AR-15 Barrel Extension Fitting

Most custom gun barrels likely have similar beginnings at the factory, starting out as blanks. Think of a big, fat, long piece of pipe with rifling inside. If you are dealing with a reputable custom barrel maker they’ll likely put equal effort and high-quality steel in all the blanks they turn out. Those blanks are purchased by gunsmiths, machinists, and gun factories. Arriving at a shop or factory, they are then cut to length, turned down some, chambered, and threaded at the back.

Barrels screwed into the front of a steel bolt-action receiver have substantial side walls to support it. Barrels used in Stoner Platform builds, on the other hand, are also turned down (perhaps skinnier than for a bolt gun), chambered (sometimes a little on the fat side), drilled for a gas port (which hopefully lands in the center of a groove), and threaded at the back. Most are likely also threaded at the front for attachment of various devices (see my July 2022 article "Muzzle Attachments" on that).

The similarity in barrels ends at this point. AR receivers are not threaded on the inside; they are made of aluminum and are usually really skinny, lacking much support for the back of the barrel. The threaded back part of an AR barrel screws into a separate massproduced part called a barrel extension.

Then the barrel, with extension attached, simply slips into the front of the AR receiver and is kept in place by a barrel nut which, at best, keeps most of the frontto-rear movement to a minimum while being less effective with up and down and left to right movement of the extension within the sloppy confines of the receiver. Regardless of the fact that the blanks that left the custom barrel shop were basically equal, the bolt action normally has more precision than the AR rifle.

We quickly skimmed past some of the barrel related reasons for this The focus of this article, however, is on updating you on ways to deal with the slop in the fit of the barrel extension to the upper receiver. Combating this slop is nothing new to me. I have been working on the problem for decades. I wrote about how I first discovered this problem (March 2013, "The Relationship Of Barrel Extension Diameter To Accuracy In The AR-15") and I’ll update what I’ve learned about this since.

The Epiphany One observation I made in the late 1980s was when the Army Reserve Marksmanship Program was at Belmont Range competing against foreign military teams in an international Service Conditions match hosted by the Australian Army Rifle Association. I remember coming back to the tent late one afternoon and everybody was cleaning their M16 rifles. One guy must have left his bore guide stateside because the back of his barrel was all slopped up with excess solvent and oil.

He discovered that if he firmly held the upper receiver with one hand and put pressures on his barrel with the other hand he could see air bubbles and movement in the excess fluid that corresponded to the flexing pressures that he applied to the front of his barrel. Like little kids, everyone else had to try it too and a fair number saw the same tell tales that their barrel extensions also exhibited a lot of slop.

We all came to the same conclusion that any movement at the back of the barrel would cause erratic and exaggerated vibrations and a less-than-stellar return to consistent registration points at the muzzle between shots. This, in turn, would result in fat groups, fliers, and wandering zeroes. What to do about the problem remained uncertain, however. Early Improvements Most of the shooters who observed that epiphany likely gave it little or no further thought once we all returned home.

However, I couldn’t let it go and spent many sleepless nights thinking about how to correct the problem. After enough Australia-induced insomnia, I decided to try using Loctite in an attempt to "weld" the outside of the barrel extension to the inside surface of the bore of the receiver. The adhesive company made a green #620 retaining compound which they said was designed for slip fits where there were "large gaps". I don’t recall them defining what constitutes a "large gap" but the #620 was easy to use.

Thoroughly degrease both the extension and the receiver, then slop some of the

retaining compound on both the extension and the inside of the receiver. Assemble as normal and wipe off any excess Loctite. Spin on a barrel nut and tighten to draw the barrel tightly into the receiver, then let everything sit for at least 24 hours before firing. When this has been performed properly there is zero visible movement between the barrel and the receiver even with the barrel nut removed.

The downside is you can not pull the barrel out of the receiver and when it comes time to replace shot out barrels they have to be heated with a torch in order to remove them. The scientist in me demanded that I perform some before and after tests of the process by shooting ten-round groups on the machine rest. I took Army Reserve Marksmanship Program uppers assembled the "normal" way without Loctite and fired groups with known lots of match ammo.

