The idea of being able to control the amount of gas operating your rifle has been a long toyed with problem in tuning certain platforms. The AR-15 pattern rifle has been the center of obsession with barbie gun builders and lego type gunsmiths. Bolt on, clamp on and drop in parts are the norm now for this ubiquitous platform that is America's favorite rifle to love and hate. Minor modifications can be done in the home workshop though that save money over the boutique parts companies offering the latest and greatest must have accessory. One such seemingly simple project is making an A2 front sight tower into an adjustable gas block, a friend of mine requested I take a look at this DIY project others had attempted and see if I could duplicate their efforts for him. Having a bag of spare A2 gas blocks helped as I quickly learned that you really can not fit a 10-24 set screw in between the barrel rings and the pathway the tube occupies when installed. My first attempt using the 10-24 set screw would have resulted in having to further modify the gas tube prior to installing the set screw.
The obvious conclusion that some had already come to and my weak google-fu later scared up was that 8-32 set screws were perfect for this job. A 6 series would also work though your restriction range may not be as great. The idea is drill the hole in the vertical path the gas takes, tap the hole and adjust the set screw so you get reliable function of the platform but not over gassing the system causing premature wear on parts and more recoil than is necessary.
My method for this project was
1. locate along the vertical gas chamber where I wanted the set screw.
2. Using a center cutting end mill I milled a flat spot for the center and drill to work off of
3. Using a center drill locate exact place to drill hole
4. Using a #29 drill bit drill hole on right side of gas block (I chose the right side so that a right handed shooter could easily adjust/monitor the set screw) I went ever so slightly into the opposite side of the gas block with the bit so as to create a seat for the set screw if it was so desired to omit all gas from the system.
5. Tap for 8-32 screw.
If so desired locate and drill hold on the front for a long 4-40 set screw. Be sure to clear all chips out of the gas block....a good oiling and de-greasing with brake cleaner a few times will ensure all machining artifacts are cleared away.
As this is an experimental item for me it'll be interesting to see how this holds up over the long term. I know for the first couple of mags it would be smart to not have anybody standing to the right of the shooter lest the screw or taped threads decide to part company with the gas block.