Home Forums Hunting & Shooting AR and Tactical Rifles Tac 20 build (pic heavy)

Gas port drilling

When I drill a gas port, I like to drill a matching set screw dimple to help with gas block alignment. To do this, I like to use a collet block, which is simply a precision ground block that holds a 5C collet in the center to allow repeatable positioning of a part in different orientations. I used a square collet block for this job, with a 7/8" 5C collet (that's why I turned the barrel to .875", rather than .850" which I don't have a collet for). I also use a stop on the vise jaw, so I can bump the collet against that to repeat the same position when the collet block is removed and replaced in the vise.

Aligning the barrel in the collet is important too; I mostly do this by eye and align it so the barrel extension pin is straight up. That can be checked in the mill once it's all in place.



Using an edge finder, I can locate both sides of the barrel to find the center. This one is an audible edge finder so it clicks when it jumps out of position as it finds the edge, that's a convenient feature.


With the center of the barrel located, I locate the barrel shoulder at the gas block journal, and move forward .300" to the gas port location.



Drilling the gas port - from research online it sounds like the appropriate gas port size for this barrel length and cartridge is usually between .088" and .093". I drilled first to .076" and then stepped up to .082"; that reduces the burr on the inside of the hole. I'll test fire it at .082" and step up the size to .088" or larger if needed.


With the gas port drilled, now I flipped the collet block upside down to drill the set screw dimple. The mill table position is unchanged, so this dimple is directly in line with the gas port, as desired.


With that part finished, I removed the barrel from the collet block and cleaned the bore in preparation for test firing.