Building (machining) an AR-15/M16 M4 barrel from a blank

Profiling, barrel extension installation, chambering, drilling the gas port, threading and crowning an AR15 barrel

May 13, 2014 Bill Marr Gunsmithing, Tutorial 0

When I left the Marines in the mid-90s I got a job working the gun counter at a small sporting goods store.  We only stocked one AR-15 rifle at a time.  Back then, it would be hard to imagine how common this rifle would become.  According to the National Shooting Sports Foundation (2013), AR-15 rifles account for nearly 20% of all U.S. firearms sales.  The AR-15 is our generation’s M1 Garand or 1903 Springfield.

I’ve always wanted to machine an AR barrel from a blank.  While it seems complicated to some, the techniques used to chamber an AR-15/M16 M4 barrel are very similar to those used to chamber a barrel for a bolt action rifle.

This project is going to start with a Shilen match barrel blank.  The blank is straight, with no contour and has a rifled bore machined through the center.  The blank is 29.5″ long and 1.200″ in diameter- it feels more like an axle then a barrel when you pick it up.

I’ll be building a rifle with a 16.5″ length, and a Special Purpose Rifle (SPR) type profile.  If you spend a little bit of time on the net, you can find some excellent drawings of barrel dimensions.   The best drawing I could find for the critical dimensions, such as gas port location, is found here.  You’ll notice the profile is different from the one I am turning, however, the other critical dimensions are the same.

So, why fabricate your own AR-15 barrel? Well, I did it for the challenge; but, if you wanted to chamber your AR for a wildcat cartridge, or faced some sort of parts shortage, your own barrel is just a few hours away on the lathe.

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I ordered the following items from Brownells:

Here is our unturned Shilen stainless steel barrel blank.
Here is our unturned Shilen stainless steel barrel blank.
I start by rough cutting the blank to length. I cut 1" off the chamber end and then cut a 17.5" section out of the blank.
I start by rough cutting the blank to length. I cut 1″ off the chamber end and then cut a 17.5″ section out of the blank.

The reason I cut off both ends of the blank is because Shilen (and most barrel makers) recommends it.  As tooling enters and exits the barrel blank, the ends can open up.  Cutting off the ends, removes these out-of-specification sections.

I face each end of the blank and then dial in the bore with a range rod.
I face each end of the blank and then dial in the bore with a range rod.
Using a piloted 60-degree center drill, I cut a counter bore to mate against the centers on the lathe.
Using a piloted 60-degree center drill, I cut a counter bore to mate against the centers on the lathe.
Here is the counterbore.
Here is the counterbore.
The barrel blank is then mounted between centers. The barrel will be driven with a drive dog I made from scrap steel.
The barrel blank is then mounted between centers. The barrel will be driven with a drive dog I made from scrap steel.
This the drive dog I am using to turn the barrel between centers.
This the drive dog I am using to turn the barrel between centers.
Note the tail stock live center securely holding the barrel in place.
Note the tail stock’s live center securely holding the barrel in place.
Since the barrel has a relatively small diameter compared to its length, I support it mid way with a steady rest and begin to rough out the blank.
Since the barrel has a relatively small diameter compared to its length, I support it mid way with a steady rest and begin to rough out the blank.
At its thickest, the barrel will have a diameter of .980" so I turn the blank to this dimension.
At its thickest, the barrel will have a diameter of .980″ so I turn the blank to this dimension.
The Brownells barrel extension I'll be using on this project.
The Brownells barrel extension I’ll be using on this project.
I measure the distance from the front of the barrel extension to the front edge of the bolt and to the bolt face in order to determine the size of the barrel extension tenon.
I measure the distance from the front of the barrel extension to the front edge of the bolt and to the bolt face in order to determine the size of the barrel extension tenon.
I then cut the tenon for the barrel extension. Most prints call for this to be .625" long. I checked this against my barrel extension to ensure it would produce satisfactory results.
I cut the tenon for the barrel extension. Most prints call for this to be .625″ long. I checked this against my barrel extension earlier (see picture above) to ensure it would produce satisfactory results.
With the major diameter cut to .812" (some sources will suggest .813"), I begin to cut the threads at 16 threads per inch.
With the major diameter cut to .812″ (some sources will suggest .813″), I begin to cut the threads at 16 threads per inch.
I check to make sure the tenon fits snug to the shoulder.
I check to make sure the tenon fits snug to the shoulder.
And I make sure the bolt will fit.
And I make sure the bolt will fit.
With the barrel extension cut, I can now use the front edge of it as a datum point to profile the rest of my barrel.
With the barrel extension cut, I can now use the front edge of it as a datum point to profile the rest of my barrel.

