View Full Version : Barrel installation "botchery"?
The carbine I built still isn't giving me the accuracy I'm expecting. I'm looking for 2-3 MOA but actual results are much worse. I initially thought the problem was a 16" Del-Ton barrel, standard hand guards, and the DPMS BCG that came with the upper, but that has been replaced with a 16" Daniel Defense barrel with LaRue rails and a BCM BCG--still on the DPMS upper.
I'm shooting my all factory ArmaLite rifle beside it as a control to make sure it isn't me. I get much better results out of the rifle. Both are being shot sandbagged from a bench with the same magnified optic (QD mount switching back and forth, paying attention to the group size and ignoring the shift in POA/POI between different ARs). Unfortunately, there are too many differences between the rifle and the carbine for me to say it's one thing or the other; examples, 1/9" vs 1/7" rifling, clamped gas block vs pinned front sight post, barrel profile, one all factory vs one built by me (this is probably the culprit heh), etc... At 25 yards, which should be very manageable, the control rifle is turning out < 1" groups (fine) and the custom build carbine is spreading them out to 2-3" groups. Remember: this is sandbagged from a bench.
I fully recognize Federal XM193 and XM855 isn't the most accurate ammunition in the world, but it should be tighter than 2-3" at 25 yards. I get the same results from both the rifle and the carbine whether I use XM193 or XM855.
I've reviewed the barrel installation procedure and I think I got it right. Zero loc-tite was applied during installation and the barrel nut took about 65 ft-lbs to get everything lined up, which I believe shouldn't be under nor over-torqued.
Could there be something funky with the dimensions of the DPMS upper, especially as it pertains to how loosely the barrel fits into the receiver (I thought torquing the barrel nut would make that not matter a bit)?
Is the Daniel Defense barrel, as tough and reliable as it is, just not going to give me the desired results?
Am I just expecting something that is not reasonable from the carbine even though the ArmaLite rifle is yielding satisfaction?
Something else I'm just not thinking of?
Will be happy to share any further details, but I think I brain-shat the right info... Thanks in advance!
Even with 193/855 at 25yds and a magnified optic there should be a 1 hole group happening. What optic are you using? Are you getting the same results with irons? I have shot a buddies Delton barreled carbine before and it would shoot fine.
drarmament
01-17-14, 20:08
Post a pic of your group and internal components and exterior. Make sure nothing is lose. Federal m193 I'm getting rounds right next to each other with the mp15. Make sure the receiver is straight. I have know people torquing down the barrel nut and put a slight bend in the receiver. Also make sure the scope you have on is secured and the sight pics is the same. I have also seen ppl bend barrels installing them.
Even with 193/855 at 25yds and a magnified optic there should be a 1 hole group happening. What optic are you using? Are you getting the same results with irons? I have shot a buddies Delton barreled carbine before and it would shoot fine.
Using a TA11F on an LT100. No noticeable difference between it and irons.
Post a pic of your group and internal components and exterior. Make sure nothing is lose. Federal m193 I'm getting rounds right next to each other with the mp15. Make sure the receiver is straight. I have know people torquing down the barrel nut and put a slight bend in the receiver. Also make sure the scope you have on is secured and the sight pics is the same. I have also seen ppl bend barrels installing them.
Then the receiver should be "suspect", along with installation... I don't see any noticeable defects. . I hope I got the important angles here. Let me know if I'm not giving you hands and eyes on what you're looking for and I'll shoot some more. Unfortunately, I did not keep the target.
Although I haven't had my booger hooks calibrated, I don't feel any catches or resistance when I separate the upper and lower receivers then slide the BCG back and forth through the upper. It just feels like one smooth, unimpeded, EWL-coated back and forth motion.
I do remember there being a very loose fit between barrel and upper receiver, but I figure the barrel nut should take care of any issue there.
Looks like I'll have to break the pics up into 3 segments...
1 of 3:
2283022831228322283322834
2 of 3:
2283522836228372283822839
drarmament
01-17-14, 22:18
If your receiver is loose it will effect accuracy and will not keep a stable sight pic. The barrel nut is use to hold the barrel so you don't shoot it down range. I would get a thing to stabilize the upper and lower. I don't remember what is called . A little loose is fine.