Then I performed the improvement discussed above, reassembling after the Loctite cured a few days with the original parts for a retest. I could see early on that groups had shrunk and I kept records. Eventually, I upgraded all 135 of the Team’s uppers, testing all of them both before and after. I had a very nice sample size and I am sure that professional statisticians would have patted me on the back. This whole process took a few years and was very work and time intensive.

When I finally analyzed my data the numbers showed an average improvement in group size of 18.6% just from using the Loctite. That is a substantial difference and many shooters told me they could see an improvement in their gun’s precision. Civilian shooters were learning about what we were doing and many of them jumped on the bandwagon.

Improving The Improvement Even before I was done with the green Loctite improvement I was beginning to wonder if there might be some "give" in the Loctite that I was unable to see with the naked eye. After all, Loctite is basically a super strong glue and not actual metal which would have less potential to compress and expand. Each of the gaps between the barrel extensions and the inside of the receiver is different between rifles due to large manufacturing differences in the diameters of extensions.

What can you expect from apart that cost about $20 at the time? There were equal variances in the inside diameters of upper receivers, creating a stack up of variances. When we used the green Loctite by itself we didn’t take these variances into account as it was just a "slap it on and assemble" procedure. I wondered if there was enough gap in most rifles to accommodate thin shim stock to be inserted along with the Loctite. I started to experiment with 0.001" shim stock which was the thinnest I could find.

Sure enough, every rifle I experimented with would accept a section of the thin shim material. I would begin by cutting the shim about 3/4" wide so there would be some clearance on both sides of it when positioned on the bottom of an extension. Then the length of each finished shim would vary quite a bit between guns depending on how much gap each rifle had between the extension and the receiver. Some shims might end up only 1/2" long while others could exceed two inches.

Regular household scissors easily cuts the shims to size. Determining this shim width and length was accomplished by trial and error and it took some time to get exactly right. If the shim was cut too long it was impossible to get the barrel extension into the receiver. A shim too short wouldn’t fill enough of the gap, still allowing movement of the extension inside the receiver. Final assembly was a little tricky. After the parts were degreased, green Loctite was applied inside the receiver and on the extension.

Then the shim was placed around the bottom of the extension and more Loctite was spread on the outside of the shim. Things got hairy at this point because the next step of sliding the extension into the receiver felt like a person needed a third hand. The Loctite was slimy and the shims wanted to migrate toward the raised portion of the extension that would eventually come into contact with the front of the receiver, sometimes called the mouth.

If that migration stopped when the shim made solid contact with the raised section there would be no problem. Often, however, as the barrel continued further into the receiver the shim would continue to move and would turn up and become trapped between the raised part of the extension and the receiver mouth. This would obviously create a high spot and prevent the extension from fitting perfectly flush. Group sizes would be compromised.

Any time I saw this taking place I would simply pull everything apart and begin over. For me, this learning curve was taking place during the time I was the Match Armorer for the Army Reserve Team so I was getting daily or weekly practice and I learned some lessons. One such lesson was to position the shim closer to the feed ramp side of the extension at the start of the assembly. This allowed for some migration to take place before the shim would hit the raised section of the extension.

Because I was doing many of these I got pretty good. As with using just the Loctite I kept good before and after shooting records and when I analyzed my numbers I found that average group sizes shrunk by a significant 35%, nearly double the improvement of just using Loctite by itself. See the test targets pictured here. Almost all of

001" shim stock. When centered on the bottom of the barrel extension this gives a little clearance on either side. Depending on the rifle, this shim could be a half inch or over two inches in length. my shooters were remarking on the noticeable improvement in precision.

Although I made a strong admonishment to my readers back in that 2013 article regarding the tendency of the shims to migrate, I worried that guys who only had to deal with one or two AR builds in a year might be challenged or perhaps forget the importance of this from one build to the next. Next in the history of this neverending saga came custom oversized barrel extensions from BAT Machine.

