View Full Version : Free-float rail flex
Does yours flex? How do you know? How did you check?
spinoff from this thread (http://m4carbine.net/showthread.php?p=902314&posted=1#post902314)
Is this barrel flex or rail flex? Does it matter? How do you tell the difference?
http://i134.photobucket.com/albums/q111/rob_s/gun%20stuff/th_IMG_1307.jpg (http://s134.photobucket.com/albums/q111/rob_s/gun%20stuff/?action=view¤t=IMG_1307.mp4)
Rob, I can't really do much to back up my statements but here is my opinion (based on some mechanical design education/background and my own experience):
What you're seeing is more the barrel flexing than the handguard flexing. The barrel "tube" is relatively small in diameter and as a result can be flexed surprisingly easily. The handguard tube, while made of aluminum and pretty thin, is relatively large in diameter so it's actually pretty rigid. In order to 'flex' noticeably, it has to deform some (meaning the tube diameter gets squished in one direction), and I don't think you're applying enough force to really see that happen.
I don't know if you saw my comment in the other thread, but my concern is the mounting method of the TRX and the interface between the handguard and receiver. I actually did the same thing on one of mine that you did in the video. What happened for me was that the handguard would move on the barrel nut and not return to original position. I could measure the change in the relationship between the end of the handguard and the barrel. I don't like this one bit, especially if you're running a low profile gas block and rail mounted sight.
FWIW, doing the same thing you did in the video on an upper with, for example, a Larue handguard will yield similar results regarding apparent flex - if the barrel is similar in profile. The difference for me is that I can do that on an upper with a handguard using a different mounting system (such as the Larue) and there's no issue with repeatability due to the handguard moving on the barrel nut. No, this 'test' isn't something the gun would normally experience, but neither is a hard drop onto the handguard. And if I can fairly easily make the handguard tube shift permanently with my fingers, then I'd expect a hard drop to have the potential to do the same.
Funny, it was not that long ago I was laughed at when I said I easily "flexed" weapons while slung in (two point) and torquing down for a long shot. You could actually see the flex as I got "set"... obviously changing point of impact. I believe at the time the topic was pencil barrels vs. my "heavy" little SBR running a 11.5" Hbar. Good job with the vid.
I only posted the video to spur discussion. I don't want to make this only about this one example. It's about worst-case scenario with a 13.0 tube and a lightweight barrel.
So in terms of barrel flex vs. rail flex, how do we measure or determine rail flex? and does it actually matter if the point, in fact, of the free float rail is so that bumps, bangs, sling tension, etc. do not translate to or effect the barrel?
I only posted the video to spur discussion. I don't want to make this only about this one example. It's about worst-case scenario with a 13.0 tube and a lightweight barrel.
So in terms of barrel flex vs. rail flex, how do we measure or determine rail flex? and does it actually matter if the point, in fact, of the free float rail is so that bumps, bangs, sling tension, etc. do not translate to or effect the barrel?
Sorry...I can delete the above post if you want.
I don't think what you're seeing in your video matters as long as everything returns to it's original relative position when no force is applied. Unless you have one really thick barrel, it's not going to take much force to flex it. The strength of a tube to resist bending goes up greatly as the outside diameter is increased - much more quickly than the diameter is increased.
Measuring the rail flex in a quantifiable way is going to be very hard for the avg person to do. I don't think any of the widely used handguards have enough flex in the tube itself that it's a problem.
No, don't delete the post, I just want to encourage others to add to your post and see the thread grow to talk about the larger issue.
I'm asking these questions more as a way to spur discussion than I am because I'm personally looking for answers to my own problems, that's all. It's an offshoot of another discussion. I probably should have skipped the video to avoid confusion.
On that note...
Looking at the larger issue I tend to agree with you but would like to hear from some others. IMHO if the barrel returns to the same place every time what you are seeing is barrel flex, which the tube system is designed to isolate. If the barrel moves within the tube and doesn't return to the same place what you're seeing is weakness in the tube mount and that *could* be a problem. Which then leads to the question of exactly what the repercussions of this problem might be.
