About rebarreling an AR15

[rickt300]

I ordered a Brownell action tool. It has slots in it that engage the locking lug slots in the Barrel extension. Watched several videos and it looks pretty easy. The barrel is a Faxon Gunner profile, just wondering if I should put a dab of locktite on the barrel shank and let it set up before I put the final torque to it and what is the recommended torque? I have found recommendations of anywhere from 30 to 80 inch pounds. Thinking 45 is a good medium since my wrench has a good distance between the square for the torque wrench and the barrel.

[LRRPF52]

That’s what I do, coat the barrel extension in Loc-tite and let it set up for a day.

Before I do all of that, I make sure the chamber, gas port, and other dimensions of the barrel are what I’m looking for.

I have 4 different dummy cartridges I use to check headspace and throat compliance with SAAMI.

Then I de-edge, blend, and polish the feed ramps. I don’t want feed ramps that are proud of the feed ramps in the upper either, since that will catch on the meplats of hollow points sometimes.

For torque spec, I have wrenches but use feel instead. Once the Loc-tite has cured overnight, I mount up the upper, then torque the barrel nut to where it feels like a dull sensation, not a resonant tuning fork. You still want to be over 35ft-lbs, which isn’t much. Many barrel nuts require tooth alignment anyway, so you can either cut teeth or shim, but I don’t like shimming barrel nuts if I don’t have to.

A lot of the barrel nuts nowadays are cylindrical and don’t require tooth-clocking, so I torque to feel for dampened harmonics.

AMU found that lightweight 20” barrels did better with 65ft-lbs, and heavy profile 20” barrels did better with 45ft-lbs, so counter-intuitive.

For tighter groups, I want a dampened relationship between the barrel and upper receiver, not one where they ring out and vibrate as much.

It’s a weird approach you won’t hear much about anywhere else, but something that comes from harmonic dampening in Aerospace, where the principles are the same.

If you have a wing, vertical stab, control surface, pylon, strake, or other protuberance from the fuselage of an aircraft that hits a certain harmonic node in-flight, it can get really bad for structures as it buffets violently. So the structures folks work really hard in design, wind tunnel testing, and early flight testing to make sure it doesn’t happen.

Several aircraft have been rushed into production without fully working-out these flutter characteristics, the F/A-18 and its vertical stabs being a great example of that. F-14 had buffeting problems with the vertical stabs as well. There are a lot of high-strength fasteners, torque specs, and adhesives used to hold aircraft structures together.

With the AR-15, I want to minimize how much barrel whip is happening so the barrel and its fitment to the upper stays as close to true as possible after each shot.

[Klem]

Rick,

Everyone has an opinion on gluing their AR's and we all look for confirmation to justify our choice. This often leads to confirmatory bias - me included.

I don't glue the action/extension interface as I don't believe the juice is worth the squeeze. In other words, I do not believe it affects precision to the extent that it is needed. Unless the interface is so sloppy as to need a filler or shim to mimic a tight fit. And if that's the case then you need better quality parts. You also have to be mindful the extension heats up during rapid fire and can melt/reflow the glue, or at least weaken the bond, which is why you chose glue instead of grease in the first place.

I use a thin film of lithium or moly grease to aid in bringing the parts together, and getting them apart later. As they are dissimilar metals, grease also prevents corrosion.

Using the Brownells action tool - Yes, it's pretty easy. Consider clamping the torque wrench in the vise so the Brownells tool stands upright. It's not only a solution in some situations but having the action vertical prevents run-out between the action and barrel that would be present if horizontal (by gravity), as you tighten the nut.
clamp - Copy.jpg

You are concerned about the distance between the wrench and the teeth of the Brownells tool; don't worry, that's not going to make any difference to the torque applied. The Brownells tool is so thick and solid that there is not going to be any twisting affecting the applied torque. And of course you know the torque extension calculation when using handguard extensions on a wrench. Here's a pic on this from Youtube.
Torque Extensions Formula.jpg

Here are a few wrench extensions supplied by handguard manufacturers for their proprietary barrel nuts. Seekins, Daniel Defence and I can't remember the third. On the right is the standard Milspec armourer's wrench. If you are using an extension with a 1/2" slot like the examples, then you can avoid having to do the above calculation by having the extension at right-angles to the torque wrench. The torque displayed on the wrench is then pretty accurate.
handguard extensions - Copy.jpg

The wide Milspec range of 35-80ft lbs is to permit the 20 teeth on a Milspec barrel nut to time a hole for the gas tube. As 52 is saying, most after-market handguards don't use barrel nuts that interfere with the gas tube however, so there's no need to crank it up to 80. I torque to the handguard manufacturer recommended setting. For example, SLR's Solo, Ion and Helix handguard's recommended torque is 35ft lbs, so that's what they get. Plus a thin coat of lithium grease on the threads. Do it up to 35, loosen it, do it back up again, loosen it, and up a third time. This beds it in properly and avoids point loading and scatter.

[rickt300]

Excellent information from both of you. My torque wrench is one of the old style that depends on a pointer being displaced by bending the spine of the wrench. So it is likely I will have my Armourer's wrench in the vice, barrel down and my torque wrench in the end of the Brownell's tool. Will have never seize on the threads and on the shoulder the barrel nut bears against to reduce the possibility of the barrel pin putting much pressure on the receiver. I also see no problem with putting some red locktite on the barrel extension and letting it cure first. My parts are in the mail coming from two places Joe Bobs and Brownells. I may clean my bench off a bit to reduce laughter and post some photos of me in process.