Has anyone else fired the rifle with the same results? Grab the upper in one hand and grab the barrel in the other and twist, there should be no movement, just the barrel itself not the rail.
Has anyone else fired the rifle with the same results? Grab the upper in one hand and grab the barrel in the other and twist, there should be no movement, just the barrel itself not the rail.
Nobody else has fired the rifle in question.
When I hold the upper receiver in one hand while separated from the lower and the barrel forward of the rails in the other and twist, there is no movement. Thanks.
Polymerhead
01-18-14, 16:40
2 of 3:
22839
Is there copper on the bore of the flash hider in this photo? Any contact between the bullet & the FH will strong shots.
Is there copper on the bore of the flash hider in this photo? Any contact between the bullet & the FH will strong shots.
Sharp eye, sir. I do think that is copper. I did spend some time with Hoppes Elite so it may be leftover from that wash-out. Is it reasonable that <500 rounds through this barrel would justify that much copper removal if bullets are not dinging the muzzle device on the way out?
I do not actually see any signs of contact when looking closer at the edge of the muzzle and inside the FH. I do see what I believe should be adequate clearance. I will certainly take her to the range with FH removed to observe results. Thank you.
Try shooting the carbine with the muzzle device removed
I had an FAL that came new with a non-concentric flash hider. The rounds hit it on the way out and I was getting four inch groups at 25 FEET. A new flash hider fixed it.
I'm not a guru when it comes to AR accuracy. Of all those I've assembled, I only test fired them at short range for function and basic targeting. Nobody ever complained. OTOH, if I was building an AR for accuracy, I'd wonder how square the front of the upper was to its bore, and take steps to correct it if it was found to be lacking. Brownells and a few other companies make squaring tools and fixtures for that. I'd be very careful to make sure the barrel was not a sloppy fit in the upper and also that the barrels bore was concentric with the upper's bore. Finally, I'd worry about the bolt carriers bore being square and concentric with the rifles bore (centered and not tilted).
The bolt gun boys shake their heads and wonder how the AR can do so well. I dunno either. But taking pains with things is not a bad thing. You've swapped barrels and BCG's. You might wanna make sure the front of the upper is square, and that the barrel nut is also good. The idea is to have that barrel un-tilted in any way, and even pressure on its shoulder.
Some smart guru of the AR accuracy will probably come correct me. I'm open to tweakage in the thought train.
The barrel extension should fit very snug into the upper receiver...
Ditch the last DPMS part and you should be GTG.
I do remember there being a very loose fit between barrel and upper receiver, but I figure the barrel nut should take care of any issue there. This is the common thing between the two barrels. It is most likely your issue.
If your receiver is loose it will effect accuracy and will not keep a stable sight pic. The barrel nut is use to hold the barrel so you don't shoot it down range. I would get a thing to stabilize the upper and lower. I don't remember what is called . A little loose is fine.
Receiver wobble doesn't do squat accuracy wise. Probably good to use the search button regarding this.
Also wedge is pointless as an fyi. And shooting a barrel down range? Really?
Please do not spread disinformation.
If your receiver is loose it will effect accuracy and will not keep a stable sight pic. The barrel nut is use to hold the barrel so you don't shoot it down range. I would get a thing to stabilize the upper and lower. I don't remember what is called . A little loose is fine.
drarmament
01-19-14, 07:19
Let me restate what i meant. The barrel nut is to hold the barrel on to the upper receiver. It will not shoot the barrel off. With the barrel aligned with the upper receiver and proper torque their should be no movement. If there is movement you have to look at the alignment pin on the barrel ext or the slot on the upper receiver to see if it's worn. If that checks out fine, look at the threads of the upper receiver and barrel nut. Also over torquing will damage the upper, barrel nut or barrel ext.
The is no affect to cause inaccuracies when the upper and lower has movement, but depending how you position yourself and the rifle will determine the out of your group. I myself hold the upper loose and a looser lower will cause me to take a longer time to acquire my sight picture. But that's just me.