PatrolRifleGroup
02-07-11, 06:25
This very topic is what makes me question Troy's new 15" TRX Extreme rail. I've got their 11" version already, and will be doing a 14.5" PA build, using their 13" rail, over the next week or so. While shopping for the 13" rail, I ran across the new 15" rail on thier site.
Now, I have absolutely zero complaints about the 11" version, and I'll hold my judgement for the 13". However, at what point does the length of the free-float tube become too much for the barrel nut? I know that LWRC ran into the same problem with their new REPR style forend.
Their solution was to do away with the barrel nut altogether. The base of their new rail threads directly onto upper receiver, cutting out the weak point on the longer rail lengths. I have yet to actually place my hands on one of these rails, but this is the gist of what I've gathered from their forums. I'm sure that there's actually more to it, in terms of reinforcement. SMGLee has posted some pics from the shot show, and you can see the example of this rail on their SPR rifle.
This is why i LIKE the Samson design as it uses the stock barrel nut which we know works fairly well as a method to secure the barrel to the upper receiver.
The barrel itself is chucked up and secured into the upper receiver via chamber extension and barrel nut. From there on out, provided you are using a free float rail, the entirety of the barrel is hanging in space.
Unless your receiver is out of spec or something is installed improperly no manner of movement via the rail itself should impact your barrel UNLESS It is causing the barrel nut itself to move.
The units that use aftermarket barrel nuts MAY allow undue flex due to improperly securing the barrel in the upper receiver.
That is pure speculation as it is not proven but the theory is sound.
Rob, i think you're over thinking this one as i dont see you experiencing any "problems" with your current setup. As the video displays, yes, the barrel is indeed moving but it is returning to its point of origin.
Go get a piece of steel, rebar, whatever. Ram it into the ground somehow and pull on the top of it. By virtue of its length you're GOING to be able to get it to bend slightly.
I think the impact of flex is far less important than shift. If the barrel and forend are both correctly secured at the rear and any flex encountered doesn't create a permanent shift, the impacts are negligible. We all know that barrels flex and it's easily seen in hi-speed video footage of the firing cycle. It's the main reason we studiously avoid resting the barrel itself on anything when firing. This is also the reason that hi-power rifle shooters equip their rifles with free float tubes. On a military KD qual course, so long as sling pressure is consistent and dope is properly applied, even rifles without free float tubes can deliver accurate hits at range.
Forends like the TRX Extreme are probably going to flex more than a heavy rail forend. There's just not as much material there to resist flex. The real question is whether the attachment point is solid and dependable. If it is, then the rail will return to it's original position as the pressure is relieved. I doubt that many TRX Extreme users are going to use traditional sling aided shooting positions for long range shooting using rail mounted BUIS. So long as they're not applying excess force to the forend of the rifle, zero should be unaffected by the minimal amount of flex encountered.
If it's not secure and reliable, then you may have issues with zero on rail mounted BUIS. The only way to really know would be to chuck up a rifle in a vice and apply gradual force to the forend, while frequently checking barrel clearance measurements until failure was detected.
I can see flex between the 11" TRX Extreme and barrel on my Noveske Rogue Hunter. Whether it's the barrel or forend, or both that flex isn't really relvant. It doesn't seem to affect accuracy of the rifle so I don't worry about it. If I discover a POI change then it will be an issue, but for now it seems to be doing it's job. If it was a part from a less reputable manufacturer, I might be a little more concerned.
One of the many reasons I like MRPs...its not one single thing like a FF rail...a barrel....an upper receiver that cause the ultimate flex in an upper, but rather the sum of all three being separate pieces and the way all three are mated together. This leaves the door wide open to flex, and requires very sound designed components and the methods used to mate them up in order to minimize as much flex as possible.
If I were to check for barrel and rail flex I would clamp the complete upper in a vise with the barrel horizontal (using the proper vise clamp).
The next step would be to establish dial indicators on 3 points of the upper.
1- on the receiver, flat top before the barrel nut.
2- forward end of rail on highest point.
3- Barrel muzzle end on highest point as well.
Set all indicators to "0" and make sure upper is secure in vise.