[A5Blaster Number 2]

I don't do any "glueing" anymore, it's not worth the effort.

What I do is buy higher end uppers known to come with tighter barrel channel specs. And do a heat/freeze barrel install.

Barrel goes in the freezer till froze and I heat the upper up and quickly install the barrel.

I do sqaure off the receiver face if it needs it.

As to how much torque to use. Most free float handguards come with a min/max torque range for their barrel nut. The last ten or so rifles I have built I just torqued them up to 35 foot lbs and called it good.

Hope this helps.

[montana]

I first check the barrel/bolt headspace (headspace gauges), gas port size (pin gauges), gas port alignment (pin gauge), proper chamber cut (function gauge), bore, chamber and crown quality (bore scope)..

I check the upper's carrier bore size (pin gauges), the barrel extension/upper fit and the upper's feed ramp interface with the barrel extension feed ramps.
I check the barrel interface fit with the upper and lap if needed.

Some people will use green or blue loctite with or without a shim for a tight fit in the upper..Some swear by it and others don't..In any case, using a loctite or shims does no harm and can be easily removed if needed..

Some people prefer thermal fit uppers and others don't..If using a thermal fit upper, then it is necessary to turn down the O.D. of the lapping tool to work..

Most newer upper receiver carrier bores I have gauged are tighter and under sized than the TDP spec requires..A tighter carrier bore will keep the carrier/bolt face straighter to the bore improving accuracy and bolt life..I have also seen newer upper receivers carrier bores that were grossly over sized and out of spec..I returned them immediately.

It is advised to use some kind of lubricant on the barrel nut threads of the upper receiver, between the the barrel extension lip and barrel nut to acquire proper torque..Mil spec requires Aeroshell 64 but any lithium/moly grease should work..I use the Colt factory Molykote G-n Metal assembly paste..

It is important to season the barrel nut/upper receiver threads by tightening/loosening the barrel nut 3 times before final torque..Colt discovered this prevents the barrel nut from loosening..

Mil spec barrel nut torque is 35 to 80 ft pounds...This large variance is to time the gas tube to the upper using the mil spec barrel nut..Always use the factory barrel nut torque values being used..

My test have show the upper receiver will compress around the barrel extension with increased barrel nut torque up to around 55 to 60 ft lbs before the compression stops..The advantages of a thermal fit upper over a standard upper has never been proven from what I can find or have experienced..

I prefer using an action rod with a sail to keep the barrel from clocking in the upper..I have personally seen this happen in two rifles and was able to correct them using the proper action wrench..

[rickt300]

I first check the barrel/bolt headspace (headspace gauges), gas port size (pin gauges), gas port alignment (pin gauge), proper chamber cut (function gauge), bore, chamber and crown quality (bore scope)..

I check the upper's carrier bore size (pin gauges), the barrel extension/upper fit and the upper's feed ramp interface with the barrel extension feed ramps.
I check the barrel interface fit with the upper and lap if needed.

Some people will use green or blue loctite with or without a shim for a tight fit in the upper..Some swear by it and others don't..In any case, using a loctite or shims does no harm and can be easily removed if needed..

Some people prefer thermal fit uppers and others don't..If using a thermal fit upper, then it is necessary to turn down the O.D. of the lapping tool to work..

Most newer upper receiver carrier bores I have gauged are tighter and under sized than the TDP spec requires..A tighter carrier bore will keep the carrier/bolt face straighter to the bore improving accuracy and bolt life..I have also seen newer upper receivers carrier bores that were grossly over sized and out of spec..I returned them immediately.

It is advised to use some kind of lubricant on the barrel nut threads of the upper receiver, between the the barrel extension lip and barrel nut to acquire proper torque..Mil spec requires Aeroshell 64 but any lithium/moly grease should work..I use the Colt factory Molykote G-n Metal assembly paste..

It is important to season the barrel nut/upper receiver threads by tightening/loosening the barrel nut 3 times before final torque..Colt discovered this prevents the barrel nut from loosening..

Mil spec barrel nut torque is 35 to 80 ft pounds...This large variance is to time the gas tube to the upper using the mil spec barrel nut..Always use the factory barrel nut torque values being used..

My test have show the upper receiver will compress around the barrel extension with increased barrel nut torque up to around 55 to 60 ft lbs before the compression stops..The advantages of a thermal fit upper over a standard upper has never been proven from what I can find or have experienced..

I prefer using an action rod with a sail to keep the barrel from clocking in the upper..I have personally seen this happen in two rifles and was able to correct them using the proper action wrench..

I would have preferred an action rod with a "sail" but the Brownells's tool was only $34. and I don't see myself doing a lot of barrels. The only reason I consider using loctite is to keep the barrel from uncentering itself when torqued. The anti-seize I plan on using is called lubriplate.The upper I plan on using is from an Aero precision build someone else put together as a 6 Grendel. I have two uppers with identical barrels, both very accurate. The barrel profiles are a bit heavier than I want for a lugging around coyote calling and maybe deer rifle. I don't have headspace gauges but I have some shim stock of .002, .004 and .006 . I know a hokey way to check headspace but hoping to feel resistance before I reach the .006 shim using factory ammo. It is likely I will use necked down new brass made from 6.5 Grendel so I have the option of using a step in the neck to perfect first round headspace.