I also agree if the round is hitting the muzzle device it will cause the round to tumble
I hope this explain it more clear then my previous comment.
I'm going to second the notion that the muzzle device has a high probability of causing your issue. Put a standard A2 on it (being sure to torque it properly) and test it out. You'll likely see your problem fixed. This also happens to be the least expensive fix if your going to throw parts at it.
Much thanks for all the advice. cjb brings up a really good point about concentricity of barrel extension, upper receiver, and bolt even though everything may be tightened down and to proper numbers and AKDoug correctly points out it is one of the last common parts... E-Man reminds me of the reputation the manufacturer of the current upper has...
In the name of good troubleshooting, I'll definitely get it out without the muzzle device to see what happens. Meanwhile, if it does happen to come down to replacing the upper, would this upper from BCM (http://www.bravocompanyusa.com/Upper-Receiver-assembly-M4-Flat-Top-p/flat%20top%20-upper%20-m4%202.htm) be a decent choice? While BCM is a trusted vendor, the name of the manufacturer of this upper is not mentioned for reputation checks. No offense to BCM here; please understand.
Thanks again.
Meanwhile, if it does happen to come down to replacing the upper, would this upper from BCM (http://www.bravocompanyusa.com/Upper-Receiver-assembly-M4-Flat-Top-p/flat%20top%20-upper%20-m4%202.htm) be a decent choice? While BCM is a trusted vendor, the name of the manufacturer of this upper is not mentioned for reputation checks. No offense to BCM here; please understand.[/QUOTE]
I have used several of those uppers from BCM with no issues. Good quality for a build. Not sure, but I believe they are the same as used on BCM builds but are not marked with the BCM logo due to being sold as a part only and not a complete upper.
Whatever upper you get, you've got a couple of barrels to swap into the end of it and see how things fit.
Some of those things I mentioned are from hanging with a bunch of bench rest shooters a long while ago. There is really no way to check some of that concentricity, say barrel to barrel-extension, without a very elaborate setup. I don't know what pains are taken in the manufacture of uppers to keep them straight and keep the ends square. Those facing cutters are made for us to use though.
Here's what I'd do.... simple simple test.
Take the fookin' muzzle device off! Then make some test firings. You'll know real quick whats up there. Just sayin!
Much thanks for all the advice. cjb brings up a really good point about concentricity of barrel extension, upper receiver, and bolt even though everything may be tightened down and to proper numbers
Unless its grossly out of spec this would not account for your issue.
In any case, concentricity issues should have significant poi shift if it was bad enough to cause your problem.
I would not do anything until you pulled the muzzle device (fh) properly and shot it. It also sounds like you used the same fh with the two barrels.
Not saying its not the upper receiver, just that most concentricity issues which could be fixed by the Brownell tool are not as extreme as what you are seeing.
But I'd sure shoot it w/o the fh before I did anything!
Sent from my PRC-104 using phonetics
Much thanks for all the advice. cjb brings up a really good point about concentricity of barrel extension, upper receiver, and bolt even though everything may be tightened down and to proper numbers and AKDoug correctly points out it is one of the last common parts... E-Man reminds me of the reputation the manufacturer of the current upper has...
In the name of good troubleshooting, I'll definitely get it out without the muzzle device to see what happens. Meanwhile, if it does happen to come down to replacing the upper, would this upper from BCM (http://www.bravocompanyusa.com/Upper-Receiver-assembly-M4-Flat-Top-p/flat%20top%20-upper%20-m4%202.htm) be a decent choice? While BCM is a trusted vendor, the name of the manufacturer of this upper is not mentioned for reputation checks. No offense to BCM here; please understand.
Thanks again.
You really don't need to know who bcm gets there stuff from, as it's illrelivant. The fact is you are getting something from BCM, and thats all you need to worry about. ALL BCM stuff is g2g. If there is an issue with a part or product they sell they will take care of you as well.
Now figured I'd mention my BCM blem upper was originally from Anchor Harvey, but even that doesn't matter. It's BCM and that is all that matters.
FYI I'd just get a blem upper. Not sure if they are still on sale for 80$. You have to install the included forward assist and door however. Great savings for really no loss.
