight edge on top of your favorite upper receiver and look if any light comes between them. Look inside to check the symmetry of the wear pattern from the bolt carrier.
3. Muzzle device. There are so many really popular muzzle devices out there that absolutely destroy accuracy that I can't help but be amazed nobody throws the bullshit flag. Never underestimate the power of advertising and the internet fanboys taking carbine classes where the longest shot is 50 yards. If you want to see what works, look at what people who shoot for money use - the ones that win consistently year after year, that is. Hint: You can start with Taran Butler and Mike Voight. If you don't like the blast and flash of a brake, check out what the real world high speed guys use on the end of their weapons.
4. Gas system length. For best accuracy, I personally like the longest length gas system that the barrel length can reliably use. It's one of those magic harmonics things you can't see. Smoother recoil pulse. No, I can't prove it.
5. Scope mount. This is a great place to get silly trying to save a couple of ounces and screw your accuracy potential long, hard, and deep. As time goes by, I'm leaning more and more toward serious overkill in this department. If Spuhr mounts didn't cost $450 each, I'd have them on everything.
5. Stock. You can get a better cheek weld with any number of carbine stocks than the PRS. With a 1.4" to 1.5" high scope mount, you have no real need for an adjustable cheek piece that can justify the increased weight in a marginally lethal caliber weapon.
6. Did you think at this point I had forgotten that single major fashion statement of the AR-15 world, the AR' forend? So long as a forend free floating, with a design where the barrel nut will not come loose under hard use, plus a means to attach a bipod, they're pretty much the same form an accuracy point of view. We can all have our favorites and still be friendly, right? Isn't a remarkable amount of engineering talent expended every year designing new ones, with new and improved features, for us to lust over?
Where all this engineering needs to be directed is not at the forend. I think making the barrel/receiver junction as rigid as possible within reasonable weight standards is way more crucial. The LMT MRP holds the crown in the rigidity department, and KeyMod equipped is no longer a pig, but getting your favorite X-Ray Blaster barrel in it is a no-go. If Karl doesn't want you to have it … The Vltor VIS is a top rigidity contender, especially nice now that it's been slimmed down with the KeyMod option. The Seekins receiver/forend system with externally threaded titanium barrel nut looks potentially good in this department (Work on that QC and CS, Seekins!). Milling a KAC URX for a hard fit on its host receiver to get that sexy no-line fit fashion statement has had the unintended consequence of a surprisingly rigid final product.
I'm sure there are many more great receiver/forend products of which I'm not even aware.
Good shooting.
Do you have any experience with the Aero Precision M4E1 upper? It mounts the handguard directly to the upper, and doesn't require indexing the barrel nut. I like how it works, but won't claim to be skilled enough to notice the difference.
Thank you for sharing. Very informative.
Interesting and thoughtful discussion - some additional points to bear in mind....
Headspace that varies a few thou either way has virtually zero affect on accuracy. In the context of this discussion, a headspace variance of say +/- .002 isn't going to show up on the target. So! I would ask those that have the their hand on the BS flag to take a breath and relax for a moment.
Consider this: In every rifle that uses a spring loaded ejector, a chambered cartridge is pushed forward in the chamber by the ejector so that the shoulder is up really close and personal with the shoulder in the chamber. When fired - and assuming the headspace is not so excessive that the firing pin can't touch off the primer, the case becomes almost plastic and sticks to the chamber walls (by design). This is evidenced by two points;
1) Excess headspace is shown by primers that are backed out of the case and
2) Cases will stretch near the back - usually about .300" up from the case head. This is where we would see insipient case head separations.
A barrel / bolt combo that is on the loose side of the headspace spectrum will almost never show any degradation in accuracy. The reality is that at the time of firing, headspace is really not a factor in accuracy. (Keep the BS flag in your pocket!) The reason behind this is again, the ejector spring pressure keeps the case all the way forward in the chamber....always. Now - uneven lug seats, bolt faces that are not perpendicular to the bore, non concentric chambers will all affect accuracy. The assumption through out this discussion is that the rifle has no other flaws / variables.
Conversely, If you ever want to open up headspace - just leave your sizing lube on a case or lubricate your chamber - it will increase bolt thrust so much that after 5 or 6 rounds, you are almost assured of an additional few thous of HS and proabably a sheared lug or two. Clean dry cases and chambers are the waypoints of operator & rifle longevity.
For those circumstances when the headspace is truly excessive - we are now dealing with a serious safety issue. Excess headspace will push those primers out too far and they will leak at best. Worse is a completely blown primer. Catastrophic, is headspace that is large enough to cause a case head separation where the cartridge has stretched so much that it separates upon extraction.
I have in the course of many many years, chambered hundreds of barrels - both in bolt guns and gas guns. For match type rifles - I always headspace to the SAAMI minimum. Not for accuracy, but rather knowing that the vast majority of shooters that want this type of rifle built are also hand loading for it too. Minimum headspace will prevent excess stretching of the cases and prolong their life.
A few other points to ponder as well. I would ask that if you have the capability to accurately measure cartridge headspace - do so for some enlightenment. In factory ammo, it is not uncommon to see match grade ammo have cartridge headspace that is .004 - .006 under SAAMI minimum. This is why virtually every round of OEM ammo will chamber and fire. (A side note to measuring Cartidge HS using the Stony Point / Hornandy or any gauge of that type - proof it first against known / verified HS gauges - I tested some Stony Point examples and they were over by +.008.) Another point to ponder - consider a .45 ACP 1911... striaght wall case, by design, on paper, headspaces on the case mouth.... In reality, it actually headspaces on the extractor - the extractor holds the case in relation the bolt/slide face. The chamber could be +.020 and the carrtridge will still fire and function.
Worse case scenarios for any bad headspace conditon is inadequate headspace, coupled with a broken or worn rifle that will fire with the bolt unlocked - almost universally results in significant operator injuries. I did a forensic eval on an M14 NM rifle that fired out of battery. The culprit was a short headspace condition (cartridge based) and a broken feature in the receiver. M14's have a bridge that connects one side of the rcvr to the other - located near the rear of the bolt. Machined into this bridge is a cam type ramp that retracts the firing pin upon bolt unlocking /extraction. It also has a notch that allows the L shapped firing pin to move forward through the notch only when the bolt is fully rotated and in the locked position. Years of firing stout 1K loads had battered the bridge and a portion of it eventually cracked and literally fell out of the rifle. This eliminated the safety functions - the bolt failed to go into battery due to a long cartridge (improperly sized handload) and the operator - not knowing the bolt was unlocked, pulled the trigger. The rifle fired out of battery. The shooter had glasses on and was peppered with brass and powder. The right bolt lug sheared off and actually went through the visor of his hat....Lucky isn't a strong enough word.
So - in the end - HS is vitally important - but it isn't an accuracy issue that some think. I personally want and build all my stuff with SAAMI minimum HS. But I also control every round that goes into those chambers. I will also cut my chambers with HS that is .001 - .002 under my target dimension - this accounts for lug set back in new extensions & bolts. If I rebarrel a bolt gun with the same rcvr/bolt - I cut my HS to exactly what I want - the rcvr/bolt have already been conditioned. I go under .002 if using a new rcvr/bolt.