I purchased these in quantity in sizes from 0.9990" to 1.003" in 0.0005" (half thousandth) incre- Author offers an improved method of securing the shim to avoiding movement. ments. I would take samples of each size with me to Camp Perry where I operated a store on Commercial Row each summer during the CMP Service Rifle National Matches.

So, if a customer brought an upper receiver into the store I could select the proper size extension for his specific application in a matter of about three minutes right in front of him. I stock gain twist match-grade barrels with different size extensions. Within about a half hour of walking in the door of my store the customer could be walking back out with a quality barrel sporting the correct size extension.

BAT was selling their custom extensions for about a measly $10 or so over the cost of the generic, garden variety extensions and properly-sized BAT extensions gave the same average 35% group shrinkage as the shim stock. Also, the BAT extensions were more user friendly for customers wanting to install their own barrels. However, BAT Machine wasn’t just manufacturing custom AR-15 and AR-10 extensions. Their main bread and butter is high quality custom bolt actions. These are high dollar items.

If you were BAT, would you be taking a man off the line and divert him to making a few custom extensions on which you are making chump change? Eventually, and unfortunately, BAT figured this out and they dropped their custom extension line. To my knowledge, no other machine shop has picked up the slack. So, for the past several years custom AR gunsmiths have had to go back to using shim stock. During these years a fair number of guns came into my shop for rebarreling.

Some of these shot out guns had shims installed by home hobbyist gunsmiths and many of those shims had migrated up between the front mouth of the receiver and the raised portion of the extension, creating high spots and some compromise of group sizes. Therefore, I was highly motivated to come up with a simple solution to this problem.

Securing The Shim The lack of readily-available oversize barrel extensions motivated me to revisit using shim stock. Here is how I do it now. Begin by dry fitting your shim as normal. Thoroughly degrease both the shim and the extension. Remember that when you permanently install it the Loctite between the shim and the extension will end up adding a little thickness. With that thickness buildup in mind, I now use Loctite #638 instead of #620.

Both are green, but #638 seems to be a bit thinner and adds less build up when combined with the shim. Coat the area of the extension where the shim will be placed with the #638 green Loctite. Also coat the underside of Temporarily hold the shim in place with one hand while wrapping a rubber band around the outside of the extension nice and tight. While doing so, make sure you cover all four edges and all four corners of the shim. If need be, don’t hesitate to use a second rubber band.

Wipe off any excess Loctite that you can get to and let everything set up for at least 24 hours. The next day, remove the rubber bands and scrape, sand, or steel wool any hardened excess Loctite from the extension. Following this, assembly of the barrel to the receiver remains pretty much unchanged. Coat the outside of the extension and shim with green Loctite. Also coat the inside of the receiver and then push together.

Based on all the problems in past history with shims moving during this step I can’t help but watch the shim to make sure it doesn’t move any. After using this technique for several years I have never had a single shim break loose or move during this step. I quickly wipe off any excess Loctite and then run on a standard barrel Place the shim with green Loctite #638 applied on the barrel extension where you want it to be permanently fixed. Hold it in place while wrapping one or more rubber bands tightly around.

Allow 24 hours to set up and remove rubber bands. Targets on the left were from the guns before any improvements were done and targets on the right were from guns improved with shims and green Loctite. the shim itself and then place the two together. nut which I torque on gorilla tight.

I give everything at least 24 hours to set up and then remove the temporary barrel nut and replace with whatever nut I am going to permanently use, torqued to a starting setting of 47 pounds for heavy barrels or 60 pounds for skinny barrels. By varying the torque setting up and down from those starting settings and shooting groups you can identify the torque sweet spot that results in the best performance. For more information on this process, see my article "AR-15 Barrel Harmonics" in the October 2016 issue.

Double-Sided Coin For decades, we in the AR community have focused our attention on just one side of the repair coin regarding fitting the barrel to the receiver, concentrating our efforts on making the barrel extension fatter. While this has worked well, wouldn’t making the bore of the receiver skinnier at the mouth work equally well? The problem has been that, to my knowledge, no receiver brand has offered a bore tight enough to be a viable cure.