Use a small bottle jack or make up a jack screw tool to be able to move and hold a position. Place this under the rail (below the #2 dial indicator) and apply pressure to move the rail and indicator a given amount, let's say .020" (about 1/2mm). Now log the data on indicator #1 (should still be "0") and #3. Apply additional pressure under rail and log again.
After a few set points remove pressure and log as well until you are back to "0" on the #2 indicator and now you have all the data to determine how much the rail movement effects the barrel muzzle.
Remember the #1 indicator should always maintain "0" to show the upper receiver is not moving as a unit. Also you will want to protect the finish of the rail where you apply pressure so make sure whatever you use doesn't deform and skew your data.
One of the many reasons I like MRPs...its not one single thing like a FF rail...a barrel....an upper receiver that cause the ultimate flex in an upper, but rather the sum of all three being separate pieces and the way all three are mated together. This leaves the door wide open to flex, and requires very sound designed components and the methods used to mate them up in order to minimize as much flex as possible.
Is this flex bad, if it does exist? If so why? What are the negative effects?
The effects of flex in terms of practical accuracy is very minimal. Most properly free floated rails that mate soundly with both barrel and upper receiver should not experience any discernible negative effects due to the amount of flex being experienced.
A big reason why I am not an AK fan is due to the overall effect that flex has on the platform due to the design of the handguard, barrel, and receiver.
http://www.youtube.com/v/L_E_GJayano
Interesting vid of the AK flexing.
Do you have a vid of a AR in slow motion and does AR have some flex?
When I got my retired sniper pre 64 Mod. 70 rebuilt and glass bedded, my gunsmith told me how even bolt rifles exhibit a degree of barrel flex when shot no matter how thick of a barrel contour.
http://img.photobucket.com/albums/v381/giantseabass/Guns/rtsidev2.jpg
Looking at the video, it looks like that rail compresses with your finger as you pull on the barrel. All barrels flex, when you sling up tight, and when you just fire it. A camera with a fast enough frame rate can detect that flex when shooting. As long as it returns to origin and it isn't loose then it should be fine.
I have 5 carbines with barrels from 10.5" to 18" all with Daniel Defense Lite rails from 10" to 14" and I think the flex is the barrel moving inside the rails, because the heavy barrels flex a lot less than the lighter profiles in the same rails.
My 16" is very heavy under the hand guards with a steep taper vs. a 14.5" BCM standard barrel both in a 12" lite rail. The standard profile barrel is very easy to flex while the Recce profile is much more difficult.
My 10.5" LMT which has a very heavy profile barely moves inside the same DD Lite rail.
Cameron
Is this barrel flex or rail flex? Does it matter? How do you tell the difference?
How would you test the flex to see if it matters??
I'm kinda confused & what we are looking for?? Rail Weakness or POI / accuracy??
When benchrest first started, all the shooters would rest the actual bbl on the sandbag...that is taboo now.
Are we concerned about the strength of the rail or the impact of the projectile??
Quiet-Matt
02-07-11, 20:19
I checked both of my free float uppers. One with a 16" military contour bbl / 12" DD omega rail, the other with a 14'5 M4 bbl / 13" Troy TRX rail. Both have very slight flex, and appear to me mostly the barrel doing the flexing. I did the test like your video Robb, but then laid the gun on the floor to support the rail and tested again only to find that the flex was still there. So, my conclusion, its more the barrel than the rail.
An Undocumented Worker
02-07-11, 20:39
From my point of view the only way to properly test rail flex is to find a way to secure the rail to an immovable object. Maybe using a cheap optics mount, mounted to the rail with the optics mount clamped in a vice. Use the position of the optics mount to simulate where on the rail forces on the rail would be input to the rail. IE slings, foregrips etc.
Once that is setup you apply force to the reciever and measure deflection at the barrel where it exits the rail. Doing it this way takes barrel flex out of the equation and allows us to determine how much the rail flexes in relation to the reciever without flexing the barrel in the process.
Pretty much the same experience with most here regarding what appears to be the barrel being "flexed" rather than the rail. This is at least what it seems to be doing on my Noveske 16" Light Recce Lo-Pro with them 11" or so SWS rails. Mine seem to "flex" up and down and side-to-side some bit, but not too much.
