Abstract: A safety device for preventing unauthorized or accidental use of a gun. The safety device includes a solid body of material, a muzzle end plug and an elongate threaded rod inter-connecting the same. The solid body is sized and shaped to simulate a live cartridge. It has an outwardly directed flange at one end for engaging an end wall surrounding entry into the cartridge chamber of the gun and a threaded bore extending inwardly a selected distance from the opposite end. An O-ring fits into a groove it the solid body and projects outwardly frictionally to engage the inner wall surface-of the gun barrel in the cartridge chamber. The muzzle plug has an outwardly directed flange at one end thereof that engages an outer end of the gun barrel.