On a recent build, I wanted to have a traditional front sight gas block, but didn’t want to deal with drilling and pinning it to the barrel. It just so happened that my LGS had a virgin FSGB laying around and upon asking about it, he just gave it to me.
I decided after looking it over that I could do some simple machining to it so that it could be fitted to the barrel via set screws. Allowing me to place the FSGB wherever I wanted on the barrel and it could be easily moved if (when) I changed my mind.
This existing hole was handy, measured .142”. I hand reamed it to .157 (as to not overload the tap) then found a 10-32 tap which I knew I could find set screws to match.
Then tapped it by hand as well.
The second hole needed to be drilled all the way through the bayonet lug, to do that I put it in a proper vise and used a mill (alt drill press) along with a .156 #22 drill and slowly drilled down through the lug until I popped through.
Then hand tapped this hole as well. Just as the first.
I cleaned up the through holes with sand paper, deburred, and cleaned the piece.
Went to the local hardware store and found two stainless 10-32 set screws (55 cent EACH!). After that I took it home and it fit my rifle perfectly! Installed the sight, detent, and spring. Then a bore sight and it’s all done. I do recommend that if you have a serious use for such a thing that you dimple the barrel so the set screws have less chance to back out. This is just a recreational upper for me so I’m just tightening them down to the barrel as is and using loc tite
All in all this went really smooth for me (luckily) and I’m glad to have this component in my collection for a more traditional build. This sort of thing has been done before I’m sure but I thought I would try my hand at it. This is by no means a comprehensive tutorial. There is a fair bit that I didn’t cover, but wanted to keep this readable. Tapping by hand and properly drilling the holes takes a fair bit of finesse but if I can do it anyone can, feel free to ask me any questions.
Bonus funny pic from the fitting process. Gotta run backups and co-witness everything.
I decided after looking it over that I could do some simple machining to it so that it could be fitted to the barrel via set screws. Allowing me to place the FSGB wherever I wanted on the barrel and it could be easily moved if (when) I changed my mind.
This existing hole was handy, measured .142”. I hand reamed it to .157 (as to not overload the tap) then found a 10-32 tap which I knew I could find set screws to match.
Then tapped it by hand as well.
The second hole needed to be drilled all the way through the bayonet lug, to do that I put it in a proper vise and used a mill (alt drill press) along with a .156 #22 drill and slowly drilled down through the lug until I popped through.
Then hand tapped this hole as well. Just as the first.
I cleaned up the through holes with sand paper, deburred, and cleaned the piece.
Went to the local hardware store and found two stainless 10-32 set screws (55 cent EACH!). After that I took it home and it fit my rifle perfectly! Installed the sight, detent, and spring. Then a bore sight and it’s all done. I do recommend that if you have a serious use for such a thing that you dimple the barrel so the set screws have less chance to back out. This is just a recreational upper for me so I’m just tightening them down to the barrel as is and using loc tite
All in all this went really smooth for me (luckily) and I’m glad to have this component in my collection for a more traditional build. This sort of thing has been done before I’m sure but I thought I would try my hand at it. This is by no means a comprehensive tutorial. There is a fair bit that I didn’t cover, but wanted to keep this readable. Tapping by hand and properly drilling the holes takes a fair bit of finesse but if I can do it anyone can, feel free to ask me any questions.
Bonus funny pic from the fitting process. Gotta run backups and co-witness everything.
Last edited: