Use whatever BCG that's in your price range. What I think is important for proper function is are you shooting subs or supers, the gas length and gas port size. Like others have said, you can fine tune with buffer weights to get consistent lock back. My barrel has a .108" gas port, carbine length on a 16" barrel. My buffer and spring system is a VLTOR A5 rifle length. This buffer system is heavier than "normal" but has always locked back on empty in part to the .108" gas port.