FYI I'd just get a blem upper. Not sure if they are still on sale for 80$. You have to install the included forward assist and door however. Great savings for really no loss.
No offense, but given his concerns the last thing I'd do is buy a blem upper. Not worth the $20 savings when you can get several top tier mfg uppers for $99-120 complete. (DD would prob be my pick, last one I bought was $107 shipped)
Sent from my PRC-104 using phonetics
I have used many blem uppers for PSA. 40 bucks and every single one has never had an install issue.
No offense, but given his concerns the last thing I'd do is buy a blem upper. Not worth the $20 savings when you can get several top tier mfg uppers for $99-120 complete. (DD would prob be my pick, last one I bought was $107 shipped)
Sent from my PRC-104 using phonetics
What are you talking about?
First... BCM IS one of the TOP brands. Second, what does a blem upper have anything to do with his issues? Blem uppers from BCM are 100% in spec. The "blem" ordeal has 0 to do with anything functional or accuracy. There is nothing your DD upper would do that a blem BCM upper wouldn't do. I just mentioned Blem uppers were available and 40$ cheaper than non blem. Please do some research, especially on BCM blem uppers before you hint at the fact they are inferior, as well as hinting at the fact BCM is not 100% top notch from a quality and qc aspect.
So much overthinking being done here. All this microscopic truing, syncing things up on a mollecular level etc. My god the bench shooter state of mind is mind boggling.
What are you talking about?
First... BCM IS one of the TOP brands. Second, what does a blem upper have anything to do with his issues? Blem uppers from BCM are 100% in spec. The "blem" ordeal has 0 to do with anything functional or accuracy. There is nothing your DD upper would do that a blem BCM upper wouldn't do. I just mentioned Blem uppers were available and 40$ cheaper than non blem. Please do some research, especially on BCM blem uppers before you hint at the fact they are inferior, as well as hinting at the fact BCM is not 100% top notch from a quality and qc aspect.
So much overthinking being done here. All this microscopic truing, syncing things up on a mollecular level etc. My god the bench shooter state of mind is mind boggling.
What is your recommendation?
What are you talking about?
First... BCM IS one of the TOP brands.
I get it, you love BCM. I hope you got a hat.
BCM makes fine stuff, no argument.
You missed my point:
1) he's chasing ghosts, and has already spent hundreds swapping stuff. Its false economy to save $20 on a blem upper at this point. I'm sure the BCM blem is just cosmetic and should be fine, but in this case its not worth saving $20.
2) he has a DD barrel. With 1st quality DD uppers available $100-120 assembled, there would be some advantage in matching the barrel to the upper mfg if there's was an issue. ( not that I think his Upper is the problem)
3) this is not a bench rest issue. Right now DD sub assemblies are a very good value. (Lpk, upper, bcg, etc)
And I'd put their quality up against anyone's. Not saying they are better, just that they are tier 1 quality currently available at a very good price.
Sent from my PRC-104 using phonetics
Actually, that del-ton (the first barrel) had a standard birdcage on it, which was sold with the barrel. The new DD barrel has a YHM 5C2.
Evidence is in. I would be surprised if anybody found these groups acceptable from a bench, on bags, with magnification.
Mixture of XM855 and XM193 with (denoted "w/MD") and without (denoted "wo/MD") flash hider. Looks like the next step is to replace the upper receiver.
22881
I know 10 shot groups are the standard for determining group sizes, but I would ask you: would you accept these groups with this ammunition out of your rifles while zeroing?
Unless its grossly out of spec this would not account for your issue.
Incorrect dimensions are likely, especially considering where it came from. I'm finding this out the hard way, which is actually the best way IMO. Now I have one more experience to go by, even though it is agreeably only one data point of many to consider.
You really don't need to know who bcm gets there stuff from, as it's illrelivant. The fact is you are getting something from BCM, and thats all you need to worry about. ALL BCM stuff is g2g. If there is an issue with a part or product they sell they will take care of you as well...
FYI I'd just get a blem upper...
Thanks for your reassurance and recommendation.
What are you talking about?
