Hi.I noticed an old comment that you'd made regarding your Remington 550 .22 semiautomatic .If you still haven't found the problem yet , try examining the rear end of the firing pin which is struck by the hammer . Years and years ago , on my gun ( now 53 years old) I found that the firing pin had peened or flared out just enough to occassionally hang in the bolt .In that state , it would project out when the next round was loaded , automatically firing off a whole series of rounds in " full automatic " fashion. Ulimately , a round would blow the end off of the brass casing and blow out the bottom cap on the receiver ,scaring the heck out of me . The cure is to simply take a jewlers file and dress the firing pin back to its original thickness .I hope that this helps. Incidently , Remington couldn't find the problem either .