Okay so I bought a used older colt upper with bcg for a song last year. Decided to build my lrb lower I had laying around and use it as a beater/plinking gun. I assembled it with the old colt a1 fixed carry handle and a offset bushing. It was rattled but ran fine. Decided to go to a flat top stag upper and keep the barrel assembly. Performed the swap without removing the gas block (I did replace the gas tube though, rifle length system). Now I'm getting ftf issues. Tried multiple mags including a new brownells mag and a new pmag. Ejects every time but sometimes does not pick up a new round, or only partially loads it(wedging). If I pull the ch all the way back it loads fine. Also does not lock back on empty. I think I was able to get 4 rounds to cycle but usually it was every round. No scoring or marks on the bcg, again the rifle ran fine before and I only removed the gas tube and reinstalled. Ammo used is American eagle 55grn factory brass loads. Rifle: Stag arms upper receiver (new) 20" colt barrel assembly (used) Lrb lower with bushmaster parts kit Colt bcg and ch Stag buffer tube and buffer/spring Symptoms make me think a gas problem, but since I never removed the gas block I doubt it's that. Gas key is staked and solid with no movement. Should I replace the buffer with a lighter one? Can new uppers really be this finicky during break in? (Only fired about 50 rounds). I lubed with rem spray since my good stuff is at work. I'm going to try to clean relube and try again. Any other suggestions?