Just built up a 9mm PCC. Was having issues with doubling (one trigger pull held to the rear and two rounds shot)
I traced my issue to my buffer. I had 7ozs in the buffer with room for the weights to move (try to reduce bolt bounce/recoil) I wasn't only getting doubling but the consistency of bolt cycle was erratic. after I added more rubber disks so that the weights don't move, It's shooting consistent and no more doubling. Although I need to remove weight from buffer, it'll short stroke if not shouldered tightly.