My headspace gauges are some of my most valuable tools - I wouldn't even attempt to chamber without them. I keep full sets in 223/556, 308/762 (.001 increments) and Go/No Go gauges for 20+ other cartridges.
I don't go near a reamer without them.
My .02
My .02
Thank you very much for sharing. Definitely great info. It always surprised me at how far below SAAMI minimum factory ammo measures with headspace gauges.
HD1911,
That sounds like an awesome rifle. About your carrier: How deep is the N/M’s grove wear in the MUR? Was difficult closing the bolt until it was broken in? Also, are you running that N/M with an H buffer or HD spring with the intermediate tube? Does the Mid-length system have a propensity for bolt bounce or high carrier velocity able to set back the ogive on the 262 Mod 0?
I’m emulating one of the older MSTN rig specs: Garden variety Noveske Recon 1-7, Larue upper & 11” hand guard. Lookin at the N/M +H2+red spring. I’m doing the homework on this thing as I graduated into precision from old-school carbines with classic FSBs.
Appreciate the advice
Sorry for taking so long to get a few pics... here they are:
This is at 2,031rnds
http://i102.photobucket.com/albums/m119/HD1911/Mobile%20Uploads/CEE5B6FC-A083-4107-B758-C2B0AE3106EA_zpsf37y9a1w.jpg
http://i102.photobucket.com/albums/m119/HD1911/Mobile%20Uploads/E705DF00-3D4D-4368-B58F-82785E79D82C_zpshaoqwitb.jpg
http://i102.photobucket.com/albums/m119/HD1911/Mobile%20Uploads/B21E31DA-CE05-489A-96C8-53B84581540E_zpsr6oeebbk.jpg
http://i102.photobucket.com/albums/m119/HD1911/Mobile%20Uploads/4F60B2D9-4744-4872-AFA0-804F627C5062_zpsnpqxyrdo.jpg
That unfortunately appears to be some poor quality anodizing. Type III Hardcoat - which is what should be used on these parts can by standard vary from a half a thou (.0005) to .0045. Carriers should NOT be a tight fit - there should be a level of "floating". Bear in mind as well that trigger assemblies can exert some measurable upward thrust to a carrier as well. Most of the the good quality uppers I have worked with and inspected also have a DFL (Dry Film Lubricant) applied to the interior. Anodizing in itself can be a tricky business - hugely dependent on operator QC. Weak baths, current variances and bath temperature are critical. Hardcoat - properly done needs to be a "cold" bath - this requires a really good system to cool it. Standard anodizing will heat a bath significantly. I suspect that this isn't a true hardcoat Type III anodizing. You can also see the machining marks from when the upper was bored for the carrier.
You may be able to have the interior treated with one of the high adhesion DFL coatings. This may slow the process of the carrier rails really wearing on the almost bare aluminum.
ETA: Please don't offense at my opinion - I would expect that an upper with that few rounds through it would show far less wear. In reading the whole post over - it appears that you were not critiquing the wear- but trying to show how slick the upper was. I have some uppers that have in excess of 15K rounds through them and they show far less wear than that.
No, I'm actually really glad you said something. I always wondered because since day one that Upper had me wondering, cause it never even had the Dry Film Lube in it, like my BCMs have all had. That is a Vltor MUR-1a Upper. I had always wondered what that Gold showing through was about it.
When lubed properly, she does feel very smooth and slick. I've never had a Malf., knock on wood.
Is this something I should be contacting Vltor/MSTN about?
Thank you for your knowledge and willingness to share it. No offense taken whatsoever.
opsoff1, lubricated cases is that likely to cause a problem? Wow. I'd consider trying that using some cheap crappy take-off components but don't have a set of headspace gauges.
It would be interesting to know exactly what the upper was made from - the default in the AR world is usually 7075-T6. The 7000 series of aluminum falls into the wrought aluminum category. All the numbers are specifications for the additives - specifically zinc. The T6 designation is that of heat treating (by solution) and artificially aged. The 7000 series is the strongest - but that depends really on the integrity of the designation. Also as written above - the quality/depth of the hardcoat anodizing. The wear that you see may be limited to the anodizing only. Without know the actual dimension prior to use - it is nothing but conjecture to speculate over actual wear.
The fact that the wear / contact areas are smooth / slick is a good sign. Outright galling would be cause for alarm.
Personally - I'd just shoot that SOB until it falls apart. Uppers / lowers are so cheap these days that they are almost disposable.
opsoff1, lubricated cases is that likely to cause a problem? Wow. I'd consider trying that using some cheap crappy take-off components but don't have a set of headspace gauges.
If you are leaving lube on cases and/or have oil or lubrication in the chamber - the potential for excessive bolt thrust is a real concern. Do it often enough and it is a guarantee that you will open up your headspace. It truly is not something to mess with. Headspace is typical of a lot of mechanical / engineering and firearm related designs in that a stacking of tolerances can get you into a bad situation. All those little things add up and eventually crosses the threshold of safety. Headspace sits squarely in that zip code. Always err on the side of caution.
The Vltor mur uppers are 7075 forgings. That color showing through is still hard coat, but the black dye has worn away. If you hard coat 7075 and don't use any dye, it's that gold/bronze color.
When a part comes out of the acid bath it is super porous, perfect to accept dye. So it goes into the dye bath, and then it's sealed.
A steel carrier is going to wear away the aluminum, so most likely what has happened is you've polished off the contact surfaces and taken the dye with it, which was only on the outside anyways. As long as you see gold/bronze and not silver, you haven't penetrated the hard coat layer.
The Vltor mur uppers are 7075 forgings. That color showing through is still hard coat, but the black dye has worn away. If you hard coat 7075 and don't use any dye, it's that gold/bronze color.
When a part comes out of the acid bath it is super porous, perfect to accept dye. So it goes into the dye bath, and then it's sealed.
A steel carrier is going to wear away the aluminum, so most likely what has happened is you've polished off the contact surfaces and taken the dye with it, which was only on the outside anyways. As long as you see gold/bronze and not silver, you haven't penetrated the hard coat layer.
I think Slippers hit the nail on the head above - I pulled some of my anodizing books out and looked at Type III Hardcoat; this flavor of hardcoat anodizing is actually really difficult to dye as the "cylinders" that grow on the surface are not optimal for dying. Without dye, the surface has a golden/copperish hue. As written above the thickness of hardcoat can be from .0005 to .0045, so if that interior is on the thin side, it may have a difficult time holding on to the dye.
Good call Slippers.
FromMyColdDeadHand
07-31-15, 00:38
Ok, that's a lot of theory, data and information on how to build an accurate AR. So, who is doing it in the real world, day after day in commercial guns?
rcoodyar15
08-11-15, 15:13
Interesting and thoughtful discussion - some additional points to bear in mind....
Headspace that varies a few thou either way has virtually zero affect on accuracy. In the context of this discussion, a headspace variance of say +/- .002 isn't going to show up on the target. So! I would ask those that have the their hand on the BS flag to take a breath and relax for a moment.
