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Brownells Gunsmith Corner - July 2013

 

 

 

AR-15 Barrel Fabrication 101 – Part 2
By Eric Kiesler, Brownells Gun Tech
Feed Ramp & Extension


Fully-chambered barrel before machining the feed ramp.

When we have reamed the chamber to depth, the next step is to machine the feed ramp. This entails machining a 30 degree countersink into the mouth of the chamber. According to Martin’s writings (see the list of sources in Part 1), the outside diameter of the countersink should be .650", but his drawing shows .570". I have measured barrels with countersinks as small as .525". The inside diameter of the lugs in the extension is .570", so to me .570" seems like the dimension to go with, and that’s what I use. The ramp is relatively subtle. You do not remove much material.


Same barrel as above with the feed ramp machined but unpolished.

Once complete, I polish it up with some 320 grit sand paper (#657-250-320). I then polish the chamber feed ramp junction to take it from a sharp corner to a nice small radius. Sorry, I don’t have a precise measurement for this - I do it by eye and by feel. Looking at a factory barrel will give you a good idea of how much to polish.

Now, the extension can be torqued on. Poyer’s book says the torque specification for the extension is 160 ft-lbs. To me, this seems excessive. Mark Hunt tells me he uses 25 ft-lbs. on his extensions. I am inclined to use the old German torque spec <em>“gooduntight”</em>and some Loctite #290 green thread locker (#532-290-006). After confirming that the headspace is correct, I can then install the locator pin (#231-000-082), which is simply pressed or driven into the hole in the extension.

Drilling the Gas Port

We can proceed to locating and drilling the gas port. Brownells sells some gas port drilling jigs (#100-006-627) that work well. The technique I learned to make sure the port is in the right spot uses a flattop upper receiver to help out. Attach the upper receiver to the barrel and tighten the barrel nut with firm hand pressure. Place a set of V blocks (#973-340-463) in the mill vise and then grab the barrel slightly in front of or slightly behind the intended port location. Now you can indicate the top of the flattop receiver and level the assembly.


Here is the set up for leveling the receiver for gas port location and drilling.


After marking the gas port, I remove the upper and let the barrel nut hang out on the barrel while I drill. I locate the port using my edge finder (#100-004-429) to find the middle and center of the barrel, the Y axis. Then I find the front edge of the flange on the barrel extension, the X axis. This is the home reference point from which we will locate the gas port.

Poyer cites a Colt specification in his book, which lists the location for a rifle port as 12.42" from home on center. Martin’s book says 12.46" from home. The DPMS barrel I recently measured was 12.425", so I go with Poyer. The port location for a carbine I measured is 7.051", a mid-length at 9.050", and an AR-15 pistol at 3.960".

Once we have the location, it’s time to drill the port. Some gunsmiths like to drill the port at a 3 degree angle leaning back toward the receiver, and some will just drill it straight in. The size of the port will vary. Poyer says .082" for a rifle, .062" for a carbine, and .092" for an 11" barrel. Martin says .093" for a rifle, but he prefers .099". I measured as follows: rifle .097", mid-length .0785", carbine .0785", and pistol .075". As a side note, the recommended port sizes for the M16 Clinic Pig Tail Gas Tube (#100-000-553) are: carbine .070"-.086", and pistol .067"-.070". Personally, I prefer a port size of .067" for a pistol.

Regardless of the size of the port, drilling it into the barrel will probably leave a burr of metal sticking out into the bore. Some gunsmiths will just remove the burr by shooting the firearm. I am inclined to remove as much of it as possible before shooting. Otherwise, I am haunted by visions of the projectile laminating the burr to my bore! The looped bristles  of a Hoppe’s Tornado brush (#699-001-022) do an excellent job of removing most of the burr, and following with some J-B Bore Compound (#083-065-025) on a patch usually gets the rest out. If you intend to turn a shoulder on the barrel for the gas block to butt up to, it should be .300" center-of-port to edge-of-shoulder.

Gas Block Fit

The other thing to consider is the fit of the gas block. Colt Armorer Instructor Ken Elmore told me that when Colt installs a gas block, they drill a hole in the sight housing, ream the hole, and then hone it. Then it is pressed onto the barrel and finally pinned in place. That sounds ideal to me. Unfortunately, I do not have an industrial-grade Sunnen Hone! So I prefer to press fit the gas block onto the barrel. I use gas blocks with setscrews on the bottom, or with pins , or both screws and pins.

I always drill dimples into the barrel for the setscrews to seat into. When turning a barrel from a blank, it is easy to get a press fit. I just measure the I.D. of my block and turn the barrel to the appropriate diameter.

If you do not have a snug press fit onto the barrel, you will likely have gas leakage. Depending on the fit, you may have to enlarge the port to get the firearm to cycle. Most guys (and some manufacturers) who bolt together ARs do not have a correctly-fitted gas block. Shooting the firearm at night and looking at the gas block will often reveal this leakage.

Final Thoughts

Machining your own AR-15 barrel from a blank is rewarding on several levels. You get the satisfaction of making it yourself. You can get barrels when the rest of the world is waiting for manufactured barrels to arrive. And you can machine the barrel to almost any contour you want, though a conventionally-installed barrel nut will limit the O.D. to a maximum of 1". You could have a heavier contour, but you’ll have to turn the barrel down to 1" in front of the extension so the barrel nut can be put on before the extension is installed. With some skill and the proper tools, there’s almost no limit to what you can imagine for your own custom AR-15 barrel.