Abstract: A gun lock including a hardened steel rod (14) threaded at each end, the threaded ends respectively engaging a cartridge shaped plug (27) and a rotatable tumbler lock (24). The lock (24) is fixed within a sleeve body (21) adapted to fit snugly around a barrel end (12) of a gun to be locked. The cartridge plug (27) fits snugly within the firing chamber (11) of the gun, in the same manner as a cartridge adapted to be fired within the gun. With the cartridge plug (27) within the firing chamber (11), and with the rod (14) threadedly engaged in the lock (24) the rod (14) can be inserted down the bore (13) of the gun and threadedly the cartridge plug (27) by rotating a mating key engaged in the lock (24), until the device is snug. Removal of the key results in securing the gun lock within the gun. A slot (26) within the sleeve body (21) engages an aiming sight ( 25) of the gun so as to prevent rotation of the sleeve (21).