Consider this: In every rifle that uses a spring loaded ejector, a chambered cartridge is pushed forward in the chamber by the ejector so that the shoulder is up really close and personal with the shoulder in the chamber. When fired - and assuming the headspace is not so excessive that the firing pin can't touch off the primer, the case becomes almost plastic and sticks to the chamber walls (by design). This is evidenced by two points;
1) Excess headspace is shown by primers that are backed out of the case and
2) Cases will stretch near the back - usually about .300" up from the case head. This is where we would see insipient case head separations.
A barrel / bolt combo that is on the loose side of the headspace spectrum will almost never show any degradation in accuracy. The reality is that at the time of firing, headspace is really not a factor in accuracy. (Keep the BS flag in your pocket!) The reason behind this is again, the ejector spring pressure keeps the case all the way forward in the chamber....always. Now - uneven lug seats, bolt faces that are not perpendicular to the bore, non concentric chambers will all affect accuracy. The assumption through out this discussion is that the rifle has no other flaws / variables.
Conversely, If you ever want to open up headspace - just leave your sizing lube on a case or lubricate your chamber - it will increase bolt thrust so much that after 5 or 6 rounds, you are almost assured of an additional few thous of HS and proabably a sheared lug or two. Clean dry cases and chambers are the waypoints of operator & rifle longevity.
For those circumstances when the headspace is truly excessive - we are now dealing with a serious safety issue. Excess headspace will push those primers out too far and they will leak at best. Worse is a completely blown primer. Catastrophic, is headspace that is large enough to cause a case head separation where the cartridge has stretched so much that it separates upon extraction.
I have in the course of many many years, chambered hundreds of barrels - both in bolt guns and gas guns. For match type rifles - I always headspace to the SAAMI minimum. Not for accuracy, but rather knowing that the vast majority of shooters that want this type of rifle built are also hand loading for it too. Minimum headspace will prevent excess stretching of the cases and prolong their life.
A few other points to ponder as well. I would ask that if you have the capability to accurately measure cartridge headspace - do so for some enlightenment. In factory ammo, it is not uncommon to see match grade ammo have cartridge headspace that is .004 - .006 under SAAMI minimum. This is why virtually every round of OEM ammo will chamber and fire. (A side note to measuring Cartidge HS using the Stony Point / Hornandy or any gauge of that type - proof it first against known / verified HS gauges - I tested some Stony Point examples and they were over by +.008.) Another point to ponder - consider a .45 ACP 1911... striaght wall case, by design, on paper, headspaces on the case mouth.... In reality, it actually headspaces on the extractor - the extractor holds the case in relation the bolt/slide face. The chamber could be +.020 and the carrtridge will still fire and function.
Worse case scenarios for any bad headspace conditon is inadequate headspace, coupled with a broken or worn rifle that will fire with the bolt unlocked - almost universally results in significant operator injuries. I did a forensic eval on an M14 NM rifle that fired out of battery. The culprit was a short headspace condition (cartridge based) and a broken feature in the receiver. M14's have a bridge that connects one side of the rcvr to the other - located near the rear of the bolt. Machined into this bridge is a cam type ramp that retracts the firing pin upon bolt unlocking /extraction. It also has a notch that allows the L shapped firing pin to move forward through the notch only when the bolt is fully rotated and in the locked position. Years of firing stout 1K loads had battered the bridge and a portion of it eventually cracked and literally fell out of the rifle. This eliminated the safety functions - the bolt failed to go into battery due to a long cartridge (improperly sized handload) and the operator - not knowing the bolt was unlocked, pulled the trigger. The rifle fired out of battery. The shooter had glasses on and was peppered with brass and powder. The right bolt lug sheared off and actually went through the visor of his hat....Lucky isn't a strong enough word.
So - in the end - HS is vitally important - but it isn't an accuracy issue that some think. I personally want and build all my stuff with SAAMI minimum HS. But I also control every round that goes into those chambers. I will also cut my chambers with HS that is .001 - .002 under my target dimension - this accounts for lug set back in new extensions & bolts. If I rebarrel a bolt gun with the same rcvr/bolt - I cut my HS to exactly what I want - the rcvr/bolt have already been conditioned. I go under .002 if using a new rcvr/bolt.
My headspace gauges are some of my most valuable tools - I wouldn't even attempt to chamber without them. I keep full sets in 223/556, 308/762 (.001 increments) and Go/No Go gauges for 20+ other cartridges.
I don't go near a reamer without them.
My .02
Well I find that if I use a body die and bump the shoulder back about 0.003 to .004 and then neck size that it does improve the accuracy of my rifle.
Very sound advice........after shooting weekend warrior "precision" for the last 10 years, I have concluded that using a quality match grade barrel and upgarded trigger are the two most important and impactful parts to a precision built AR.
Now put 15 more shots on one target and submit your score to the MOA challenge........I will already pencil you in at the #1 spot!!! :cool:
rcoodyar15
08-12-15, 13:21
Here is an interesting article from someone who does it for a living
http://bulletin.accurateshooter.com/2013/07/building-an-accurate-ar-robert-whitley-explains-what-works/
Building an Accurate AR — Robert Whitley Explains What Works
In our Shooters’ Forum, one member posed the question: “What makes an AR accurate? What parts on an AR can really affect accuracy — such as free-floating handguards, barrels, bolts, bolt carriers?” He wanted an honest, well-informed answer, not just sales pitches. Robert Whitley posted a very comprehensive answer to this question, based on his experience building and testing dozens of AR-platform rifles. Robert runs AR-X Enterprises, which produces match-grade uppers for High Power competitors, tactical shooters, and varminters.
AR-X AR15 Upper
Building an Accurate AR — What is Most Important
by Robert Whitley
There are a lot of things that can be done to an AR to enhance consistent accuracy, and I use the words “consistent accuracy” because consistency is a part of it (i.e. plenty of guns will give a couple great 5-shot groups, but won’t do a very good 10- or 20-shot groups, and some guns will shoot great one day and not so good on others).
Here are things we think are important to accuracy.
1. Great Barrel: You’ll want a premium match-grade barrel, well-machined with a good crown and a match-type chambering, true to the bore and well cut. The extension threads must also be cut true to the bore, with everything true and in proper alignment.
2. Rigid Upper: A rigid, heavy-walled upper receiver aids accuracy. The typical AR upper receiver was made for a lightweight carry rifle and they stripped all the metal they could off it to make it light to carry (which is advantageous for the military). The net result are upper receivers that are so thin you can flex them with your bare hands. These flexible uppers are “strong enough” for general use, but they are not ideal for accuracy. Accuracy improves with a more rigid upper receiver.
AR-X AR15 Upper
3. True Receiver Face: We’ve found that truing the receiver face is valuable. Some may argue this point but it is always best to keep everything related to the barrel and the bore in complete alignment with the bore (i.e. barrel extension, bolt, upper receiver, carrier, etc.).
4. Barrel Extension: You should Loctite or glue the barrel extension into the upper receiver. This holds it in place all the way front to back in the upper receiver. Otherwise if there is any play (and there typically is) it just hangs on the face of the upper receiver completely dependent on the face of the upper receiver as the sole source of support for the barrel as opposed to being made more an integral part of the upper receiver by being glued-in.
