AR barrel seating/bedding

[BGDave]

Good post, except you're not really current on high-accuracy AR work. The following will focus on that, as I don't presume to know much about heavy full-auto usage.

Several of the best accuracy builders in AR history either glue their uppers, and/or use larger extensions, to reduce barrel joint movement, then still torque to spec.

There's been quite a pile of development work done with the AR since 1984. The basic design remains unchanged, but because of some very open-minded people whose names are now "household", what we can reasonably expect from an AR has changed dramatically.

To your main points:

1) No, it doesn't.

2) Please cite the specification that called for over a mil of freedom between extension and upper receiver bore.

3) That is possible, in theory. If you manage to create a 0.002-0.003" difference that has to be "thermally fit" (as is the rage term now), then it is probably possibly for that to become a failure point in some modes of fire.

Glue doesn't do that though. It only occupies the existing space, and would melt LONG before a receiver ring failure from heat. Course..if you ever managed to get that part of the rifle to over 750 degrees, the barrel would probably have failed first.

Now what I will say is that glue creates a field armorer's nightmare. Removal and cleanup for rebarrel takes a lot longer, and at least one more tool, if not more. If you are a competition shooter maintaining only one or a few uppers, and replacing 1-2 barrels a year, it's not a big deal, but glue in police or military uppers is probably not feasible. The military M4/16 should righteously be expected to be rebarrel-able on a stump with no more power than what comes in an MRE.

Last thing: if the barrel nut won't line up, guys can get into a bad situation when they over-torque the joint, and/or permanently twist their receivers. Shims are available, sure, but the easier solution is to just torque it to spec, mark what needs, and then remove the nut to cut/grind off the offending tooth or teeth. That is, assuming the use of a standard barrel nut, which can't easily be assumed anymore.

-Nate

I'm going with this. Only to add, the goal is to make the barrel, barrel extension, and the bolt and carrier group repeatable to the receiver sights. Sorry if I missed this point anywhere among this very informative thread.
Very interesting thread. Thanks.

 

[bwframe]

Greasing and seasoning the threads at least three times has always worked for me. If and when barrel or handguard changes are needed, this is pretty essential to me. I have yet to outshoot my AR's, with midrange name brand barrels, so ultimate accuracy is not worth the deconstruction in my case.

If I ever get there, Nate would be my go to guy for advice.

Hey Nate; What is the best expected round count for an accurate AR barrel?


.

 

[russc2542]

Torquing to spec with grease/oil never made sense to me.
You’ll stretch your lugs doing that!
Maybe this is different, but I’m not seeing it.

Because the spec for lug nuts is with dry threads. If you lube a fastener and torque to dry spec, you're 100% right it's overtorqued. Specs for barrel nuts are (generally) for wet threads. You don't put your head bolts in dry do you? If you torque set-specced fastener with dry threads, it'll be undertorqued.

 

[ditcherman]

Please don't take offense, but I think you're leaving out some important context for that note. BCM recommends grease or moly paste on the threads of the barrel nut.
From the posted .pdf:

Haha not offended at all. The only thing that really matters is the truth, and apparently the only thing that sucks worse than a barrel nut that has to be timed is my reading comprehension skills.
It’s been a while since I’ve built anything, but I don’t remember using grease. They haven’t loosened, that’s all I know.

 

[natdscott]

Greasing and seasoning the threads at least three times has always worked for me. If and when barrel or handguard changes are needed, this is pretty essential to me. I have yet to outshoot my AR's, with midrange name brand barrels, so ultimate accuracy is not worth the deconstruction in my case.

If I ever get there, Nate would be my go to guy for advice.

Hey Nate; What is the best expected round count for an accurate AR barrel?

Thanks for that, though I'm not sure I deserve it. I have a lot (lot) of books and tools, have shot with some very knowledgable smiths and shooters, and have pushed limits with the little gun. I do very much admit that most of my experience regards mid-to-heavy bullets, trying for extreme accuracy. Same on the rimfire uppers.

"Premium" broach or cut rifled SS match barrels (e.g.: Krieger, Bartlein) seem to be reliable to about 4,000 rounds, to draw a conservative line in the sand. With today's wait times, you better be ordering a tube at 3,500-4,000 if you shoot much. When they start showing wider groups, slightly off-call shots, and vertical at 5-600 yards, we've always trashed them for any serious match.

Some of the premium buttoned barrels (e.g.: Hart, Shilen, Lilja, Broughton, Schneider) have been rumored to be doing about the same, and can be crazy accurate. Mr. Schneider, for example, has had the M40 contract for 20 some years, and is the only supplier D. Tubb has used in years. Shilen Supermatch ratchets are unreal.