First... BCM IS one of the TOP brands. Second, what does a blem upper have anything to do with his issues? Blem uppers from BCM are 100% in spec. The "blem" ordeal has 0 to do with anything functional or accuracy. There is nothing your DD upper would do that a blem BCM upper wouldn't do. I just mentioned Blem uppers were available and 40$ cheaper than non blem. Please do some research, especially on BCM blem uppers before you hint at the fact they are inferior, as well as hinting at the fact BCM is not 100% top notch from a quality and qc aspect.
So much overthinking being done here. All this microscopic truing, syncing things up on a mollecular level etc. My god the bench shooter state of mind is mind boggling.
I didn't get the impression he was hinting at any level of inferiority toward the blemished uppers offered by BCM. I read that only as an empathic statement, and I think that's as far as it was meant. When the dollars come hard and spend easy, it's easy to relate to somebody else getting burned by flashy marketing and poor product delivery.
Of course, time will tell the difference poor product delivery on their part (not BCM; I mean the suspect upper receiver's brand here) and hideous wrenching on my part. I've learned much more than expected from building this carbine and wouldn't do it differently if I had the choice. Experience complements education.
I get it, you love BCM. I hope you got a hat.
BCM makes fine stuff, no argument.
You missed my point:
1) he's chasing ghosts, and has already spent hundreds swapping stuff. Its false economy to save $20 on a blem upper at this point. I'm sure the BCM blem is just cosmetic and should be fine, but in this case its not worth saving $20.
2) he has a DD barrel. With 1st quality DD uppers available $100-120 assembled, there would be some advantage in matching the barrel to the upper mfg if there's was an issue. ( not that I think his Upper is the problem)
3) this is not a bench rest issue. Right now DD sub assemblies are a very good value. (Lpk, upper, bcg, etc)
And I'd put their quality up against anyone's. Not saying they are better, just that they are tier 1 quality currently available at a very good price.
Sent from my PRC-104 using phonetics
That was my point in also mentioning a way to save money. There is no "tier 1" stuff. This isn't call of duty lol.
I do like BCM because of their qc and reputation, plus their excellent cs. Do I have other brands of stuff, yes, including DD and Noveske. They are all right up there quality wise.
Matching brand of barrel to upper means 0. I have a DD barrel in a BCM upper and is flawless. If itqs in spec it's in spec. Not sure what brand matching has anything to do with it other than OCD.
IMO a BCM blem is just as "tier one quality" as a DD. May just have a scratch etc here or there, which one hard day at the range will quickly make any mint receiver look 10x's as bad.
IMO as long as he gets any reputable brand it shouldn't matter who it was from, or if it was blem or not.
But anyways gone enough off topic with this.
Let us know if the muzzle device removal has any major impact.
I believe the muzzle device is not at fault. Took it out a few hours ago. Shot some, marked the groups shot with muzzle device and ammo type. Removed muzzle device, shot some more, marked the groups without muzzle device and ammo type.
Groups circled in red were shot without the muzzle device. Groups circled in thin black were shot with the muzzle device attached.
Vertical distance from the most obvious POA for each group can be explained by me shifting the holdover for off-topic reasons. Please ignore the variations in vertical distance of each group from what the most obvious point of aim should be.
22891
Your groups are stringing. They are not round. If this isn't due to some error on the part of the shooter, this indicates some kind of mechanical shift, either from the bedding or the sight.
Yes, an AR can have a shift in it's bedding. The AR is bedded where the barrel extension fits inside the upper reciever. If the barrel extension shifts around in there, it will cause a shift in the POI.
It could also be the sights. The mount or rings could be a poor fit or loose. There could be an internal problem.
In this case, my gut says it's the bedding, that the barrel extension to upper receiver fit is loose. Some guys will use some kind of epoxy to take any slop (real or imagined) out of the fit. I think this is a bad idea as it will complicate removal at a future date and epoxy tends to break down from shock, vibration, heat & exposure to chemicals. Some will use a facing tool to square the upper. This will remove the anodizing. Aluminum is self healing, that is, with exposure to oxegen, the surface will convert to aluminum oxide but it will not be as deep or as even as the aluminum oxide surface from anodizing.