AR-X AR15 Upper5. Gas Block: You want a gas block that does not impose pointed stress on the barrel. Clamp-on types that grab all the way around the barrel are excellent. The blocks that are pinned on with tapered pins that wedge against the barrel or the slip on type of block with set screws that push up from underneath (or directly on the barrel) can deform the bore inside of the barrel and can wreck the accuracy of an otherwise great barrel.
6. Free-Float Handguard: A rigid, free-float handguard (and I emphasize the word rigid) really makes a difference. There are many types of free-float handguards and a free-float handguard is, in and of itself, a huge improvement over a non-free-float set up, but best is a rigid set-up. Some of the ones on the market are small diameter, thin and/or flexible and if you are shooting off any type of rest, bipod, front bag, etc., a rigid fore-end is best since ARs want to jump, bounce and twist when you let a shot go, as the carrier starts to begin its cycle before the bullet exits the bore.
7. Barrel Contour: You want some meat on the barrel. Between the upper receiver and the gas block don’t go real thin with a barrel (we like 1″ diameter if it’s workable weight-wise). When you touch off a round and the bullet passes the gas port, the gas system immediately starts pressuring up with a gas impulse that provides vibrations and stress on the barrel, especially between the gas block back to the receiver. A heavier barrel here dampens that. Staying a little heavier with barrel contour through the gas block area and out to the muzzle is good for the same reasons. ARs have a lot going on when you touch off a round and the gas system pressures up and the carrier starts moving (all before the bullet exits the bore) so the more things are made heavier and rigid to counteract that the better — within reason (I’m not advocating a 12-lb barrel).
8. Gas Tube Routing Clearance: You want a gas tube that runs freely through the barrel nut, through the front of the upper receiver, and through the gas key in the carrier. Ensure the gas tube is not impinged by any of them, so that it does not load the carrier in a stressed orientation. You don’t want the gas tube bound up so that when the gas tube pressures up it immediately wants to transmit more force and impulse to the barrel than would normally occur. We sometimes spend a lot of time moving the gas block with gas tube on and off new build uppers and tweaking gas tubes to get proper clearance and alignment. Most gas tubes do need a little “tweaking” to get them right — factory tubes may work OK but they typically do not function optimally without hand-fitting.
9. Gas Port Tuning: You want to avoid over-porting the gas port. Being over-gassed makes the gas system pressure up earlier and more aggressively. This causes more impulse, and increases forces and vibration affecting the top end and the barrel. Tune the gas port to give the amount of pressure needed to function properly and adequately but no more.
10. Front/Back Bolt Play: If accuracy is the game, don’t leave a lot of front/back bolt play (keep it .003″ but no more than .005″). We’ve seen factory rifles run .012″ to .015″ play, which is OK if you need to leave room for dirt and grime in a military application. However, that amount of play is not ideal for a high-accuracy AR build. A lot of front/back bolt play allows rounds to be hammered into the chamber and actually re-formed in a non-consistent way, as they are loaded into the chamber.
11. Component Quality: Use good parts from a reputable source and be wary of “gun show specials”. All parts are NOT the same. Some are good, some are not so good, and some aftermarket parts are simply bad. Don’t be afraid to use mil-spec-type carriers; by and large they are excellent for an accuracy build. Also, remember that just because a carrier says “National Match” or something else on it does not necessarily mean it’s any better. Be wary of chrome-plated parts as the chrome plating can change the parts dimensionally and can also make it hard to do hand-fitting for fit and function.
AR-X AR15 Upper
12. Upper to Lower Fit: A good upper/lower fit is helpful. For quick and dirty fit enhancement, an Accu-Wedge in the rear helps a lot. The ultimate solution is to bed the upper to a specific lower so that the upper and lower, when together, are more like one integral unit. For the upper receivers we produce, we try to get the specs as close as we can, but still fit the various lowers in the market place.
13. Muzzle Attachments: Don’t screw up the muzzle (literally). Leave as much metal on the barrel at the muzzle as you can. People like to thread the muzzle for a flash hider, suppressor, muzzle brake, or some other attachment, but if you really want accuracy, leave as much metal as you can there. And, if you have something that screws on, set it up so that it can be put on and have it stay there without putting a lot of torque and stress on it right where the bullet exits the bore. If you are going to thread the end of the barrel, make it concentric with the bore and make sure what you screw on there is as well. For all muzzle attachments, also ensure that the holes through which the bullet passes through are dead true to the bore. Many aftermarket screw-on things are not so good that way. Anything that vents gas should vent symmetrically (i.e. if it vents left, it should vent equally right, and likewise, if it vents up, it should vent down equally). Uneven venting of gas can wreck accuracy.
14. Quality Ammunition: Ammo is a whole story by itself, but loads that are too hot typically shoot poorly in an AR-15. If you want accuracy out of an AR-15, avoid overly hot loads. Shown below are test groups shot with four (4) different uppers, all with moderate loads. These four uppers all pretty much had the same features and things done to them as explained in this article, and they all shot great.
AR-X AR15 Upper
Robert Whitley
www.6mmAR.com
Failure2Stop
08-12-15, 16:18
Incorrect information in the above post: The bolt does not reach operating pressure until after the projectile leaves the bore.
Front/back bolt play? Is this referencing headspace?
From Tapatalk:
Jack Leuba
Knight's Armament Company: Military/Govt Product Liaison
F2S Consulting: Director of Shooting Stuff
rcoodyar15
08-12-15, 16:39
Incorrect information in the above post: The bolt does not pressure until after the projectile leaves the bore.
Front/back bolt play? Is this referencing headspace?
From Tapatalk:
Jack Leuba
Knight's Armament Company: Military/Govt Product Liaison
F2S Consulting: Director of Shooting Stuff
so what keeps the gasses from going through the gas block and into the receiver after the bullet passes the gas port in the bore?
Failure2Stop
08-12-15, 16:45
The pressure wave moves at the speed of sound. The projectile is moving 2-3x the speed of sound when it hits the port, depending on the port distance from the chamber. As long as the distance from the port to muzzle is less than 2x as long as the distance from the port to the gas key, the projectile exits before the expansion chamber reaches pressurization.
From Tapatalk:
Jack Leuba
Knight's Armament Company: Military/Govt Product Liaison
F2S Consulting: Director of Shooting Stuff
rcoodyar15
08-12-15, 17:19
The pressure wave moves at the speed of sound. The projectile is moving 2-3x the speed of sound when it hits the port, depending on the port distance from the chamber. As long as the distance from the port to muzzle is less than 2x as long as the distance from the port to the gas key, the projectile exits before the expansion chamber reaches pressurization.
From Tapatalk:
Jack Leuba
Knight's Armament Company: Military/Govt Product Liaison
F2S Consulting: Director of Shooting Stuff
so you are saying the hot gasses from the burning powder are expanding at a slower rate than the projectile they are propelling down the barrel?
We know when the rifle is fired the pressure in the chamber is about 50 to 55 thousand psi. That is some high pressure gas. I believe the gasses would pass the bullet if it would get out of the way.
The pressure wave moves at the speed of sound. The projectile is moving 2-3x the speed of sound when it hits the port, depending on the port distance from the chamber. As long as the distance from the port to muzzle is less than 2x as long as the distance from the port to the gas key, the projectile exits before the expansion chamber reaches pressurization.
