Ok, I've been tinkering with ARs for oh a few years. I ran across something last night that really surprised me.
I probably should have posted this in the general AR discussion, because I don't think it's pistol specific, but it happened on my Pistol lower/upper combo.
I usually use RRA 2 stage triggers in my builds and at one time I had bought a CMMG 2 stage combat to see how it compared. Well I got my 300BK upper all finished and went out to shoot it with my pistol lower that I had put the CMMG trigger in some time back. (no issues with other uppers and BCG)
Anyway long story short after having a failure to lock back on last round on supersonic ammo, I started working parts combos, (ie, buffers, springs, etc) and then I noticed even on hand cycling it wouldn't lock back.
So I figured either BCG or Bolt catch issue. Changed both out no effect. Changed the complete lower out and it locked out fine. Went back and changed the trigger out of my original pistol lower and it locked up just find. WTF? So I started doing to checking and for some reason the design of the CMMG and this particular upper causes this issue. Wow.
Never had this happen before. Never would have suspected a Hammer would put enough upward force to not allow the bolt catch to engage properly.
Anyway just thought I'd share.
I was really expecting it to be a gas issue and it really thru me off. Took hours to find the problem.
Anyway the upper and lower are functioning flawlessly with supersonic ammo now. will be working up some subsonic loads soon.
SS33
Read on below I was totally incorrect