AR barrel seating/bedding

[BoilerWes]

Working on my first AR build and scrutinizing over the details. What’s the consensus around here on using some sort of compound/adhesive when seating the barrel? What do you use?

 

[JEBland]

Working on my first AR build and scrutinizing over the details. What’s the consensus around here on using some sort of compound/adhesive when seating the barrel? What do you use?

Not an adhesive.

Just a little grease. There are many options. Next time I buy an ADM lower, I'll probably buy their grease:

Aeroshell 33MS Grease

AeroShell 33MS aerospace grease that meets mil-spec MIL-G-21164D for barrel nut assembly on AR platform upper receiver threads. As used on ADM Rifle builds, offered here in a 1/4oz jar for your own AR-15 projects. Unless you run your own shop, this small jar will likely last you through several...

www.admmfg.com

Their ambi lower is the bee's knees for this AR noob. (There's a thread on triggers.)

 

[natdscott]

I use a bigger extension (BAT, WOA), or a smaller receiver (BCM).

That not being an option, yes, you can loctite it, in a vertical position. Very, very common in match uppers. Then again, we're used to the need for heat guns and hammers to get barrels back out.

 

[WhiskE TangO]

You will get many different recommendations from manufacturers. Some will advise you to just install the barrel, and then apply some form of lock preventative compound on the barrel nut. Torque to spec and done. But you really cannot go wrong any route you take.

 

[bstewrat3]

I used stainless shim stock and a thermal seating on a 6.5 Grendel build I did back in 2020. I premeasured the clearance between the barrel extension and the upper and then used shim stock that was slightly larger. The thermal fit involved heating the barrel channel of the upper with an induction coil and chilling the barrel in the deep freeze for a couple hours. I wrapped the shim stock that I had precut around the barrel extension with the seam at the top by the locating pin and held it in place with a rubber band while the barrel was cooling. Once you get to the seating part it is a a very time sensitive process. I had my wife heat the receiver while I pulled the barrel straight from the freezer, removed the rubber band and quickly slid the barrel into the receiver as my wife moved the heating coil. The barrel was seated solidly and will not come out without using a lot of heat.

 

[BigMoose]

If I remember right I used a bit of High Temp synthetic wheel bearing grease on my threads. But that was it.

 

[natdscott]

I used stainless shim stock and a thermal seating on a 6.5 Grendel build I did back in 2020. I premeasured the clearance between the barrel extension and the upper and then used shim stock that was slightly larger. The thermal fit involved heating the barrel channel of the upper with an induction coil and chilling the barrel in the deep freeze for a couple hours. I wrapped the shim stock that I had precut around the barrel extension with the seam at the top by the locating pin and held it in place with a rubber band while the barrel was cooling. Once you get to the seating part it is a a very time sensitive process. I had my wife heat the receiver while I pulled the barrel straight from the freezer, removed the rubber band and quickly slid the barrel into the receiver as my wife moved the heating coil. The barrel was seated solidly and will not come out without using a lot of heat.

Last one I did that way, I had no wife to help, so a 2x4 was involved.

Worked, but a BCM A4 paired with an already slightly larger barrel tenon means that I am pretty sure whomever tries to get that one loose will have to split the receiver ring to do it.

C'est la vie. Receivers are cheap. That barrel was not.

 

[Squid556]

I remember reading about gaining some accuracy by lapping the front of the receiver face so the barrel extension shoulder has a nice true surface to mate to.

Havent done it myself yet but plan to if i do another accuracy build.

Never used anything on the threads and am yet to have a problem getting them off. Although I've never gotten my rifles super hot either. Mileage my vary.

 

[BiscuitsandGravy]

I remember reading about gaining some accuracy by lapping the front of the receiver face so the barrel extension shoulder has a nice true surface to mate to.

Havent done it myself yet but plan to if i do another accuracy build.

Never used anything on the threads and am yet to have a problem getting them off. Although I've never gotten my rifles super hot either. Mileage my vary.

Yes and I believe the lapping also may help remove imperfections in the anodizing process. I've used a piece of glass with some wet sandpaper. Also, apply a bit of anti-seize to the threads. Pick up a couple extra 'ketchup' packs of anti-seize the next time your at your local auto parts store. Its handy to have. YMMV.

 

[natdscott]

I remember reading about gaining some accuracy by lapping the front of the receiver face so the barrel extension shoulder has a nice true surface to mate to.

Havent done it myself yet but plan to if i do another accuracy build.

Never used anything on the threads and am yet to have a problem getting them off. Although I've never gotten my rifles super hot either. Mileage my vary.