From Tapatalk:
Jack Leuba
Knight's Armament Company: Military/Govt Product Liaison
F2S Consulting: Director of Shooting Stuff
I'm not sure how you are explaining this - i.e. the terms used - but respectfully, I am going to disagree with your explanation.
The pressure wave moves at speeds way beyond the speed of sound. Data from the Sandia National Labs - specifically their Aeroballistics Division ran tests using spark shadow graphs. Actual tests show that muzzleblast pressure was in excess of 5000 psi for a 6mmBR and 11,500 for a .270 Win. How can you have a pressure wave that is the driving force to propel the projectile moving at the speed of sound and yet drive said projectile at 2-3X that? I can show you all the calculations and formulas as well as the actual data. This is supported by enormous data troves developed by Krupp and explained in painful detail in Thermodynamics of Firearms; LTC Clark Shove Robinson, Dept of Chemical Engineering, MIT, McGraw Hill NY 1943. Harold Vaughn has extensive data as well. (He of Sandia Nat Labs)
The gas system of a rifle such as the AR is charged and pressurized within milliseconds as the bullet passes the port. The rifle bore and chamber/cartridge case depressurizes when the bullet leaves the bore and this depressurization allows for extraction. Actual measurements of the gas port pressure in M16's & M4's show pressures between 13,000 & 26,000 PSI. Exothermic chemical reactions - read gunpowder, exhibit gas velocities up to a physical maximum of about 6,000 fps. Actual data from modern smokeless powders indicate that maximum gas velocities are in the neighborhood of 5,000 fps. This is all born out in the issues that the M4 experienced with extraction with very high chamber pressures as well as the velocity of the carrier and the rotational speed of the bolt.
Regarding the front to back bolt play - this is in reference to fitting the barrel extension to the barrel and the bolt lug depth. The distance between the bearing surface (backside) of the locking lugs in the extension to the breach face of the barrel should be greater than the depth or length of the locking lugs on the bolt. If this distance from the breach face to the back of the extension locking lugs is smaller that the bolt lug length - the bolt cannot rotate. The premise is that if the dimension is excessive, the bolt has the added distance to pound the cartridge case into the chamber and possibly deform the shoulder/neck. The gap should be held to nominal .002 - .005". Don't give the bolt extra space to "take a run at the case" Additionally, in precision AR's many builders / shooters opt for lighter/shorter ejector springs. This effectively reduce the shock absorber effect when the cartridge is chambered. The reduced gap also provides for a better supported chamber - if only a few thousandths.
This has nothing to do with head space - totally different set of datum points.
So - nothing personal - presented in the spirit of healthy discourse.
so you are saying the hot gasses from the burning powder are expanding at a slower rate than the projectile they are propelling down the barrel?
We know when the rifle is fired the pressure in the chamber is about 50 to 55 thousand psi. That is some high pressure gas. I believe the gasses would pass the bullet if it would get out of the way.
The gasses actually do pass the bullet - spark shadow graphs show the gasses exiting the muzzle before the bullet - the bullet is about 3" from the crown when the gasses "blow by".
Failure2Stop
08-12-15, 23:57
I'm not sure how you are explaining this - i.e. the terms used - but respectfully, I am going to disagree with your explanation.
Your objection to my simplified explanation is understood.
I'll give a more detailed response tomorrow when I have access to resources and time to reply.
Regarding the front to back bolt play - this is in reference to fitting the barrel extension to the barrel and the bolt lug depth. The distance between the bearing surface (backside) of the locking lugs in the extension to the breach face of the barrel should be greater than the depth or length of the locking lugs on the bolt. If this distance from the breach face to the back of the extension locking lugs is smaller that the bolt lug length - the bolt cannot rotate. The premise is that if the dimension is excessive, the bolt has the added distance to pound the cartridge case into the chamber and possibly deform the shoulder/neck.
Gotcha.
Seems to me that it would take a significant gap for that to be an issue, but eliminating that variable isn't without benefit when looking for absolute precision.
So - nothing personal - presented in the spirit of healthy discourse.
Absolutely nothing personal.
I made a statement that is in conflict with your data. Understandable that you would want clarification.
From Tapatalk:
Jack Leuba
Knight's Armament Company: Military/Govt Product Liaison
F2S Consulting: Director of Shooting Stuff
Failure2Stop
08-13-15, 12:03
Ok, finally rested from my 24 hours in airplanes and airports, and have access to my source data.
I can't get too specific, as there are proprietary matters, however, I can disclose that we use high-speed video and pressure sensors that are accurate to silly small measurements, and have really smart engineer guys that translate that data for knuckle-draggers like me. I was corrected in that speed of sound was the local speed of sound, not the ambient speed of sound. Mea culpa.
The collected physical data shows that the bolt carrier begins rearward movement in about twice the time it takes for the projectile to travel from the chamber to the muzzle.
As the carrier begins its rearward movement, it travels approximately 0.075" before the cam-pin hits the first angle of the cam path, which gives additional delay to unlocking.
The gasses actually do pass the bullet - spark shadow graphs show the gasses exiting the muzzle before the bullet - the bullet is about 3" from the crown when the gasses "blow by".
So gases produced by powder ignition pass the projectile before the bearing surface of the projectile seals with the lands and grooves just past the leade? Or do gases pass the projectile while it travels down the barrel? With a force fit between the barrel and projectile in a good barrel, I am having difficulty understanding how the gas gets past the projectile in the barrel.
I agree with your comment about headspace and accuracy. A few of my best shooting rifles had long headspace and throat erosion, but still shot well. I believe that muzzle crown wear and the wear in the last few inches of barrel behind the crown have more of an effect on accuracy than headspace or throat erosion.
So gases produced by powder ignition pass the projectile before the bearing surface of the projectile seals with the lands and grooves just past the leade? Or do gases pass the projectile while it travels down the barrel? With a force fit between the barrel and projectile in a good barrel, I am having difficulty understanding how the gas gets past the projectile in the barrel.
I agree with your comment about headspace and accuracy. A few of my best shooting rifles had long headspace and throat erosion, but still shot well. I believe that muzzle crown wear and the wear in the last few inches of barrel behind the crown have more of an effect on accuracy than headspace or throat erosion.
I wish I could post the spark shadow graph pictures - but to address your comment, the bullet actually never completely seals the bore - expanding gas begins to pass the bullet from the instant it leaves the case and will exit the bore prior to the projectile itself. I tried to google image search it - but nada. I have the books at home. The photos I have at home indicate, along with the data that the bullet is about 3" from the muzzle when the gases exit the bore.
It is also interesting to note - that some manufacturers over the years, have incorporated a "gas ring" at the base or at the bearing surface/boattail juncture. This was done in an effort to promote a better seal.
I'll see if I can copy and post the pictures - without brealing any copyright laws.
The crown on a precison rifle barrel is HUGELY important. It is also the area that is most prone to damage from either handling, cleaning or even shooting. I have brought a fair number of barrels back from demise by very careful recrowning. All done on a lathe - always. The hand tools that are available a junk. Over time, simple gas blow by when the bullet exits the barrel imparts erosion to the crown. I use different crowns for different applications as well.
Hope that helps.
