We'd need more information. Does it rotate and lock up properly then the hammer is cocked? If so, I'd say it sounds like hammer rebound. The cylinder cannot rotate when the hammer is down on the cap so it must be rebounding when the gun fires. What load are you shooting? If it's a heavy load of 777, that may be the only problem. Try about 15 grains of Goex or Pyrodex and see if that still happens. If so, I'd first remove the grip frame and make sure the screw which holds the mainspring is tight, if loose you don't get much spring pressure to hold the hammer down. Then check the nipples, if the hole is too large it will allow enough back blast to throw the hammer back a bit. The nipple hole, at the bottom, should not be larger than about .030".