Interesting vid of the AK flexing.
Do you have a vid of a AR in slow motion and does AR have some flex?
When I got my retired sniper pre 64 Mod. 70 rebuilt and glass bedded, my gunsmith told me how even bolt rifles exhibit a degree of barrel flex when shot no matter how thick of a barrel contour.
http://img.photobucket.com/albums/v381/giantseabass/Guns/rtsidev2.jpg
I have not seen a slo-mo vid of an AR exhibiting flex.
I would agree w. your gunsmith and in my first post if you note I said.."in order to minimize as much flex as possible". Even the MRPs with a 100% 1piece chassis and a beautifully designed barrel capturing aspect that incorporates a proprietary barrel extension will flex to a minor degree.
Good way to think about this is:
Take a barrel by itself and try and bend it w. both hands, not gonna happen.
Now take a barreled upper receiver with a FF rail and take one hand on the barrel and the other on the rail or the upper receiver and you are gonna detect at least some minor play or movement.
The reason why movement was present in the second scenario I'm sure is obvious to most...its the mating of the barrel to the upper receiver/FF rail. Some accuse me of being a bit friendly with MRPs but this topic really makes the system shine in regards to rigidity and flex.
Nice rifle by the way:)
I believe this could be tested with a laser bore sight ,the kind that goes in the chamber. Assuming that the device sends the beam down the center of the bore, if the barrel is flexing more than 1/2 the diameter of the bore the laser should disappear or it will be wildly distorted by reflecting off the inside of the barrel. Might be worth a try.
You could also try to flex the barrel up in the vertical plane (relative to the receiver). If the barrel is flexing, the gas tube should move deeper into the receiver when this is done.
Vince
My DD rails don't flex really, my TRX-E rails flex a ton...
Take a barrel by itself and try and bend it w. both hands, not gonna happen.
Now take a barreled upper receiver with a FF rail and take one hand on the barrel and the other on the rail or the upper receiver and you are gonna detect at least some minor play or movement.
The reason why movement was present in the second scenario I'm sure is obvious to most...its the mating of the barrel to the upper receiver/FF rail.
I don't agree with your analysis of the situation.
Some accuse me of being a bit friendly with MRPs but this topic really makes the system shine in regards to rigidity and flex.
We get it.
But to what advantage? Didn't you yourself say that flex doesn't matter?
On a more general note...
I think that what we really have is at least 4 different potential issues, and I'm going to define them this way
Barrel flex. The condition whereby the barrel moves from it's centered location, under an external force, and returns to the centered location.
Forearm flex. The condition whereby the forearm moves from it's centered location, under an external force, and returns to the centered location.
Barrel displacement. The condition whereby the barrel moves from it's centered location and does not return to the same centered location.
Forearm displacement. The condition whereby the forearm moves from it's centered location and does not return to the same centered location.
I further submit that # and #2 not only don't matter, but are also unavoidable. Even if you think they aren't happening, they are. #1 and #2 may be an issue if the barrel is being flexed at the moment of the shot, or if the forearm is being flexed and the front iron sight is being used and is attached to the top rail of the forearm. Still, these are only temporary problems as once the external force is removed the flexing part will return to it's centered state.
#3 and #4 are the problems. If parts are moving out of alignment and not coming back to the same position, or very nearly the same position, every time, then accuracy (even gross accuracy) cannot be repeatable. If a barrel nut is allowing the barrel to be displaced in the upper, or a forearm mounting system allows the forearm to be displaced about the barrel, you may have issues. But even then, forearm deflection is only an issue if iron sights, attached to the forearm, are being used. Repeated displacement may be disconcerting, and instill a lack of confidence in one's equipment, and may even be indicative of a situation that could lead to part failure or separation (barrel or forearm coming loose from the gun, depending on where the loose part is), but in and of itself it is a non-issue.
