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.
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.