Thankfully, a couple of years ago the industry finally caught up to our needs. I’m aware of two companies now offering true tight bore flat top uppers. If you want a mil spec upper, contact Bravo Company USA (BravoCompanyUSA.com, I am using their flat tops in all the uppers I am building for persons wanting to shoot Service Rifle matches overseen by either the NRA or CMP.

To install a barrel on a Bravo receiver, start by using a torch and focus the tip of the flame at the junction of the front threads and the receiver. While doing so, rotate the receiver to heat uniformly around it. The heat will serve to expand the mouth of the bore enough for the extension to enter. After roughly 30 seconds of heating, see if the extension will enter the mouth. It may go part way and then stop or it may not

enter at all. If so, apply more heat and try again. I often hold the barrel in one hand while tapping the back of the receiver with a wooden mallet to drive it into place. Just make sure that the front of the receiver makes hard contact with the raised portion of the extension with no gaps. As soon as the heat is removed the metal will attempt to "spring back" to its original, tight dimension.

So, once I have made that contact between the receiver front and the raised section of the extension Author recommends that you NOT use one of the "Receiver Lapping Tools" pictured on the bottom as they can actually add run out as opposed to correcting it. I quickly run a standard barrel nut into place and then tighten it gorilla tight using a cheater bar over my wrench. I usually allow about an hour for the parts to completely cool before removing the temporary barrel nut and then proceed with normal assembly.

Months or years down the road when that barrel shoots out and you are removing it to install a replacement you will have to heat the receiver back up to get it to release the extension. And when you are ready to install that new replacement barrel you will have to apply heat again in order to get the receiver to expand enough to accept it. The process I have just described is often called "thermo fitting".

While it is the flavor of the week it appears to me that it is equal but not better with regard to precision as compared to fat extensions or extensions with shims and Loctite. The other company offering tight bore flat top receivers is JP Enterprises (JPrifles.com, although I don’t believe their receiver would pass NRA or CMP muster for shooting Service Rifle as there are some cosmetic differences between it and regular mil spec.

Therefore, because I concentrate on Service Rifles I have not had occasion to try one. That said, I have used a wide cross section of other JP parts over the years and found them to be well designed and of quality manufacture. I am hoping that shooter demand will cause additional receiver makers to offer tight bore options in the future.

Receiver Run Out Even those of us who don’t work on or shoot bolt actions are probably aware that gunsmiths who custom build these rifles for use in precision events pay attention to the machining of the front ends where the barrel comes to rest. Top-end custom receivers like those made by BAT Machine come from their shop perfectly squared. Cheaper actions often come from the factory with run out having one side being taller than the opposite side. Custom gunsmiths correct this by machining on a lathe.

I thought it might be worthwhile to look at this with regard to our AR platform. I measured the amount of run out in receivers from a variety of AR manufacturers. With one excep-

tion (Sun Devil), none of them were square at the mouth. There was some variance in the severity of run out, however, the average run out was similar to mass produced bolt actions. If custom bolt gun builders were seeing benefits to squaring the front ends might gas guns benefit also? I set up a study. First, I built up temporary guns with receivers just as they came from the factory. Then I dismantled the guns and squared the front mouths on a lathe just the same as bolt gunners.

I reassembled the temporary test guns and using the same ammo lots I machine rest tested them a second time. I did this about 30 times starting with a different receiver selected from a variety of brands each time and then averaged the group reduction that squaring produced. I came up with a significant average group size improvement of 14% after the receivers had been squared. As noted, Sun Devil was one brand of upper receiver whose mouths came properly squared right from the factory.

Their receivers were machined from solid blocks of aluminum as opposed to being cast. The bores on Sun Devils were also tighter than most but not as tight enough. The walls of Sun Devil receivers are also thicker than regular mil spec so they aren’t legal for either CMP or NRA Service Rifle shooting. Their lowers had the first Accu Screws, a device that I have mentioned in previous articles used to tighten the fit of the upper to the lower.

