300 blk gas port size?

[az doug]

My son gave me a CMMG 16" 300 blk barrel with a pistol length gas system. I plan on cutting it down to 12 inches on Monday. I want to run it with everything from subsonic unsuppressed (local subgun match) to sonic suppressed, although I am not certain when I would run sonic suppressed.

My research shows the gas port should be .082 to .085. Is this correct?? I will start with .082.

R. Silvers, I understand if you do not respond, but I hope you do.

Thanks

Doug

ETA: I may cut the barrel shorter in the future, but I wanted to start with the longest barrel that fits my needs.

[PRGGodfather]

For both subsonic and hypersonic .124-.125

[az doug]

Thanks

[az doug]

I shortened my CMMG 16" pistol length gas system barrel to 12". It uses a .936 gas block. I opened the gas port up to .082 using a #45 drill bit. With a carbine buffer the gun would cycle subsonic 220 gr SMKs, loaded by Southwest Ammunition, but would not lock the bolt open after the last round.

Since I want to use this upper on a full auto lower with an H2 buffer (to eliminate bolt bounce), I skipped the .086 #44 drill bit and opened the gas port up to .089 using a #43 drill bit. With my H2 buffer, the gun cycles fine with the 220 gr subs and locks the bolt open after the last round. In hindsight I probably should have tried the #44 drill bit, but having a gas port .003 too large isn't the end of the world, especially if it functions reliably.

I tried some Barnes 110 bullets loaded by Southwest Ammunition and they cycled fine as I knew they would. My concern was whether or not the gun was over gassed for this ammunition. Felt recoil was fine.

I have not had the opportunity to chrono any of this ammo yet. (I need to get my chrono back from my son.)

If anyone is wondering why I cut the barrel to 12" instead of 12.5" it had to do with where CMMG machined a shoulder in the barrels profile. I did square up their shoulder as I do intend to use this with a suppressor, eventually.