I would carefully inspect and tighten the sights, mounts & rings as applicable (check the easiest part first), then remove the barrel nut to check the fit of the barrel extension in the upper. Leave the muzzle device off until you isolate your problem and keep the muzzle threads ptotected. When you do get to re-installing it, do so with minimum torque. Over torquing the MD will distort the barrel and impair accuracy
The name of the manufacturer is irrelevant and BCM doesn't list them for a good reason. But, I can tell you that once they are received in house, they do check them. Anything found out of spec is sent back.
People assembling AR's need to remember that many uppers and lowers are only made by a handful or companies. BCM is not going to disclose that they buy them from xxxx for a good reason.
Meanwhile, if it does happen to come down to replacing the upper, would this upper from BCM (http://www.bravocompanyusa.com/Upper-Receiver-assembly-M4-Flat-Top-p/flat%20top%20-upper%20-m4%202.htm) be a decent choice? While BCM is a trusted vendor, the name of the manufacturer of this upper is not mentioned for reputation checks. No offense to BCM here; please understand.
I have used several of those uppers from BCM with no issues. Good quality for a build. Not sure, but I believe they are the same as used on BCM builds but are not marked with the BCM logo due to being sold as a part only and not a complete upper.[/QUOTE]
Aside from the issue at hand, shooting 3 rounds serves no purpose. Try shooting 5 or 10 round groups. In order to troubleshoot this, you are going to need to isolate the issue. That means following the advice given here about checking everything from A-Z otherwise you are chasing your tail.
I believe the muzzle device is not at fault. Took it out a few hours ago. Shot some, marked the groups shot with muzzle device and ammo type. Removed muzzle device, shot some more, marked the groups without muzzle device and ammo type.
Groups circled in red were shot without the muzzle device. Groups circled in thin black were shot with the muzzle device attached.
Vertical distance from the most obvious POA for each group can be explained by me shifting the holdover for off-topic reasons. Please ignore the variations in vertical distance of each group from what the most obvious point of aim should be.
22891
...In this case, my gut says it's the bedding, that the barrel extension to upper receiver fit is loose...
I would carefully inspect and tighten the sights, mounts & rings as applicable (check the easiest part first), then remove the barrel nut to check the fit of the barrel extension in the upper...
Thanks for your detailed analysis. Rest assured epoxy or loc-tite between the barrel extension and receiver is a hard negative ;)
I took the opportunity to break it down, re-inspecting every pinned and threaded point of interest. Everything was tight except for the fit between the barrel extension and upper receiver, as predicted. They fit together and separate without effort and there is a slight wobble between barrel and receiver with all retaining devices (barrel nut and family) removed.
...But, I can tell you that once they are received in house, they do check them. Anything found out of spec is sent back...
Perfect. Thank you for confirmation. This is exactly what I was looking for.
Next step is to replace the upper and take it for another test run without muzzle device.
drarmament
01-20-14, 17:41
Sounds like the slot that the barrel ext slides into is worn.
Hard to understand how it wore, unless the barrel was removed and replaced frequently. Sounds like tolerance stacking. Hopefully replacing the upper receiver will eliminate looseness between the barrel extension and upper receiver. If the looseness persists, other than replacing the barrel/extension assembly, an option is to use something like loctite 609. Not a permanent mating like epoxy, but will need to be heated up with a propane torch to disassemble. This method is not unheard of. Believe Alexander Arms uses a loctite product in assembling uppers.
Edited to change loctite 620 to 609, a lower temp product
drarmament
01-20-14, 20:32
Hard to understand how it wore, unless the barrel was removed and replaced frequently. Sounds like tolerance stacking. Hopefully replacing the upper receiver will eliminate looseness between the barrel extension and upper receiver. If the looseness persists, other than replacing the barrel/extension assembly, an option is to use something like loctite 690, which is designed to fill in gaps up to .015". Not a permanent mating like epoxy, but will need to be heated up with a propane torch to disassemble. This method is not unheard of. Believe Alexander Arms uses a loctite product in assembling uppers.