Entirely unnecessary, unless used in the case of fixed-position breech bolts like dedicated .22 uppers. Maybe.

Think about it: the entire BCG floats, and can to some extent align itself to the barrel extension. If not immediately, then it surely does after a few hundred rounds of wear-in on the lug surfaces.

This is one of those logical holdovers from bolt rifle "truing" that just doesn't actually accomplish much in the AR.

Oh, and you can't do it with glass, or some other flat sheet. If you're going to gain anything, that action face has to remain (or be made to) orthogonal to the threads, which we hope are parallel to the receiver bore. That requires a lathe with jig, or second best, a mandrel setup. All that sheet lapping does is flatten a pre-existing surface orientation...and I assure you that any normal commercial upper was plenty flat enough when you bought it.

If you decide to lap the thing, be careful to go SLOWLY. Carbide cuts that anodizing VERY quickly. Do NOT take off the entire face...all you wanna do is get 1/2 to 2/3 of the face cleaned up, and leave the remaining black anodizing alone...that's plenty "good" enough unless you have a lathe.

If you had a lathe, you wouldn't be asking, because you'd have done it already.

 

[dieselrealtor]

I used stainless shim stock and a thermal seating on a 6.5 Grendel build I did back in 2020. I premeasured the clearance between the barrel extension and the upper and then used shim stock that was slightly larger. The thermal fit involved heating the barrel channel of the upper with an induction coil and chilling the barrel in the deep freeze for a couple hours. I wrapped the shim stock that I had precut around the barrel extension with the seam at the top by the locating pin and held it in place with a rubber band while the barrel was cooling. Once you get to the seating part it is a a very time sensitive process. I had my wife heat the receiver while I pulled the barrel straight from the freezer, removed the rubber band and quickly slid the barrel into the receiver as my wife moved the heating coil. The barrel was seated solidly and will not come out without using a lot of heat.

I have done similar stainless shimming, heating & cooling to install barrels. I used a thin coat Aeroshell 33 on both sides of the shim as well as the threads.

In the unlikely event that I run out of things to do. I will assemble one without shimming, break in the barrel & shoot some groups. Then pull it apart, "fit" the barrel & see if there is an appreciable improvement.

 

[ditcherman]

I’ve always assembled dry, using BCM’s barrel nut onto a no name receiver. Might of had to heat a receiver up once if I recall.

 

[Runngun01]

A little red loctite around the barrel extension. Good grease on the receiver threads and torque to spec. You’re all set!

 

[ditcherman]

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.

 

[mike4]

WHAT THE ACTUAL HELL?

This is not an area of mystery or debate or need for varied and innovative technique or experimentation. This is long established procedure where I am going reference for the second time in recent days military experience in the initial production and servicing of, no exaggeration, millions of rifles.

People are actually talking about creating whole new potential problems if anything, for a procedure that is straight forward and very well documented for decades. There are a lot of things that are complex, or uncertain, or debatable; this is not one of them. There are a lot of things that are subject to careful testing to determine and validate what works best. Already done for us long ago.

Some folks have correctly noted the bolt locks into the barrel extension so that the connection between the barrel and the upper receiver only needs to the keep the barrel attached and the receiver channel where the bolt carrier reciprocates generally in a straight line with the barrel axis. There is no need to have the barrel extension with an extremely tight fit into the upper receiver, or have adhesives or shims present here which are only going to do some of three things if anything, none of which are helpful: 1) You are going to impede the free movement of the barrel extension seating back into the receiver so that it jacks up the barrel nut torque which is your indicator for that operation on initial mounting, or future remounting with now setup adhesive present in that space. 2) You are screwing up clearances that in part accommodate the differing thermal expansion rates of two very different alloys in conditions from the Arctic to the maximum recommended rate of full auto fire. 3) You may be stressing a relative thin circumference of much weaker aluminum surrounding the steel barrel extension.

Finally the torque ratings for AR barrel mounting assumes lubricated threads. In fact, I have seen through first hand experience where torquing dry threads resulted in the barrel nut loosened with thermal cycling where the gas tube passing through the barrel nut was the only thing preventing the rifle from disassembling itself.

Quoting here from the closest reference I could reach on the shelf, October 1984 USMC M16A2 technical manual. Apply molybdenum disulfide grease to upper receiver barrel nut threads. Assuming your barrel wrench has a similar offset from the barrel axis to the square hole for a standard torque wrench as the USGI barrel wrench, torque to 31-35 ft-lb. Loosen and repeat. Loosen and repeat. Third torquing is the charm, hopefully plus or slightly minus enough rotation to line up the barrel nut to allow proper alignment of gas tube.

