Using high quality bbs? Not the shitty cyber gun and wallyworld/crappytire ones.
Check that the hop nub is centered properly? Check that there's no debris in the barrel?
Check that the hop rubber doesn't get pinched when reassembling the gun?
mk43, ump.45, p90, barret, m14, g33, k98, m4 tac, m4 swat, mp5 navy, vsr10, p226, m9.