I probably miss read it, but that's the first time I ever heard of Loctite, how does it work?
https://www.m4carbine.net/showthread.php?119904-Applying-loctite-to-increase-accuracy
Basically used to "bed" the barrel extension in the upper. Does not "glue" it in, just like bedding does not glue in the action in a bolt action rifle.
In the link Rob Jennsen among others mention using this method in an accuracy build, unlike the situation presented by the OP.
Only mentioned it because he described significant movement of the barrel extension in the upper prior to tightening barrel nut. This method just might help avoid the need to buy another upper, if it works.
Not much to lose, maybe $10-15 on Amazon.
Another very interestting bit of info.
I saw a reference to the 10/22 in that thread, and will mention that I've personally used AccraGlass with stainless dust embedded into it as a way to tighten up the barrel in a 10/22 receiver. Those don't get nearly as hot as an AR, so I don't suggest using it. But it was nice to see the same concept being used.
To the OP: What is the outer size of the barrel exstenion and inside diameter of the upper at the barrel end? I wonder how much slop there really is. The numbers will tell.
drarmament
01-20-14, 21:04
Thanks.
Another very interestting bit of info.
To the OP: What is the outer size of the barrel exstenion and inside diameter of the upper at the barrel end? I wonder how much slop there really is. The numbers will tell.
A fine question, sir.
inside diameter of the upper at barrel end x-axis: 0.996 in
inside diameter of the upper at barrel end y-axis: 0.996 in
barrel extension x-axis: 0.990 in.
barrel extension y-axis: 0.990 in.
I'm additionally taking 2 measurements for each diameter ("y-axis" is top to bottom and "x-axis" is side to side) to address previous considerations in warpage from excessive tightening.
The wiggle isn't due to tolerance stacking, it's due to one or both parts being out of tolerance. Since two barrels have had the same problem, my guess is it's the upper.
Green Loctite won't fill the gap on a barrel extension to upper receiver fit if it wiggles. It only works if the fit between the parts is tight
Did you use the same gas tube on both builds?
The blueprint spec on the barrel extension is .9987 -.0005 Diameter shall not exceed .999 after assembly, before finish is applied.
Upper is 1.000 bore +-.002
So lets say 1.002 and .9982 as the sizes.... just under .002 slop. Assuming the errors in gauging, you've got .006 slop.
In fact, the OD is much easier to meaure, and the ID is a bit more difficult. You'd want a bore gauge for the upper, and a 1" micrometer, not calipers, for the barrel extension.
Continuing with the thought process, its more likely if you used quality instruments, your .990 barrel extension is more correct, and is undersized. Can't say on the upper, but if it was in spec, you'd be talkin' .008 to .012 slop.
Pause for thought.... dunno what to say.
Did you use the same gas tube on both builds?
Different gas tube.
...Continuing with the thought process, its more likely if you used quality instruments, your .990 barrel extension is more correct, and is undersized. Can't say on the upper, but if it was in spec, you'd be talkin' .008 to .012 slop...
I totally dig where you're going with this, but my equipment just isn't up to the cause. I only have inexpensive calipers. I did notice the measurement changed depending on what part of the "teeth" the calipers got a "bite" with. I tried to take the measurements from the same place on the calipers as I could, so they're consistent, but who is to say I took them at the right place for the most real number? Finally, I don't have anything known to the 1/1000th of an inch to calibrate/zero these calipers on. I just have to zero from "all closed".
I've spent enough time troubleshooting complex systems to understand exactly how correct Iraqgunz is. If this isn't approached with controlled and methodical isolation, there will be much chasing of the tail--especially without tools to trace the system, and double especially without a necessary level of experience in the system. Great troubleshooting skills can only go so far before we're left branching out across multiple possibilities where experience would otherwise lead us.
Try putting the carbine upper on the rifle lower and shoot another group. Sometimes the difference in stockweld (cheek contact with the stock) between the rifle and carbine stocks can have an effect on accuracy, especially when using optics. Consistent placement of the eye behind the scope is very important when shooting groups.
In reading the entire thread did I miss if you reused the old barrel nut or got a new one for the DD barrel? Barrel nuts are like crush washers use it once and do not reuse again.