Caveat here. I'm not talking about gnat's-ass, benchrest, sub-MOA shooting. Not only do I have no interest in same, I'm not qualified to talk about it due largely to that lack of interest leading to a lack of pursuit. I'm talking about chrome-lined, Govt, lightweight, or M4 profile, carbon steel, button-rifled or hammer forged, standard AR barrels shooting the kinds of ammo that most of us find ourselves shooting most often such as Wolf, XM193, Prvi, PMC, Sellier & Bellot, etc. in the kinds of applications the guns are intended for.
for some that have not seen barrel flex in an ar.
this a a promo vid for LAV carbine vid,
1:37 sec shows some barrel flex/whip etc in a 16' light weight profile carbine.
http://www.youtube.com/watch?v=5hKaPkvOtEo
Pressing the barrel as done in Rob's video will cause both the barrel and the forearm to flex. In this case, the barrel should show more flex than the forearm.
The reason a barrel will flex while mounted in a receiver and will not when attempting to bend an unmounted barrel with just your hands is a matter of leverage and deflection. Greater leverage is available when flexing the mounted barrel. Also, the mounted barrel has a longer length from it's fulcrum and the deflection will show more displacement with the greater distance.
The main purpose of free floating the barrel is to eliminate any interference with the natural flexing of the barrel during the shot. On an AR, there will be some from the gas tube, but that should be minimal. Secondary is to eliminate flexing the barrel and changing the point of impact through pressure from a sling, vertical grip or simply from being gripped tightly by an adrenalin pumped shooter.
Pressure on a free float forearm will be transferred to the upper receiver and possibly cause the upper to flex. How much? Here are two links to actual tests performed by K.L. Davis
http://m4carbine.net/showthread.php?t=57
https://www.m4carbine.net/showthread.php?t=58
I don't offer these tests as a final conclusion but as facts for further discussion
Like MistWolf said, the amount of static force on the hand guard or bbl (like from using a sling or VFG) can be substantial. Distance from handguard attatchment point/bbl nut to pressure point multiplied by force, even 14 inches down putting 20 lbs of pressure is like putting 280lbs of weight on it.
You would probably bend the upper/strip threads or break the forearm before you permanently (plastic-ly) bent the barrel though. The links above do a good job of showing that.
Actually, 20 lbs of force with 10 inches of arm is 200 in/lbs of force. That's 16.7 ft/lbs
Actually, 20 lbs of force applied 10 inches from the fulcrum is 200 in/lbs of force. That's 16.7 ft/lbs
My thought process was "the ratio of the sum of the forces to their distance is equal on a beam in static equilibrium..." so 20*10:200*1.
Cesiumsponge
02-28-11, 20:27
Hi folks,
I decided to get some figures on this rail flex issue for free-float systems. I've only got a Mk12 Mod 1 SPR clone on hand to test so I used that as my testbed. It sports the correct KAC rifle-length free float system and is not a particularly heavy or light example for a 12" rail.
I set up a dial indicator (Compac 213GLA .0005" indicator and later Interapid 312B-3 .0001" to compare readings) on a magnetic indicator (Brown & Sharpe) base with a vee-groove. The rail is aluminum so I attached the base to the barrel, centering it with the vee-groove and butting it up against the thread protector on the Ops Inc muzzle brake so it won't move around. The indicator is at the 12 o'clock position. Without setting the gun on anything to prevent any and all possible deflection, I zero the indicator on a flat spot on the front BUIS. I did this with the gun's buttstock on the ground, the firearm vertically oriented. This allows me to measure the total movement between the barrel and rail, but requires me to differentiate if the barrel itself or the rail is moving.
http://farm5.static.flickr.com/4078/5487147919_2533b8fce8_o.jpg
For reference with these small numbers, a sheet of printer paper is .004" thick.
The first thing I noticed is the barrel sags from gravity alone. It is noteworthy that the SPR barrel is 18" long and utilizes a heavier contour than seen on a typical carbine. With the dial indicator zeroed to the rail, there was an immediate .0075" reading when I changed the orientation of the rifle from perpendicular to parallel with the ground. The rifle is being supported by the receiver with both hands. I am not supporting it at the rail with my support hand at all which might deflect the rail and invalidate the figure. To make sure this wasn't the base coming loose, I held the rifle vertical again and the indicator returned to zero. Then I held it upside down and the dial indicator read .0055" the other direction. This means the barrel is sagging from it's own weight. There is a discrepancy of about .002" between the amount the barrel sags depending if I hold it normally or inverted. I am guesing this is because the gas tube might lend stiffness to the barrel when it's in compression (when the rifle is inverted upside down) versus when it's in tension (held in a normal position brought to bear).
