Retention Of AR-15 Barrel Extensions

Very few shooters, including even the most active competitors, will ever have a barrel come loose. That is a good thing! For those of us who have experienced a barrel unscrewing, I can assure you it is an event that will never be forgotten. The first time it happened to me was back in the early 1970s, not long after my return from Viet Nam. I was working in law enforcement at the time. Revolvers were the staple for police duty and I remember paying the outrageous price of $190 for a brand new Colt Python.

I quickly became interested in shooting PPC (Practical Police Combat) competitions and by the time my Python was reaching about the 200,000 round count (that is two tons in bullets alone) it was still shooting well as we used soft lead bullets with light "talcum powder" propellant charges. A rifle shooter would only get a tiny fraction of that kind of mileage. One day while training, I noticed my groups shift to the left. I blamed it on grip strength and tried to compensate.

As the week wore on, however, I found my zero continuing to shift further and further to the left. Eventually, toward the end of the week, I was running out of windage correction on my rear sight. Finally, while gripping the unloaded firearm normally in one hand and twisting the barrel with the other, I discovered that I could easily screw it in either direction. I unscrewed the barrel completely and degreased everything.

The following day, after everything had sufficient drying time, I doused the back threads of the barrel liberally with red Loctite and screwed everything back together as stoutly as I could by hand. After allowing a couple of days for set up, I rezeroed the gun. The rear sight ended up back near the middle of its travel, so I felt my repair job was successful—although you real pistol smiths may tell me if I goofed it up. Anyway, I went on to carry that gun every day and staked my life on it.

I killed a six point whitetail buck with it and earned my first of three Distinguished metals with it. Now, about 40 years later, it still holds zero and shoots well but, unfortunately, not often enough due to my AR-15 gunsmithing workload. Perhaps I have a dark cloud hanging over my head because this wasn’t my last time to get unlucky. The next time was with an AR-15.

At the time, a number of top AR gunsmiths in the country were advocating screwing on flash suppressors only hand tight and using lots of red Loctite. Most of us who work on the various Stoner plat-

forms still use some variation on this theme. I have found, however, that just a little mechanical torque helps me as I have had a number of flash suppressors which were held on only with the red Loctite come loose. The theory is that too much torque, like most factories apply, will crush the muzzle. Somehow that theory, which does make sense, also got applied to the other end. At least one notable barrel maker was using hand tightened barrel extensions and red Loctite.

That worked for awhile but eventually heat (remember, we are right at the chamber here) and age conspired to weaken many of these assemblies and mine was one of them. I cured it in a manner very similar to the Python and the joint held to the end of the barrel’s life. I know it would make for a much more interesting story if I said I earned my second Distinguished medal with it. Honestly, however, I earned that one with an issued M14 years prior.

And no, I didn’t kill any six pointers with it or carry it into battle either. In an interesting post script, I have learned that this barrel manufacturer in question abandoned the practice of just hand tightening their extensions and now torque them on at 150 footpounds. As the Armorer for the US Army Reserve Marksmanship Program (ArmyReserveMarksman.info) for a little over a decade, I would occasionally run into these loose extensions at arate of about one or so per year.

Because of my hard learned personal lesson with a loose extension, I was able to quickly diagnose the problems and get the rifles repaired. As William Bell once said, "In any search for knowledge there are always unintended consequences." Suddenly, a couple of years before I left the Team to pursue my civilian gun business I noted a significant up-tick in loose AR-15 barrel extensions. All of the barrels in question had been salt bath nitrided, usually under the trade names Melonite or Isonite.

When I removed the extensions to repair the barrels I noted that the male threads on the all of them were perfectly coated with the chemical and looked like there had been no extension covering them at all during the nitride processing. I spoke with Joel Kendrick who is associated with MMi Tru Tec, one of the companies who performs the nitriding process. He indicated to me that they use temperatures closely approaching 1,000 degrees during the processing of barrels.

We both agreed that such a temperature would easily melt out even the strongest Loctite and jeopardize retention. It was also apparent to me that as the Loctite was melted out it was replaced with the Isonite compound which, unfortunately, had no bonding characteristics. To make matters worse, as I was repairing these barrels whose extensions had come loose I inquired with their makers as to the original torque they used.

I found that when I duplicated their original torque I invariably "over timed", meaning the extensions went too far and actually passed top dead center. So, while there are tangible benefits to having barrels salt bath nitrided (which I will cover in detail in a future article) as William Bell said, there can also be unintended consequences. Frank White at Compass Lake Engineering turns and chambers a lot of my barrel blanks for me. I was discussing the problems I was having with him not too long ago.

I mentioned Derrick Martin of Accuracy Speaks a few months ago. Derrick is one of those folks that I tend to listen to regarding AR-15 gun plumbing. Frank White is another such person who has earned my professional respect. Here is what he has to say about what the high temperatures are doing to barrel extensions and why they "overtime" if reinstalled at the original torque. "If the nitriding process heats the barrels to 1000 degrees The barrels

and extensions are stress relieved and they lose their torque. Re-torquing will cause the extension to rotate past correct alignment. The draw temperature of the extension is exceeded by several hundred degrees which results in decreased hardness and strength of both the core and case of the barrel extension. I recommend removing the extension before nitriding. Punch mark the barrel just in front of the extension pin and when reinstalling the extension align the pin with the punch mark.

When the extension is reinstalled, the nitrided threads of the barrel will have a different coefficient of friction so 150 foot pounds may not get the extension to the same spot." Frank and I agreed on a strategy for dealing with the extension retention dilemma. We agreed that Compass Lake would continue to install extensions as done previously. Like some others in the AR-15 barrel business, they use the increasingly popular 150 foot pound torque setting when timing their extensions.