On Midway's site it is claimed that 609 will fill in up to a .005" gap and 620 up to .015" gap. If the fit is tight there is no gap to fill.
A new barrel nut will not eliminate the slop between extension and receiver.
Try putting the carbine upper on the rifle lower and shoot another group. Sometimes the difference in stockweld (cheek contact with the stock) between the rifle and carbine stocks can have an effect on accuracy, especially when using optics. Consistent placement of the eye behind the scope is very important when shooting groups.
Thanks for the suggestion. Will do (along with 10 shots per).
In reading the entire thread did I miss if you reused the old barrel nut or got a new one for the DD barrel? Barrel nuts are like crush washers use it once and do not reuse again.
I did not know this. Thanks! It's actually a different barrel nut. The old rifle had old-school handguards on it, and this one has LaRue free-float rails. I have emailed LaRue to ask about reusage of this nut and will update with their answer as soon as I have it.
On Midway's site it is claimed that 609 will fill in up to a .005" gap and 620 up to .015" gap. If the fit is tight there is no gap to fill.
A new barrel nut will not eliminate the slop between extension and receiver.
From watching videos of barrel installations, it looks like there should be a tighter fit than what I have between barrel extension and receiver. I'm hoping a new upper will fix this. Would like to avoid putting anything but moly-grease in there, but I will definitely keep this in mind. Thanks.
I need to take a couple weeks to get moved across the country and will update with new information as soon as I get it. I really appreciate all the help and advice here. It's worth a million dollars.
To-do:
* update with LaRue's answer about reusing the barrel nut on LT15-7
* update with any notes about felt differences re fit between DPMS upper vs BCM upper with DD barrel
* replace upper with BCM and re-try with 10 shot groups without muzzle device
* swap upper group to rifle's lower and make some more 10 shot groups; compare.
From Dan at LaRue: "As long as the barrel nut is not damaged, you can re-use it."
From Dan at LaRue: "As long as the barrel nut is not damaged, you can re-use it."
Thank you for the research. I have been re-using barrel nuts for some time.
A pleasure.
This only speaks for the barrel nut as packaged with the LT15 series of LaRue handguards. In quick further research, I found a few forum posts stating it was fine to reuse standard barrel nuts as long as the threads aren't damaged and it has enough teeth to torque it to spec.
Varminting or target ARs may well have different expectations.
Thanks for the suggestion. Will do (along with 10 shots per).
*SNIP* Would like to avoid putting anything but moly-grease in there...
Good job for paring some of the advice to go full retard sprinkled about in this thread. Proper procedure is to replace the out of spec upper receiver with one that is in spec and use moly-grease when assembling.
On Midway's site it is claimed that 609 will fill in up to a .005" gap and 620 up to .015" gap. If the fit is tight there is no gap to fill.
A new barrel nut will not eliminate the slop between extension and receiver.
I've used green Loctite to install new bearing in the rod ends for the control surfaces of different aircraft, There's a big difference between how much gap they claim can be filled and how much gap it fills in actual use. A special primer is also used before applying the green Loctite. If there is a gap, the Loctite does nothing to hold the part in place. Green Loctite does help hold parts in place when there is a close slip fit or a press fit. Even with polished surfaces, there are peaks & valleys. You may not be able to see them without a microscope, but they are there. When the two parts or fitted together, they only make contact on the peaks. Using adhesive filles in the valleys increasing the grip.
None of this is to suggest that green Loctite should be used when installing a barrel to an upper. It will make removing the barrel more difficult. Of course, this doesn't matter if you plan to get a new upper when swapping barrels
KF5MLT, KD6IJG
thanks for the info, never too proud or old to learn.
sounds like green loctite is for a more or less permanent application, not a good choice for an ar barrel.
Thanks for all the help. The upper receiver was swapped out for one from BCM and that seems to have fixed everything up. (The SSA is the 77 gr. OTM variety.)
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Yes, I would have gone nuts if I had to wait another week to get out and test this thing. ;)
Glad swapping the receivers worked out for you, next time this should help you steer away ==> D.P.M.S. = Don't Purchase Monkey Shit
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