Next, I put my support hand under the rail as if I was shooting offhand without a sling using a Magpul AFG, pulling the rifle against the pocket of my shoulder. The indicator reading increases .0025" to read a total indicator reading of of .010". That is, the initial .0075" from the barrel sagging plus .0025" of rail deflection holding the rifle off-hand.
Next, I attached a sling and cinched it so my support hand was pushing against the AFG to get tension on the sling. I get a .010" total indicator reading, nothing different. I reoriented the indicator to the 9 o'clock position to see how much the rail is deflected away from the barrel with the sling pulling on it. I zeroed the indicator with the rifle in the vertical orientation. Bringing the rifle back to horizontal did not change the reading because gravity wasn't pulling the barrel to the side. With the sling tensioned up, I managed to deflect the rail itself .005-.010" horizontally, which varied depending on how I positioned the elbow on my support hand wrapped in the sling.
Next, I wanted to use the bipod. I am using the Harris bipod located at the very end of the rail. When I set the rifle down on a hard table with the bipod deployed so the rifle free-stands on the bipod and buttstock, the indicator reading grows to .013". Subtracting the .0075" sag of the barrel, we can see that the rifle resting on the bipod deflects the rail .0065" with only a BUIS on the end of the 12" rail. If you hang lights, lasers, grenade launchers, NVS, this will undoubtedly increase the deflection on the rail itself if it rests on a bipod.
Out of curiosity, I wanted to see how weight affected the barrel. I didn't have room to fully thread on my 12th Model because the indicator was affixed to the barrel, but I did just slip it over the muzzle so the can hung off the muzzle brake. The barrel deflected the indicator .0025" due to the additional weight hanging off the barrel. The barrel itself appears to sag .010" overall with the Ops Inc 12th Model can, which is all stainless and pretty hefty.
With the can removed, once I lift the buttstock off the table, the reading increases from .013" to .015" because the center of gravity changed and there is a weight shift towards the bipod. I did notice when I went prone and got behind the gun, the reading did not change any, even if I preloaded the bipod with some force and tried various ways of supporting the buttstock with my support hand.
It seems .010" of horizontal deflection can be created by sling use for my particular example and about .0075" vertical deflection of the rail. I would not consider the initial .0075" of sag from the barrel as part of the equation for overall deflection because you'd zero the rifle with the barrel horizontally, not pointing at the Sun. Rail deflection appears to be worst with sling use, followed by bipod use, and lastly off-hand use. A rifle that uses a bipod like the Mk12 Mod1 will be zeroed with the bipod deployed and fired with the bipod most of the time. I've never actually fired my Mk12 off-hand because it's a hefty beast so I don't know what, if any POI changes there would be.
Theoretically a vertical deflection on the rail of .0075" over an 18" long barrel would equate to .005" of divergence per foot traveled, or 1.5" at 100 yards. A maximum horizontal deflection of .010" from sling use should mean a divergence of .0075"/foot or 2.25" at 100 yards. I don't know if this translates because I've never attempted to document POI shifts with various shooting positions. Anyone care to chime in?
Good post. It would be interesting to see how these would compare against a Service Rifle with a bull barrel and a steel free float tube. Assuming it too would deflect, the main thing is that the deflection is consistent from session to session and that the load is tailored to the barrel whip characteristics of the barrel (i.e., bullet exiting at a "full up" or "full down" node).
cobravenom39
02-28-11, 21:27
for some that have not seen barrel flex in an ar.
this a a promo vid for LAV carbine vid,
1:37 sec shows some barrel flex/whip etc in a 16' light weight profile carbine.
http://www.youtube.com/watch?v=5hKaPkvOtEo
Jeebuz! No I know why the military rocks the a2
My thought process was "the ratio of the sum of the forces to their distance is equal on a beam in static equilibrium..." so 20*10:200*1.
