Blackout help please

[Jjed]

I built a 300 blackout upper and have nothing but trouble, what I have figured out is the ring that holds the fore stock behind the gas block is too thick and from being stamped out is not flat, I measured the distance from the shoulder on the barrel to the gas port, did the same on the gas block the difference is .027 so I removed the fore stock ring reinstalled the gas block using a feeler gauge to set the distance from the shoulder on the barrel to the gas block too ,027 and tested the gun, it ran fine . I marked the center of the holes on both pieces so I know they where lined up. Measured the ring it’s .031 but with the slight bends from being stamped it could be as much as .040 I have built uppers before and never had this problem. The difference of .004 is causing the problem and all the other rings I have looked at are the same. Any ideas short of cutting the shoulder on the barrel back would be appreciated.
Jim

[Frozone]

ovalize the port in the gas block until it clears the hole in the barrel - it's the size of the hole in the barrel that matters.

[Jjed]

Frozone
Thanks I should have thought of that, I took a drill that was only a little bigger than the gas port on the gas block drilled it and it works better, loaded up some 147 gr fmj with 17 gr h110 they cycled droped the load to 16 gr they didnt do so well. I think I will step up one drill size to see if that helps. thanks for your help.