AR15 Barrel Threading

With the lathe running at its lowest setting, we make a series of cuts, gradually advancing the tool into the barrel until the thread is formed. Note: you can also complete this cut with a die and a thread starter.

We recently decided to add a muzzle device to our factory cut and crowned Colt 16 5/8″ barrel.  As always, the following is presented to document our projects and is for information purposes only.

We are a big fan of heavy, contoured (.875′) 16″ barrels on ARs.  Once common, acquiring one nowadays is a difficult task, especially if you want a chrome lined parkerized barrel.  Fortunately, Brownells offer a stripped Colt barrel and front sight assembly in their catalog.  The only downside is that the muzzle is not threaded to accept a muzzle device.

All lathe work is conducted on a Grizzly gunsmith’s lathe.

We used the following supplies from Brownells:

Here’s how we did it:

The front sight assembly is removed from the barrel assembly, along with the barrel nut. The portion of the barrel behind the muzzle is carefully wrapped with painters tape to prevent marring the finish and the barrel is secured in a three-jaw chuck. It should be noted, that if we we were adding a muzzle device to the end of a precision rifle, we would have “dialed in” the barrel using a four-jaw chuck, spider and readings from the bore since the bore is seldom concentric to the outside diameter of the barrel.

.223/5.56mm rifles like this typically have a 1/2″x28 thread on the muzzle end. Before we turn the muzzle down to the major diameter, we make a witness mark on the barrel to mark the location of the newly formed shoulder.

The end of the barrel is turned using a carbine insert tool until it reaches the major diameter of our thread, in this case .497″, or .003″ less then the .500″ diameter.

With our 60 degree threading cutter installed in the tool post, we use a 60 degree center gauge to ensure the cutter is perpendicular to the surface.

With the lathe running at its slowest speed, we make a series of cuts, gradually advancing the tool along the barrel until the thread is formed. Note: you can also complete this cut with a die, a thread starter and some elbow grease.

With the thread complete, we test fit an A2 compensator (that’s what its called in the USMC FM) on the end of the muzzle.  Perfect fit!