You are set now unless you ever need to remove that barrel and it will be simple to do without damaging anything once the gas tube is removed. The only grief in a higher mileage upper is sometimes driving the damn gas tube pin out of the front sight base, if so equipped.

 

[natdscott]

WHAT THE ACTUAL HELL?

This is not an area of mystery or debate or need for varied and innovative technique or experimentation. This is long established procedure where I am going reference for the second time in recent days military experience in the initial production and servicing of, no exaggeration, millions of rifles.

People are actually talking about creating whole new potential problems if anything, for a procedure that is straight forward and very well documented for decades. There are a lot of things that are complex, or uncertain, or debatable; this is not one of them. There are a lot of things that are subject to careful testing to determine and validate what works best. Already done for us long ago.

Some folks have correctly noted the bolt locks into the barrel extension so that the connection between the barrel and the upper receiver only needs to the keep the barrel attached and the receiver channel where the bolt carrier reciprocates generally in a straight line with the barrel axis. There is no need to have the barrel extension with an extremely tight fit into the upper receiver, or have adhesives or shims present here which are only going to do some of three things if anything, none of which are helpful: 1) You are going to impede the free movement of the barrel extension seating back into the receiver so that it jacks up the barrel nut torque which is your indicator for that operation on initial mounting, or future remounting with now setup adhesive present in that space. 2) You are screwing up clearances that in part accommodate the differing thermal expansion rates of two very different alloys in conditions from the Arctic to the maximum recommended rate of full auto fire. 3) You may be stressing a relative thin circumference of much weaker aluminum surrounding the steel barrel extension.

Finally the torque ratings for AR barrel mounting assumes lubricated threads. In fact, I have seen through first hand experience where torquing dry threads resulted in the barrel nut loosened with thermal cycling where the gas tube passing through the barrel nut was the only thing preventing the rifle from disassembling itself.

Quoting here from the closest reference I could reach on the shelf, October 1984 USMC M16A2 technical manual. Apply molybdenum disulfide grease to upper receiver barrel nut threads. Assuming your barrel wrench has a similar offset from the barrel axis to the square hole for a standard torque wrench as the USGI barrel wrench, torque to 31-35 ft-lb. Loosen and repeat. Loosen and repeat. Third torquing is the charm, hopefully plus or slightly minus enough rotation to line up the barrel nut to allow proper alignment of gas tube.

You are set now unless you ever need to remove that barrel and it will be simple to do without damaging anything once the gas tube is removed. The only grief in a higher mileage upper is sometimes driving the damn gas tube pin out of the front sight base, if so equipped.

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

 

[55fairlane]

After reading this consult your machinery hand book or mechanical engineer for a further indeapth, dumbed down to advanced college explanation

Aluminum is soft and "gummy" if you try to mechanically fix (thread) two Aluminum articles together (with any kind of persition fit) the surfaces will "gauld" together and never come back apart, there for lubrication is needed & threads must have exseive clearance in them.

We anodize Aluminum with a hard coat to give the surface a coating that won't oxide, and will allow a tighter thread spec to be used, but once again, a lubrication is needed, a high pressure, but still low viscosity.

You tighten the nut up, let it sit, relieve the nut, and torque to proper spec, threads "grow" so we want to stretch them (under a controlled environment) then retorque so nothing comes loose.

Last comment, lap anything together on an AR? Not necessarily, there are stresses on the upper/barrel/bolt/ ect, but there very different stresses then the old blued steel & wood rifles. These things are erector sets with all the variables know, no need to reinvent how to do things

 

[BoilerWes]

Thanks for all replies! Great information!

 

[ditcherman]

The instructions for using BCM’s barrel nut, required with a BCM handguard, state that oil MAY ease installation, and to never use grease.

https://www.bravocompanymfg.com/downloads/kmr_installation_instructions.pdf

These barrel nuts do not have to be timed to let the gas tube through, I actually had no idea the original design did. That would kind of suck. Just shows we don’t know what we don’t know.

ETA Strikethrough. Read on.

 

[JEBland]

The instructions for using BCM’s barrel nut, required with a BCM handguard, state that oil MAY ease installation, and to never use grease.

https://www.bravocompanymfg.com/downloads/kmr_installation_instructions.pdf

These barrel nuts do not have to be timed to let the gas tube through, I actually had no idea the original design did. That would kind of suck. Just shows we don’t know what we don’t know.

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:

3. Apply white lithium grease or moly paste to the threaded area of the inside of the KMR barrel nut (Item No. 1)
[...]
9. Note: Application of oil to the exterior of the barrel nut may ease installation. NEVER use grease of any type on the exterior of the barrel nut.