So gases produced by powder ignition pass the projectile before the bearing surface of the projectile seals with the lands and grooves just past the leade? Or do gases pass the projectile while it travels down the barrel? With a force fit between the barrel and projectile in a good barrel, I am having difficulty understanding how the gas gets past the projectile in the barrel.
I agree with your comment about headspace and accuracy. A few of my best shooting rifles had long headspace and throat erosion, but still shot well. I believe that muzzle crown wear and the wear in the last few inches of barrel behind the crown have more of an effect on accuracy than headspace or throat erosion.
That would be my opinion as well.
C4
rcoodyar15
08-13-15, 13:44
Ok, finally rested from my 24 hours in airplanes and airports, and have access to my source data.
I can't get too specific, as there are proprietary matters, however, I can disclose that we use high-speed video and pressure sensors that are accurate to silly small measurements, and have really smart engineer guys that translate that data for knuckle-draggers like me. I was corrected in that speed of sound was the local speed of sound, not the ambient speed of sound. Mea culpa.
The collected physical data shows that the bolt carrier begins rearward movement in about twice the time it takes for the projectile to travel from the chamber to the muzzle.
As the carrier begins its rearward movement, it travels approximately 0.075" before the cam-pin hits the first angle of the cam path, which gives additional delay to unlocking.
Ok here is my understanding of what happens in an AR in less that the blink of an eye. Explain what I am getting wrong
cartridge is fired in the chamber and the bullet starts its travel down the barrel
bullet passes the gas port in the bore and instantaneously 20,000 psi of pressure travels through the gas block, gas tube and into the bolt carrier.
at this point pressures are so high that they actually push the bolt into the chamber as well as pushing the carrier back balancing the pressures and not allowing the bolt to unlock
when the bullet leaves the barrel pressures drop in the chamber and bore and allow the bolt to unlock and and the carrier to travel rearward removing the spent brass.
that is the way I understand it.
The crown on a precison rifle barrel is HUGELY important. It is also the area that is most prone to damage from either handling, cleaning or even shooting. I have brought a fair number of barrels back from demise by very careful recrowning. All done on a lathe - always. The hand tools that are available a junk. Over time, simple gas blow by when the bullet exits the barrel imparts erosion to the crown. I use different crowns for different applications as well.
Hope that helps.
I would not use a hand tool to resurface a muzzle crown on a bolt action rifle, but I have had some success with the PTG muzzle crown facing tool on M1 Garand barrels. The last barrel crown I resurfaced turned a 5 MOA Garand to a sub 3 MOA shooter. The PTG tool is definitely not for the impatient or ham fisted and a lathe would be preferred, but I no longer have access to precision machines.
Failure2Stop
08-13-15, 15:24
The forward pressure on the bolt during expansion is less than it sounds like.
It's pretty much just enough to reduce the contact friction that is held at the lugs from initial set-back and the case sticking in the chamber from obturation.
If pressure is applied to the rear of the bolt, pushing it forward, that would aid in unlocking, just as what happens when the carrier is pushed rearward.
The pressure at the gas key and gas key end of the gas tube can be tracked and timed against the timing of the pressure at the port.
The physical data shows pressure levels rising in different parts of the barrel and gas system at measurable and consistent time intervals.
I would not use a hand tool to resurface a muzzle crown on a bolt action rifle, but I have had some success with the PTG muzzle crown facing tool on M1 Garand barrels. The last barrel crown I resurfaced turned a 5 MOA Garand to a sub 3 MOA shooter. The PTG tool is definitely not for the impatient or ham fisted and a lathe would be preferred, but I no longer have access to precision machines.
Have you used the Manson Muzzle Crown Refacing Tool Kit? Excellent piece of kit and in some ways better than a lathe.
C4
In case you might need to clean your gas tube I point the barrel down at about a 45 degree angle and spray into the tube brake cleaner with the tiny tube attached to the cans push nozzle. The brake fluid should flow freely out of the barrel to be cleaned properly. I usually do this every time I am cleaning the action with brake fluid then let it dry good and lubricate like you normally would do. I have never had a problem with anything since I started doing this 15 years ago.
I can't get too specific, as there are proprietary matters, however, I can disclose that we use high-speed video and pressure sensors that are accurate to silly small measurements, and have really smart engineer guys that translate that data for knuckle-draggers like me. I was corrected in that speed of sound was the local speed of sound, not the ambient speed of sound. Mea culpa.
Again - no disrespect, but I have no idea what you are trying to say. Local speed of sound? Ambient speed of sound? Ambient is the immediate surrounding - so I am assuming you are referring to the pressure and velocity of the gas at the muzzle?
Basic chemistry and more specifically, thermodynamics as related to firearms and ammunition adhere to all the same laws of physics as does everything else in the world. With that in context, small arms propellants are classified as low order explosives. The most powerful of these low order explosives are these same smokeless powders. In low order explosives, the process of burning or "decomposition" is termed deflagration. This process produces heat, light and a subsonic pressure wave. (Much to our testosterone fueled delight) However, it is important to note that this is in an uncontained state - like pouring a pile of 4895 on the ground and sparking it up. The pressure wave is subsonic, meaning it does not produce sound waves in the infrasound (very low) or the ultra sound (very high) frequencies (neither of which is in the range of human hearing) - so all you hear is the fizzing of the burning. There is no blast. So, when uncontained - the pressure wave or sound wave presents itself as subsonic - it doesn't exceed the speed of sound (1126fps in dry air at 68 deg F) However, when contained in small vessels (cartridge cases & bores) the pressures generated can exceed the material strength of the barrel / firearm. These extreme pressures and rapid expansion of the decomposing propellant is what we see exiting from the muzzle as the bullet clears the barrel. High pressure,supersonic sound waves - give us the muzzle blast and accompanying explosive like sound report. Measurements and calculations indicate that the velocity of these gases exiting from the bore (known as the velocity of the reaction front) hover around 5200 fps for a long gun.
The process of burning the powder and building the pressure to move the bullet down the bore is a Pressure/Volume/Temperature problem. Smokeless powder contains its own oxidizing agent - it can feed itself. More heat builds pressure, more pressure builds heat, and so on - the variable is the volume of the vessel which changes as the pressure moves the bullet down the bore.
In your original response, you stated that the pressure wave moves at the speed of sound - this is reasonable statement for propellant that is burning in an uncontained state. However, everything changes in a closed vessel. Since we have all sorts of burn rates, single base, double base and even triple base powders, the velocity of the reaction front at the muzzle is accepted within the engineering and physics circles to be 4000 - 6000 fps. As stated above and earlier - the mean is accepted to be approximately 5200fps for a rifle. Once clear of the bore an in an uncontained state, this gas pressure front cools rapidly and dissipates.
The collected physical data shows that the bolt carrier begins rearward movement in about twice the time it takes for the projectile to travel from the chamber to the muzzle.