Frank agreed to put a distinctive punch mark at top dead center right in front of where the extension pin is located. When barrels are returned to me from Compass Lake, I simply spin off the extensions from those that are being sent out for the nitriding process and label them so they go back on their respective barrels. By not allowing the extensions to be subjected to those near 1,000 degree temperatures, their characteristics are not changed.

When the barrels are returned about a month later I thoroughly degrease both the extension and the back of the barrel (the male threads.) Then I do a trial fitting using 140 foot-pounds of torque and visually inspect to see if the extension pins line up with the punch marks at top dead center. Note, I am starting with 10 foot-pounds less than the original 150 because of the lowered coefficient of friction on the male threads of the barrel that Frank White cautioned about.

Finally, I triple check my work using what I calla "bastard application" of a Gas Port Drilling Jig available from Brownells (#100-006- 626EJ). As originally designed, the tool consists of two flat aluminum jigs. The first fits over the barrel extension indexing on the pin and

the flat surface sits level on a work bench or other flat surface. Then, the second jig slides back on the front of the barrel until it abuts against the shoulder of the barrel near where the gas port is to be drilled. When both the front and rear jigs are sitting flat on the work surface the front one is locked in place with a set screw. There is a bushing on the front jig that aligns the drill to where the port needs to be drilled at top dead center.

It is a handy device in its own right to do the job it was designed to perform. Please note that one weakness of this fixture, however, is that it fails to take into consideration the location of the lands of the barrel so there is a high probability of cutting a land when drilling the gas port. We will cover this topic in greater detail in a future article on gas ports. I simply reverse the order.

I put the front jig on first and carefully drop the shank of the appropriate size drill (commonly a 0.093" but 0.100" and 0.098" are also used) down thru the bushing and into the gas port. To avoid damage to either the gas port or the bore I suggest you do not insert the drilling side of the bit. It doesn’t hurt to carefully slide a cleaning rod into the bore first to keep the drill from contacting the bottom of the bore.

Then, tighten the set screw on the front jig and ensure that it is sitting level on a flat work surface. Slide the rear jig over the barrel extension. If it fails to sit flat on the work surface it will be immediately evident whether you need to increase or decrease torque. In the event that I have to lighten up the torque much below the original 150 foot-pounds, I unscrew the extension and apply red Loctite to the threads and then torque everything back up. Too easy!

Joel Kendrick from MMi Trutec and the folks at Northern Competition came up with an alternate method of dealing with retention. Their fix is also very straightforward. They recommend timing up the barrel extension to top dead center as normal. Before tapping in the pin, however, drill down into the top of the threads 1/8". Use a pin that is 1/8" longer (1/8" diameter x 5/16" long) than normal and gently tap it into place. Obviously, an extension thus pinned to the barrel can’t unscrew.

Northern Competition sells a lot of Badger AR-15 barrels and so far they report 100% success with this method. A warning. If you pound that pin too hard as it bottoms out in the hole in the barrel, the bottom of the pin will mushroom out a little making it almost impossible to get back out. I am using BAT Machine match barrel extensions exclusively in my custom guns. While they are very reasonably priced given their significant contribution to accuracy, they area little more expensive than the garden variety.

Barrel extensions way out last barrels so I want to recycle my BAT’s onto anew barrel when the first barrel shoots out. If you pound too hard on that pin and mushroom the bottom of it, you will likely need to drill it out when you want to remove it. I recommend that you tap it in and check to see if that a barrel nut will clear it. Done as I have described, you should be able to pull such a pin back out and save some time later when you want to recycle the extension.

Northern Competition also cautions that hammering too hard on the pin will flare the top if it as well, often making it impossible to fit into the corresponding slot in the front of the receiver. Should that happen, dress the sides of the pin back to spec with a file until everything fits as normal. Now, some comments on what won’t work.

See Jethro Gibbs, Rule #51: "Sometimes you’re wrong" and Rule #8: "Never Assume." When I first noticed this tendency for nitrided barrels to unscrew from their extensions, as many of you are probably thinking, I figured I could just set them vertically with the muzzle down and apply some American Gunsmithing Association

wicking-grade Loctite to the joint where the back of the barrel comes through the extension. I assumed that the Loctite would do its job and migrate down into the threads and form enough of a bond to cure my retention problems. Gibbs would have been able to say, "I told you so," twice. My assumption was wrong! The wicking Loctite just pooled up and wouldn’t soak down into the threads at all.

Remember me stating earlier how the salt bath solution had so effectively seeped into the space between the male threads of the barrel and the female threads of the extension? In doing so it apparently filled up all the gaps and voids leaving no room for Loctite to wick or migrate into. It would have been a whole lot easier fix had it worked, but it didn’t. I have found that 100% of extensions put on hand tight with Loctite will come loose if subjected to the nitriding process.

About 50% of extensions put on at 75 foot-pounds will come loose. Even 150 footpounds of torque is not foolproof. So that leaves you with the two very viable alternatives described above. Both take time, but not that much. Spin it off and try again with 10 foot-pounds less torque. Continue backing off torque until the proper setting is established. Apply red Loctite to the threads if the final torque is much below 140 foot-pounds.

Chances are that only a fraction of your customers will want their barrels nitrided anyway due to the significant added cost. Securing the extensions right the first time will save you time in the long run, however, as opposed to inconvenience to the customer and you doing a repair job later on when the barrel unscrews. AG