Abstract: A firearm locking mechanism comprising block or body having a conventional pin-tumbler or cylindrical lock mounted generally vertically therein. The block or body is received within the exposed area between the breech and open breech block in a firearm directly above the magazine, with an engagement member connected to the bottom of the lock being received within the top of the magazine and rotated by the lock. The engagement member engages beneath and between the cartridge-retaining surfaces at the top of the magazine to secure the lock and body to the top of the magazine, thereby preventing the breech block from closing or the magazine from being removed. The locking mechanism similarly prevents moving the firing pin assembly into proximity with any cartridge remaining in the barrel or magazine.