Your math is correct. The mistake was giving your answer as foot pounds instead of inch pounds. For 20 lbs of weight to exert 200 ft/lbs, you'd need an arm of 10 feet :)
I should have shown my math:
20 lbs x 10 inches = 200 in/lbs.
To convert in/lbs to ft/lbs, divide in/lbs by 12. 200 divided by 12 = 16.666 or 16.7
superstratjunky
03-01-11, 07:27
So does this equate to loss of accuracy during rapid fire, that should be a concern, or metal fatigue of the upper itself? The DD vid showed a substantial amount of flex in the barrel during firing, enough that even I would be concerned about fatigue & or accuracy. I have a DDL 14" that I bought so I could extend my arm out as far as possible & to extend the sight picture. I trust the way that it's mounted & how it cinches down. My barrel is a BHW 16" M4 profile & the upper is a BCM. All quality parts that I have confidence in & I'm not all that worried about these parts failing any time soon. With all of the guys running 3 gun & tactical courses, if it was a concern in terms of accuracy & failure, I think one of the guys here would've seen any form of fatigue or failure at some point. With all the different combination's & configurations, it would take years to gather enough info to make solid data available. So, what's up Rob? Why's it a concern?
SSo, what's up Rob? Why's it a concern?
That's a great question, and kind of the reason I started this thread to begin with.
We've gotten a lot of discussion on what flex is, what deflection is, what displacement is, but still not a lot on whether they matter, or whether one matters more than another.
I think we may be headed down the rabbit hole (or circling the drain?) without any end or purpose in sight.
Does flex (see definition below) matter? If so, what is the negative effect in the short term single shot), mid-term (multiple shots) and long-term (life of the rifle)? Are the effects of flex the same or different for barrels vs. rails?
I think it's fair to say that displacement, flex without the return to center, is a problem in both systems.
On a more general note...
I think that what we really have is at least 4 different potential issues, and I'm going to define them this way
Barrel flex. The condition whereby the barrel moves from it's centered location, under an external force, and returns to the centered location.
Forearm flex. The condition whereby the forearm moves from it's centered location, under an external force, and returns to the centered location.
Barrel displacement. The condition whereby the barrel moves from it's centered location and does not return to the same centered location.
Forearm displacement. The condition whereby the forearm moves from it's centered location and does not return to the same centered location.
I further submit that # and #2 not only don't matter, but are also unavoidable. Even if you think they aren't happening, they are. #1 and #2 may be an issue if the barrel is being flexed at the moment of the shot, or if the forearm is being flexed and the front iron sight is being used and is attached to the top rail of the forearm. Still, these are only temporary problems as once the external force is removed the flexing part will return to it's centered state.
#3 and #4 are the problems. If parts are moving out of alignment and not coming back to the same position, or very nearly the same position, every time, then accuracy (even gross accuracy) cannot be repeatable. If a barrel nut is allowing the barrel to be displaced in the upper, or a forearm mounting system allows the forearm to be displaced about the barrel, you may have issues. But even then, forearm deflection is only an issue if iron sights, attached to the forearm, are being used. Repeated displacement may be disconcerting, and instill a lack of confidence in one's equipment, and may even be indicative of a situation that could lead to part failure or separation (barrel or forearm coming loose from the gun, depending on where the loose part is), but in and of itself it is a non-issue.
Caveat here. I'm not talking about gnat's-ass, benchrest, sub-MOA shooting. Not only do I have no interest in same, I'm not qualified to talk about it due largely to that lack of interest leading to a lack of pursuit. I'm talking about chrome-lined, Govt, lightweight, or M4 profile, carbon steel, button-rifled or hammer forged, standard AR barrels shooting the kinds of ammo that most of us find ourselves shooting most often such as Wolf, XM193, Prvi, PMC, Sellier & Bellot, etc. in the kinds of applications the guns are intended for.
Robb Jensen
03-01-11, 07:39
Does yours flex? How do you know? How did you check?
spinoff from this thread (http://m4carbine.net/showthread.php?p=902314&posted=1#post902314)
In that thread I used the term flex/shift which isn't the correct term.