Unfortunately, even though Sun Devil was good, they went out of the receiver business a few years ago. Customers have told me that they are sometimes successful in still finding them on e Bay or Gun Broker. Most home hobbyists don’t have lathes so I recognized how attractive a simpler method would be for them. Chances are that most of you have seen those mandrels that fit in a 3/8" hand drill. They are sometimes called touted as "AR Receiver Lapping Tools". They claim to square receiver mouths.

Like many of you, I have one and I had previously tried it on some receivers that I still had in stock. I dug those receivers out and took readings of their run out. None of them had less run out than new untouched receivers from the same manufacturers! Worse, many of them had even more run out. I took a few new receivers and measured their run out. Then, using the instructions provided by the maker of the lapping tool, I attempted to square the mouths of these receivers.

Not one of them ended up with less run out and most of them had more run out using the mandrel, meaning this tool not only failed to properly lap the receiver, it made the problem worse. Those of you scientists in the audience are probably positing that perhaps Joe’s mandrel was just a one of a kind that was manufactured with some error at the factory. Or maybe Joe just wasn’t operating it correctly. These are fair reservations and I was asking myself the same questions.

I rebarrel a fair number of uppers each year. When the existing shot out barrel is removed it is fairly easy to visually identify receivers that the owners have tried the lapping tool on. So, I started gauging such receivers and found that none were square and that most had more run out than factory new receivers of the same manufacturer. This is exactly the same as I found when I using my receiver lapping tool.

My conclusion is that this mandrel method fails to remove run out and often makes matters worse for a significant number of home operators. I don’t know of any custom bolt gun craftsmen using a mandrel to square their receivers; they use a lathe. If you don’t have a lathe perhaps you know a machinist that will help you out. D. Wilson Manufacturing (DWilsonMfg.com is one custom gunshop that offers lathe squaring. I have had them lathe-square receivers for me and their work is good.

They also offer mandrel squaring so when contacting them you will need to strongly clarify that you want only the lathe method. If you happen to have a receiver that has been subjected to a mandrel most are salvageable by doing it over the right way in a lathe. Proper squaring is a once-and-done procedure and does not need to be repeated each time you rebarrel. I believe it is a good investment.

Conclusions To my knowledge, Army Reserve Team shooters made the first discovery regarding the looseness of fit in the front ends of Stoner Platform rifles in the late 1980s, somewhere roughly in the midpoint of the life of that rifle. I would like to say that the epiphany was made using sound scientific testing methods. In reality, however, it was just plain dumb luck. Correcting the problem in subsequent years involved correct science.

It’s possible that someone elsewhere had noticed this prior but since this was before the Internet was available to most of us so such information would have likely not have traveled. In that regard, the Internet has been a good thing. These days, unfortunately, I see more misinformation on the Internet regarding the Stoner Platform.

For example, I watched a video on front end receiver squaring where the presenter was holding a flat top upper receiver in his bare hand (no glove) while he drove one of those mandrels attached to a hand drill down the bore with his other hand. Is it any wonder that we gunsmiths are encountering receivers in our shops with more runout than when they originally left the factory? Before leaving the topic of on-line videos and chat I will leave you with a caution.

It seems that most anybody sitting in an air conditioned man cave with their feet propped up can get a video published. The Scientific Method of testing is seldom adhered to and it is obvious that even actual range testing is often abandoned in

favor of just speculation and theory. I personally stay off those sites and I recommend that you do likewise and rely more on reputable publications like American Gunsmith for information. From time to time I get decadesold original manufacture 1960s uppers sent to me. When I dissemble them I nearly always find the fitment of the extension to the receiver to be noticeably tighter than recently manufactured uppers. So, in the ensuing decades fit has gotten more slop, not less.

Back in the ‘60s about the only factory making ARs was Colt. Apparently, their factory specs for the associated parts resulted in tighter fits. Today it seems that almost everybody and their brother is turning out AR parts. So, when you combine them, the fit can be very sloppy. Therefore, the general trend has been toward more slop which has been going in the dead wrong direction.

