1. IN A CONVERTIBLE GUN, THE COMBINATION OF: A RECEIVER; A BOLT CARRIER IN SAID RECEIVER; A REVERSIBLE MOUNT SECURED TO SAID BOLT CARRIER; REVERSIBLE GUIDE AND DRIVE MEANS SECURED TO SAID MOUNT; A BELT FEED MECHANISM DEMOUNTABLY SECURED TO SAID RECEIVER FOR ENGAGEMENT BY SAID REVERSIBLE GUIDE AND DRIVE MEANS; A FIRING PIN IN SAID BOLT CARRIER; AND DETENT MEANS ON SAID BOLT CARRIER FOR HOLDING SAID FIRING PIN AGAINST MOVEMENT WITH RESPECT TO SAID BOLT CARRIER WHEN SAID REVERSIBLE GUIDE AND DRIVE MEANS IS ENGAGED WITH SAID BELT FEED MECHANISM, SAID FIRING PIN BEING OPERATIVELY CONNECTED TO SAID MOUNT TO ENGAGE SAID DETENT MEANS WITH SAID FIRING PIN AND PREVENT RELATIVE MOVEMENT BETWEEN SAID BOLT CARRIER AND SAID FIRING PIN.