The problem I saw with 2 Samson rails was that the rail fit too lose on the 2 inserts and fit too lose on the bbl nut. This allowed the rail to shift around on the barrel nut and the rail would stay in that position. Not very good just as lose lug nuts on your car isn't good.
In that thread I used the term flex/shift which isn't the correct term.
The problem I saw with 2 Samson rails was that the rail fit too lose on the 2 inserts and fit too lose on the bbl nut. This allowed the rail to shift around on the barrel nut and the rail would stay in that position. Not very good just as lose lug nuts on your car isn't good.
Ah, so we have a third term. was this shift fore/aft movement? rotational? could it be described as "wiggle" or "wobble"?
I know these terms seem like minutia, but this thread and topic probably explains more clearly than any other why words do matter.
Robb Jensen
03-01-11, 07:46
Ah, so we have a third term. was this shift fore/aft movement? rotational? could it be described as "wiggle" or "wobble"?
I know these terms seem like minutia, but this thread and topic probably explains more clearly than any other why words do matter.
It allowed the rail to be pushed laterally side-to-side with about 6lbs of pressure.
It might work okay on an Airsoft gun. ;) Since even UTG / Legion Rails are tighter...
I don't know where this rabbits going but all rifles flex. Take any rifle, attach a bi-pod on it and shoot off the ground or bench. Vary your cheek weld pressure and watch your groups open up. Usually they will go higher due to flex. A free floated barrel is less affected but still affected.
Does it matter? No. How you deal with it does.
superstratjunky
03-01-11, 10:21
Nature of the beast in other words? With this being the soul reason that AK's & mini 14's having thin barrels are the results of lack of consistent accuracy, it would then make sense that the proper sizing & profile of AR barrels, would make them less prone to the affects of flex. How thin is to thin? It's going to be a compromise regardless. Even then the AR is still accurate. Now with the AR being as accurate as it is to begin with, {even with standard HGs} it now focuses on how the FFT/Rs are mounted to the rifle & the strength of the way in witch it was made. The way the DDL rail attaches to the UR is pretty stout & on top of that, the profile it's self is inherently made more stiff. It all boils down to how the FFR/T attaches to the UR. You now have to make an educated decision as to how light you want to make your rifle. The lighter it is, the more prone it is to flex. IMHO anyway.
Cesiumsponge already partially worked this out but: If you have a CompM2 that's a 4 MOA? red dot. The guys who actually practically tested this were getting deflections in the .01 of an inch range at most. 1 MOA at the muzzle of a 16" barrel is 32pi or about 100.531/21600(minutes in a circle) = 0.00465. So 4 MOA is about 0.0186 of deflection at the muzzle of a 16" bbl.
So for a carbine it's minimal, but possibly notable. It could affect BUISs attached the the very front of a free float rail a lot though.
(My math may be wrong, but Im fairly sure of the theory.)
This is a really interesting thread since I expected there to be some POA change when the front sight is mounted to a floating handguard. But it is not as bad as I expected for a working rifle application.
So I took Cesiumsponge's measurements (https://www.m4carbine.net/showpost.php?p=924812&postcount=32) and plugged it into a spreadsheet and here is what I got:
Sight radius: 18"
Elevation POA change
Off hand support: 0.5 MOA
Sling: 0.5 MOA (2 MOA windage change)
Bipod: 1.3 MOA
The interesting thing to me is that the POA change using a bipod is larger than off hand. So if I zero the rifle on the bench or bipod, my POA would actually be low if I switch to off hand or sling. 0.8" high at 100 yds is really not a big deal and even 2.4" at 300 yds is well within man size target. It does get a little less acceptable pass 300 yds but that depends on application. In the end, a 2 MOA working rifle might be closer to 3 MOA depending on how you hold it.
albatrossarmament
09-10-13, 18:24
Didnt see this posted, here's a slow-mo vid of the AR15 barrel whip when free-floated, both heavy and pencil suffer:
http://www.youtube.com/watch?v=0Xr_RG_lcNk
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