Mid-grade barrels like Wilson-blanked WOA, Green Mountain-blanked Keystone Accuracy, etc. are reliably judged at 2,000 rounds. Most all will go that long, but they can really fall apart by 2,500, even at 300 yards.

Lower grade than that are probably not suited to much true accuracy work, but might be gotten chromed (Criterion makes some chrome ones) and still shoot pretty darn well for a long time.

Gunshow specials should be left to somebody else's wallet. If they cannot answer "Whose blank was used?"...just keep walking.

 

[jrh84]

Torquing to spec with grease/oil never made sense to me.
You’ll stretch your lugs doing that!
Maybe this is different, but I’m not seeing it.

Not sure what you mean by stretching the lugs...how so?

I assume the torque spec was developed with having the threads lubricated with aeroshell grease.

 

[cosermann]

Using the spec grease (ex. Aeroshell, post #2) is key. Using the wrong grease can cause “issues.” For example using a grease containing graphite, will setup a galvanic action between the aluminum and steel - not good.

 

[natdscott]

Just for giggles, anybody interested, here's a round of 1,000 yard Berger 90 beside a round of greentip 5.56 in a 20 rd mag:

 

[55fairlane]

Just for giggles, anybody interested, here's a round of 1,000 yard Berger 90 beside a round of greentip 5.56 in a 20 rd mag:

View attachment 194088

Hey Nate, either use a pair or good wire cutters or a really big hammer and you can fit that inside your magazine.

Old Colt magazine?

 

[BIGE7.62]

Just for giggles, anybody interested, here's a round of 1,000 yard Berger 90 beside a round of greentip 5.56 in a 20 rd mag:

View attachment 194088

The Valkyrie would love that 90 gr with a
1/6.5 twist and 22 " barrel

 

[natdscott]

Hey Nate, either use a pair or good wire cutters or a really big hammer and you can fit that inside your magazine.

Old Colt magazine?

Hahaha!! That's the kind of open mind we need sometimes. In all truthfulness, I have trimmed 77 SMK back "aways" to fit just a littttttle but more black stuff behind them to try to hit MK262 speeds for a guy. It worked, though not really super advisable.

 

[natdscott]

The Valkyrie would love that 90 gr with a
1/6.5 twist and 22 " barrel

Ha! It sure would; a but more than the .223 anyway. It's honestly really suboptimal in the little case. Just too far outside the parameters, too much of the time, unless you have a really long barrel.

My 2nd LR barrel--for that load--was/is an 8-6.5 left twist 5R CM chambered at 0.110 FB, 1.5-degree leade. It shot the Berger slightly better than the Sierra, but both are premier-level bullets. Swampy's JLKs were still better than either (RIP).

My personal opinion though is that the Valkyrie does not go far enough beyond .223 to make me interested for a LR cartridge. It's neat, and fits in the little AR, but if I want a "long range" 22 (which I really, really don't), then it would be a .22-250, 22x47 Lapua, or something like that. Takes a lot of "go fast" to make a 90 into anything but a cruiser.

ETA: If I didn't say it would be Bartlein's BB steel, I did now.

 

[ditcherman]

Not sure what you mean by stretching the lugs...how so?

I assume the torque spec was developed with having the threads lubricated with aeroshell grease.

I was speaking specifically about auto/truck lug bolts in that comment; see post 23 for a better explanation but basically some torque specs are dry, some are wet, and I was ignorant of the classic instructions and misread my current instructions.
Sorry for the confusion.

 

[churchmouse]

Because the spec for lug nuts is with dry threads. If you lube a fastener and torque to dry spec, you're 100% right it's overtorqued. Specs for barrel nuts are (generally) for wet threads. You don't put your head bolts in dry do you? If you torque set-specced fastener with dry threads, it'll be undertorqued.

Bingo.

 

[Frontiersman]

Having spent just a few hours in a military armory, we used an anti-seize compound. We weren't building sniper rifles, just good ole GI in mudhole reliability.

 

[jrh84]

Gotcha, I get

I was speaking specifically about auto/truck lug bolts in that comment; see post 23 for a better explanation but basically some torque specs are dry, some are wet, and I was ignorant of the classic instructions and misread my current instructions.
Sorry for the confusion.

Gotcha, Im with you now.

 

[jrh84]

Ha! It sure would; a but more than the .223 anyway. It's honestly really suboptimal in the little case. Just too far outside the parameters, too much of the time, unless you have a really long barrel.

My 2nd LR barrel--for that load--was/is an 8-6.5 left twist 5R CM chambered at 0.110 FB, 1.5-degree leade. It shot the Berger slightly better than the Sierra, but both are premier-level bullets. Swampy's JLKs were still better than either (RIP).

