AR-15 Barrel Extension Fitting

An updated look at improvements and new products for barrel extension fitting precision AR-15 rifles.

by Joe Carlos

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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 mass-produced 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 front-to-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 above: Outside contour, gas port, chamber, and front end attachments – all of these variables impact rifle precision and I have given tips on how to deal with them since I started writing for this magazine way back in the early 2013. 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.

Read more in the March 2023 issue.

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