To my thinking, this calls for more attention to the problem for us gunsmiths if we are building a rifle for a customer wanting a high degree of precision. I believe this article has provided the tools to accomplish this work easily and relatively quickly. By the way, when I machine rest test those old 1960s uppers using 55 grain ball ammo I also get much better groups than with most recently manufactured uppers.

Part of that is likely due to the tighter fit at the barrel extension and also probably the correct 1:12" twist barrels that adequately stabilize the short 55 grain bullets. There has been an industry-wide trend in recent years toward faster twist barrels needed to stabilize longer and heavier bullets. Some of those barrels, such as the 1:7" twists, spin 55 grain bullets at double the RPMs required to properly stabilize them. Overspinning bullets results in fat groups and fliers.

I have admonished readers many times regarding the importance of using the correct twist rate so I will (temporarily) climb down from my soap box now. Using the techniques described in this article you can make a $300 or $400 properly-bedded barrel shoot as precisely as a $600 barrel just slapped into the receiver the old way. If you install that $600 barrel correctly the resultant precision can be downright stellar! Barrel selection is very important when trying to build the highest quality rifles.

To that end I have been working with gain twist barrels for decades—about half of my life, and I ain’t young. The finest of those gain twists have doubled their twist rates from the front of the chamber to the muzzle. For example, the 20" Service Rifle barrels I stock begin their twist at 1:13" at the back and end at 1:6.5" at the muzzle. I consider this gain twist topic to be of such importance that I wrote about it in my second article for this magazine.

See "Revisiting Gain Twist" in the November and December 2013 issues. For more information on barrel selection see "Barrels For Custom AR-15s" in the July and August 2017 issues as well. Please remember that the precision of any barrel can be ruined in seconds if you use the wrong attachment to the front threads. Certain screw on recoil reducers and combination flash suppressors/ quick mounts for silencers can ruin groups in a heartbeat. See "Muzzle Attachments" in the July 2022 issue for more on that.

We began this article by mentioning that barrels installed on bolt action rifles normally shoot tighter than those on gas guns. That precision gap can be closed by a bunch in AR-15 rifles as well. I think of the main methods of doing so as the three legs of a tripod. The first leg of that tripod is the barrel extension to receiver fit that was the topic of this and earlier articles. The second leg is getting the upper-to-lower fit nice and tight. I wrote about this in April 2017 ("The Accu-Wedge").

The final leg involves the bolt carrier and its tendency to tilt down at the back setting the bolt face, putting it and a cartridge attached to the bolt face at an angle in the chamber. This results in bullets entering the rifling of the barrel off center and deforming. This issue has been known since Stoner’s time and still hasn’t been 100% resolved to my mind. I wrote about some cures in "Match Bolt Carriers" (May 2014) and "Combating AR-15 Bolt Carrier Tilt" (May 2017).

Getting the bolt carrier to line up properly with the bore is another of those two-sided coins. The other side is split between gas tube alignment covered first in March 2014 ("Gas Tube Alignment") and updated in May 2018 ("Gas Tube Alignment Revisited"). Cartridge concentricity is also involved, which I covered in "Straight Bullets" (December 2018) and "Better Ammunition" (April 2016).

It used to be that when gas gunners showed up at precision and long-range events the owners of expensive bolt rifles sort of turned their heads and snickered. Using the techniques described in the articles I have just cited and others, those snickers seldom occur these days because those bolt gunners have had their you-know-what handed to them often enough.

The growing popularity of 90 grain bullets which allows the wimpy Mouse Gun to compete with more ballistically-efficient calibers found in bolt rifles has also helped. Mouse Guns will shoot the hotter 22 Nosler which drives bullets faster than the 223 Remington can also help, as detailed in "22 Nosler" (March 2019). The one variable that we gunsmiths have no control over is the proper application of marksmanship fundamentals by the gun owner. This involves lots of hours on hot, dusty ranges honing skills.

I hope that if you area gun owner reading these pages you will keep that in mind. As always, any comments and questions can be addressed to me at G