I had to really think about this - and this is understandable, but it is very misleading. Establishing a point of measurement from the chamber to compare carrier movement is unfair as there is nothing to move the carrier. Nothing should be moving other than the bullet in the bore until the bullet passes the gas port. Once this happens, the expanding gas enters the gas tube and expansion chamber in the carrier. In a full sized 20" AR/M16, the port is 6.825" from the muzzle. This allows for a pressurization of the bore, chamber, gas tube and carrier for only the time that the bullet is between the port and the muzzle. That is a very very short amount of time. I have seen data that indicatates it is south of 1 millisecond. As soon as the bullet clears the muzzle - the pressure drops. What allows for the whole mechanism to work is the momentum of the gas "charge". This is where the port tuning becomes so important. Port location dictates the pressure available at the port. (13,000 psi for an M16, 26,000 psi for an M4) Port diameter dictates the volume of the gas that gets sent down the tube. In a crude analogy for port diameters, if you have a baseball traveling at 1000 fps and a bowling ball traveling at 1000 fps - which is going to do more work? Conversely, when we attach a can to the muzzle - we are in effect lengthening the bore and this pressurizes the system longer and cause all sorts of issues, not the least of which is hard recoil. Again - I point to the M4 problems - all directly related to the gas port location change and the accompanying spike in port pressures.
ETA - on the flip side and as an example of momentum - look at an M1 Garand - the port is an inch from the muzzle - that bullet barely even travels one caliber before it clears the barrel - so the propellant gas gets a RHCH nanosecond to impart a punch to the oprod - its all momentum baby!
As the carrier begins its rearward movement, it travels approximately 0.075" before the cam-pin hits the first angle of the cam path, which gives additional delay to unlocking.
I absolutely agree and understand that. This is engineered into the design This delay (aka dwell) is required to allow chamber pressures to subside in order for extraction to occur. Momentum is force multiplier! This dwell is also part of the inherent accruacy of this system. If a shooter can exhibit proper follow through with an AR type rifle, there is no mechanical disadvantage to the system other than the slow lock time, but that is up front, prior to ignition. Unfortunately this isn't the case with an M4 type system - the early bolt unlocking and high rotational speed of the bolt actually begin when there is still significant pressure in the chamber.
As a qualifier, I am a bit reluctant to say that this isn't merely a hobby. I don't like to get into resume' competitions - but I offer this as a bit of a qualification to the information presented, this isn't stuff I pulled out of a comic book while fondling myself in my mothers basement - I work for one of the premier DOD R&D/S&T labs in the country and my section deals with ballistics, blast protection, terminal performance, body armor and individual protection and individual equipment as well as ballistic threat assessments. We live it, breathe it and eat it everyday. Hopefully the information will provoke rational thought and discussion of these wonderful little rifles.
Failure2Stop
08-14-15, 13:22
Alrighty, had to tap one of my engineers for the following response since I am not an engineer.
Some of the reply had to be edited to remove proprietary knowledge and design information.
Again - no disrespect, but I have no idea what you are trying to say. Local speed of sound? Ambient speed of sound? Ambient is the immediate surrounding - so I am assuming you are referring to the pressure and velocity of the gas at the muzzle?
RD engineering analyst responding, I don’t want to hurt any feelings, my goal is to educate, but please realize I cannot give any proprietary knowledge.
The gas field in a gas system consists of constantly varying pressures, velocities, and temperatures, where every location and point of time has a different speed of sound.
Basic chemistry and more specifically, thermodynamics as related to firearms and ammunition adhere to all the same laws of physics as does everything else in the world. With that in context, small arms propellants are classified as low order explosives. The most powerful of these low order explosives are these same smokeless powders. In low order explosives, the process of burning or "decomposition" is termed deflagration. This process produces heat, light and a subsonic pressure wave. (Much to our testosterone fueled delight) However, it is important to note that this is in an uncontained state - like pouring a pile of 4895 on the ground and sparking it up. The pressure wave is subsonic, meaning it does not produce sound waves in the infrasound (very low) or the ultra sound (very high) frequencies (neither of which is in the range of human hearing) - so all you hear is the fizzing of the burning. There is no blast. So, when uncontained - the pressure wave or sound wave presents itself as subsonic - it doesn't exceed the speed of sound (1126fps in dry air at 68 deg F)
Nothing wrong here other than sound emanating from this burn occurs at the speed of sound( not less than) and this has nothing to do with combustion in firearms.
However, when contained in small vessels (cartridge cases & bores) the pressures generated can exceed the material strength of the barrel / firearm.
This would mean that the gun is exploding, but since we know what these pressures are we are able to design for them, and alternately a light finger press could break a small enough part, but this really isn't pertinent to the discussion.
These extreme pressures and rapid expansion of the decomposing propellant is what we see exiting from the muzzle as the bullet clears the barrel. High pressure,supersonic sound waves - give us the muzzle blast and accompanying explosive like sound report. Measurements and calculations indicate that the velocity of these gases exiting from the bore (known as the velocity of the reaction front) hover around 5200 fps for a long gun.
The process of burning the powder and building the pressure to move the bullet down the bore is a Pressure/Volume/Temperature problem. Smokeless powder contains its own oxidizing agent - it can feed itself. More heat builds pressure, more pressure builds heat, and so on - the variable is the volume of the vessel which changes as the pressure moves the bullet down the bore.
The combustion is an internal energy equaling PVT problem.
Only at immediate muzzle exit do you have a transition to supersonic, based on rapid acceleration behind the initial shock wave as energy contained in this wave dissipates it slows to a simple expanding sound wave. The time scale on this event is about 10^-6s. The muzzle blast is a pressure release not an explosion, there is no additional energy being released.
In your original response, you stated that the pressure wave moves at the speed of sound - this is reasonable statement for propellant that is burning in an uncontained state. However, everything changes in a closed vessel. Since we have all sorts of burn rates, single base, double base and even triple base powders, the velocity of the reaction front at the muzzle is accepted within the engineering and physics circles to be 4000 - 6000 fps. As stated above and earlier - the mean is accepted to be approximately 5200fps for a rifle. Once clear of the bore an in an uncontained state, this gas pressure front cools rapidly and dissipates.
Burn rate has nothing to do with gas velocity and again at the muzzle there is no continued explosion, only the rapid expansions of gas into an ambient environment.
I had to really think about this - and this is understandable, but it is very misleading. Establishing a point of measurement from the chamber to compare carrier movement is unfair as there is nothing to move the carrier. Nothing should be moving other than the bullet in the bore until the bullet passes the gas port. Once this happens, the expanding gas enters the gas tube and expansion chamber in the carrier.
We have the capability to see the pressure and motion and link them to a common time accurate to 1/200,000 s. The combustion can be seen as well as time pressure response at gas port and beginning of the gas tube.
The misconception is that everything needs to occur the same way a balloon fills, flow has inertia and once gas has started it will continue. This will occur at a predictable velocity based on pressure expanding into free air with a substantial wall friction.
In a full sized 20" AR/M16, the port is 6.825" from the muzzle. This allows for a pressurization of the bore, chamber, gas tube and carrier for only the time that the bullet is between the port and the muzzle.
This is another gun lore misconception, as said below gas has momentum and does not need to have a sealed system to work. Using this example, projectile in bore time is 2.29*10^-4. Gas travel time in gas tube is substantially greater.