My personal opinion though is that the Valkyrie does not go far enough beyond .223 to make me interested for a LR cartridge. It's neat, and fits in the little AR, but if I want a "long range" 22 (which I really, really don't), then it would be a .22-250, 22x47 Lapua, or something like that. Takes a lot of "go fast" to make a 90 into anything but a cruiser.

ETA: If I didn't say it would be Bartlein's BB steel, I did now.

I shot my Ruger Precision using 88 ELD-M's in the Ireland USMC match in 2019. One of those $!#@ Illinois guys that like to come **** on our parade had a match rifle in .224 Valkyrie, 26" barrel, and he was shooting the Hornady factory 88 ELD ammo. His rifle was throated to put mag length 88's 0.010" off the lands. Not sure where the typical .224 Valkyrie chambering puts them, but anyway....

My long-loaded 88's out of a 20" Ruger Precision barrel was slinging them downrange 10 fps faster on average than his mag-length .224 Valkyrie at 1000 yards. Granted, he was still able to mag-feed, and was using factory ammo vs. my well-over book max loads using Varget, but still....the .224 Valkyrie is a neat round, but its not gaining THAT much over a .223 if you aren't tied to magazine length.

 

[mike4]

Have been working overtime on work deadlines and just now looping back to comment on post #15 and #16. The entry point for my comments on post #15 was divergent thread advice (using Loc-tite, interference fit and shrink-fit with or without metal shims, debate over lubing barrel threads) in response to a "first AR build" question. Not one reference to the standard M16 series barrel mounting procedure which I provided so the OP or anyone reading the thread now or years later had some sound advice with a reference to it's source.

There is a clear distinction to me between the people competently chasing maximum accuracy, and builds intended for military, law enforcement and defensive use. OP certainly opened the door to those accuracy techniques asking about using some sort of compound/adhesive when seating the barrel. Regarding natdscott's series of posts and some others, I'm critical of a lot of haphazard techniques implemented without consistency or prior experience by a first time builder. I'm not critical of competent gunsmiths, careful handloaders and highly skilled shooters that are testing their techniques to close the loop with evidence of what really works.

As to natdscott stating my first point is wrong, I said one or more of these things can come into play, meaning with one or more of the scattering of procedures that had been mentioned. I understand unset adhesive is not going to create resistance to barrel seating that has any effect on correct barrel nut torque. If you FULLY seat the barrel in the receiver with a shrink-fit, or an interference fit ideally press-fitting and not hammering, where you have zero further movement of the barrel into the upper, that has no effect on correct barrel nut torque. If someone does not know what they are doing through lack of experience with shimming or other interference fit, or the temperature of their shrink-fit parts are converging back toward room temp rapidly enough to start increasing friction, and they start depending on the torquing of the barrel nut to fully seat the barrel, then at a certain magnitude of insertion friction force stacking on top of thread seating torque, your torque wrench IS going to click "done" before you want it to.

Again, I know the AR accuracy world is it's own world with a whole range of techniques evolved, yet consistent application of experience-proven techniques is equally important there, and I'm not sure that is first time builder territory. Although without more specifics in original post, maybe that's the direction they want to go based on the question?

Finally in direct reply to natdscott comments "except you're not really current on high-accuracy AR work" and "There's been quite a pile of development work done with the AR since 1984". No where did I suggest I was not talking about high-accuracy AR work, or I would not be citing that mil assembly standard. I'm aware from direct experience some poor choices creep into some military system designs, but the quality of specs and procedures for fully developed programs of record, and long attention and eventual field feedback on long serving systems actually produces some of the most carefully considered results to be found in any field of applied engineering. I indicated the 1984 Technical Manual was just the first within reach, but the key point is that it is not "out-dated", it is "long established". And proven even in 1984 from 28 years of AR refinement, and 20 years in service through the evolution of M16, M16A1, XM variants, and M16A2. (I've tended toward more SOCOM-like personally owned configs since the late 90s.) I cited it because it is probably relevant to 95%+ of AR builds, and far superior to any half-assed methodology for anyone not doing a high accuracy build.

A post script to the useful technical discussion: I'm setting my Internet vocabulary diplomacy filter to 11 to select phrasing; I'm 'deeply unappreciative' of your two above comments combined. I have not been living under a rock since 1984. I've been building AR-15s and M-16s for 32 years, with a Noveske Switchblock barrel sitting here today for a SOCOM556-RC QD host build.

It is correct that my experience and interest is military, LE, and defensive AR applications where accuracy is only one variable in the mix with carry weight, handling, reliability, and durability over time in a wide range of field environments. So I do defer entirely on methods to those with solid experience where the goal is extreme accuracy, although I would expect that is a minority niche of AR applications. Not a knock at all on that pursuit and interest and specialized skillset, just that most people are going to have more practical use for a reliability/durability oriented "milspec-ish" AR.

 

[indysims]