I turned the first 2.25″ in front of my barrel extension to a diameter of .980″, then reduced to .840″.  The gas block will sit on a section of barrel .750″ in diameter.  This gas block shoulder is located 8.750″ from the front edge of the barrel extension and is 2.000″ long.  A detailed drawing of a Bergara mid-length barrel can be found here.

While the barrel was still between centers, I used some 220-grit abrasive cloth coated in oil to polish the finish.

Once the barrel is profiled, I dial it in in my four-jaw chuck.
Profiling complete, I dial the muzzle end in, in my four-jaw chuck.
To cut the chamber I am using a fixed pilot Manson 223 Wylde reamer in a Manson floating reamer holder. I take small passed until the reamer stop hits the back edge of the barrel.
To cut the chamber I am using a fixed pilot Manson 223 Wylde reamer in a Manson floating reamer holder. I take small passes until the reamer stop hits the back edge of the barrel.
After my initial cut, I insert a "go" gauge into the chamber, and screw the barrel extension in place with the bolt. I then measure the space between the barrel extension and shoulder. This is the depth I still need to cut for the chamber.
After my initial cut, I insert a “go” gauge into the chamber, and screw the barrel extension in place with the bolt. I then measure the space between the barrel extension and shoulder. This is the depth I still need to cut for the chamber.

With the correct headspace achieved, I cut a small feed ramp on the rear of the barrel.  Unfortunately, I forgot to take a picture of it, however, a picture of this recess can be found in this article on the Brownells website.

With the barrel securely held in a barrel vise, I use a barrel extension tool and a torque wrench to secure it in place. Torque specs are listed all over the place. I've seen them as low a 30 ft-pounds and as high as 150 ft-pounds. I went with 100 ft-pounds. I also used thread locker to help keep the barrel in place. At this point, the barrel extension pin can be pressed or driven into place.
With the barrel securely held in a barrel vise, I use a barrel extension tool and a torque wrench to secure it in place. Torque specs are listed all over the place. I’ve seen them as low a 30 ft-pounds and as high as 150 ft-pounds. I went with 100 ft-pounds. I also used thread locker to help keep the barrel in place. At this point, the barrel extension pin can be pressed or driven into place.
The barrel extension pin is simply pressed into the hole in the barrel extension. This indexes the barrel in the proper position.
The barrel extension pin is simply pressed into the hole in the barrel extension. This indexes the barrel in the proper position.
I cut the barrel to the final length, in this case, 16.5" and dial in the bore. I turn a .500" tenon .625" long.
I cut the barrel to the final length, in this case, 16.5″ and dial in the bore. I turn a .500″ tenon .625″ long.
Threads are cut at 28 threads-per-inch.
Threads are cut at 28 threads-per-inch.
Finally, I use a .420" target crown tool held in a floating reamer holder to crown the end of the barrel.
Finally, I use a .420″ target crown tool held in a floating reamer holder to crown the end of the barrel.
This is the set up I use to drill the gas port. I secure barrel in "V" blocks and slide an upper onto the end of the extension. I place a level on the top of the upper to align the barrel so the gas port location is at 12 0'clock.
This is the set up I use to drill the gas port. I secure the barrel in “V” blocks and slide an upper onto the end of the extension. I place a level on the top of the upper to align the barrel so the gas port location is at 12 0’clock.

The gas port is located .300″ from the rear edge of the gas block cut step.

Finally, I drill my gas port hold. I decided on a .076" gas port since this is a mid length gas system on a 16" gun. Most specs I've seen for this gas system and barrel length run from .075-.085".
Finally, I drill my gas port hold. I decided on a .076″ gas port since this is a mid length gas system on a 16″ gun. Most specs I’ve seen for this gas system and barrel length run from .075-.085″.
Looks pretty good, doesn't it?
Looks pretty good, doesn’t it?
Here is my mid-length barrel next to a section of the unturned blank I started with.
Here is my mid-length barrel next to a section of the unturned blank I started with.  I’ll use the left over to make thread protectors for my other rifles in case I need to remove a brake.

Machining an AR-15 barrel from a blank was a rewarding endeavor.  If you ever wanted to give it a try, go for it, you’ll be happy you did!

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