That is a very very short amount of time. I have seen data that indicatates it is south of 1 millisecond. As soon as the bullet clears the muzzle - the pressure drops. What allows for the whole mechanism to work is the momentum of the gas "charge". This is where the port tuning becomes so important. Port location dictates the pressure available at the port. (13,000 psi for an M16, 26,000 psi for an M4) Port diameter dictates the volume of the gas that gets sent down the tube. In a crude analogy for port diameters, if you have a baseball traveling at 1000 fps and a bowling ball traveling at 1000 fps - which is going to do more work? Conversely, when we attach a can to the muzzle - we are in effect lengthening the bore and this pressurizes the system longer and cause all sorts of issues, not the least of which is hard recoil. Again - I point to the M4 problems - all directly related to the gas port location change and the accompanying spike in port pressures.
This is actually not the case, the analogy is much closer to stepping on a garden hose and flow decreasing.
Ok, me back:
The net of the discussion is this: original claim that "ARs want to jump, bounce and twist when you let a shot go, as the carrier starts to begin its cycle before the bullet exits the bore". Physical evidence proves that this is not the case unless intentionally producing a system that does so.
Jack, again with respect,
The gas field in a gas system consists of constantly varying pressures, velocities, and temperatures, where every location and point of time has a different speed of sound.
Agreed - but specific to a gas system as there are restrictions (ports) vents, leaks etc. In a closed bore - like a bolt action rifle, the pressure is not linear, but rather when plotted as time vs pressure, it generates a parabolic curve.
I am not sure what you are talking about when you say "a different speed of sound". Sound travels at a constant; 343 m/s or 1126 fps. Frequency??
Nothing wrong here other than sound emanating from this burn occurs at the speed of sound (not less than) and this has nothing to do with combustion in firearms
I didn’t say it was less – what I was trying to impart was specific to the frequencies of the pressure wave. We were discussing the velocity of the reaction front – AKA the pressure wave that emanates from the muzzle generates the noise we hear. The noise we hear is at “sonic” or the speed of sound levels – not supersonic or subsonic - which we can’t hear. Uncontained powder burns – it doesn’t go bang was my point.
This would mean that the gun is exploding, but since we know what these pressures are we are able to design for them, and alternately a light finger press could break a small enough part, but this really isn't pertinent to the discussion.
Not what I was trying to say - the context of this was that different powders can generate dangerous / catastrophic pressure levels. Do you think an AR would fare ok with a case full of N310? In more applicable terms, if we load too fast of a rifle powder, the pressure at the port is insufficient to cycle to system – and conversely, too slow a powder can impart port pressures that are far more than needed, usually resulting in damage to the rifle. (Just like M1 Garands w/ heavy bullets and slow powder – they bend op rods)
The combustion is an internal energy equaling PVT problem.
I stated this two posts ago “The process of burning the powder and building the pressure to move the bullet down the bore is a Pressure/Volume/Temperature problem. Smokeless powder contains its own oxidizing agent - it can feed itself. More heat builds pressure, more pressure builds heat, and so on - the variable is the volume of the vessel which changes as the pressure moves the bullet down the bore.”
Only at immediate muzzle exit do you have a transition to supersonic, based on rapid acceleration behind the initial shock wave as energy contained in this wave dissipates it slows to a simple expanding sound wave. The time scale on this event is about 10^-6s. The muzzle blast is a pressure release not an explosion, there is no additional energy being released.
Agreed again – however with the caveat that you used the right powder and it was consumed in the bore. Too slow a powder and you get obnoxious muzzle blasts – think of a .22LR vs .338 LM. And to be specific, I wrote: “give us the muzzle blast and accompanying explosive like sound report.”
Burn rate has nothing to do with gas velocity and again at the muzzle there is no continued explosion, only the rapid expansions of gas into an ambient environment.
Actually this is false – burn rates do affect the velocity of the reaction front, AKA gas velocity, as does the amount of powder being burned. I suspect that we may be trying to explain two different things here.
We have the capability to see the pressure and motion and link them to a common time accurate to 1/200,000 s. The combustion can be seen as well as time pressure response at gas port and beginning of the gas tube.
The misconception is that everything needs to occur the same way a balloon fills, flow has inertia and once gas has started it will continue. This will occur at a predictable velocity based on pressure expanding into free air with a substantial wall friction.
Ok – cool.
This is another gun lore misconception, as said below gas has momentum and does not need to have a sealed system to work. Using this example, projectile in bore time is 2.29*10^-4. Gas travel time in gas tube is substantially greater.
So this was in response to: “In a full sized 20" AR/M16, the port is 6.825" from the muzzle. This allows for a pressurization of the bore, chamber, gas tube and carrier for only the time that the bullet is between the port and the muzzle.”
This is actually fact – it’s not lore. I didn’t say it needed to have a sealed system to work – what I wrote was this is the only time the system gets pressurized. It doesn’t need to be sealed to operate. I wrote; “What allows for the whole mechanism to work is the momentum of the gas "charge". Maybe inertia is a more accurate term vs momentum.
This is actually not the case, the analogy is much closer to stepping on a garden hose and flow decreasing.
Ok – so bad analogy on my part. Bottom line – available port pressure is a function of location and gas volume (which is tied to the inertia) is a variable driven by port diameter. Too small a port at the same location and the rifle won’t cycle. Too big a port at the same location and the same rifle is overgassed and beats itself silly. The pressure doesn't change the volume of the gas through the restriction does.
Ok, me back:
The net of the discussion is this: original claim that "ARs want to jump, bounce and twist when you let a shot go, as the carrier starts to begin its cycle before the bullet exits the bore". Physical evidence proves that this is not the case unless intentionally producing a system that does so.
Who have I been sparring with all this time? LOL
Great discussion – time to ring the bell. I need to hydrate and towel off. Read Cap’t & coke and a poolside nap. Have a great weekend.
Failure2Stop
08-14-15, 16:05
Who have I been sparring with all this time? LOL
As I stated at the start of the post, since I am not an engineer (I'm pretty much just an educated end-user), I had one of my R&D engineers write out a response. He's one of the "smart guys" that feeds me usable data.
Anyway, as it's closing time on Friday here I won't be able to get a detailed reply from him until Monday at soonest, and that's assuming that he isn't busy doing his real job.
I can address this:
Jack, again with respect,
Agreed - but specific to a gas system as there are restrictions (ports) vents, leaks etc. In a closed bore - like a bolt action rifle, the pressure is not linear, but rather when plotted as time vs pressure, it generates a parabolic curve.
I am not sure what you are talking about when you say "a different speed of sound". Sound travels at a constant; 343 m/s or 1126 fps. Frequency??
The speed of sound is variable, and constant only at specific conditions.
True, at 68 degrees F in dry air at sea-level, the speed of sound is 343.2 meters per second, but change those parameters and the speed of sound changes.
http://www.engineeringtoolbox.com/elevation-speed-sound-air-d_1534.html
This is pertinent to me, as an end-user, and I am pretty well versed in it.
Big reason, for me, is knowing where a projectile will go trans-sonic as it distinctly affects stability characteristics.
It's indicated in red on Applied Ballistics trajectory software with regard to the environmental inputs.
Have you used the Manson Muzzle Crown Refacing Tool Kit? Excellent piece of kit and in some ways better than a lathe.
C4
I have used Manson reamers, but not their muzzle crown resurfacing kit. I have my eye on a LeBlond engine lathe that will be coming up for sale.
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