+1 on montanas comments. I take a precision square and mark a line from gas block shoulder out past where end of gas block will be. Then mark center of gas hole in the block, on block so the marks are visible and can be lined up. After assembly but before installing handguard, remove bolt from carrier and run carrier in and out of upper to detect any interference between the two. Turning upper upside down will enable you to observe. The reason for leaving handguard off is in case you need to tweak barrel nut. That should get you pretty good. If is centered in opening in nut and the opening in upper, like Montana said, tweak with a flat head screwdriver.
Edit: +1 on blasters loc tite. I built one upper that wouldn't shoot until I loctited gas block with 242 and barrel extension with the 680 bearing mount.
Richard
Edit: +1 on blasters loc tite. I built one upper that wouldn't shoot until I loctited gas block with 242 and barrel extension with the 680 bearing mount.
Richard
Member since 2011, data lost in last hack attack