Abstract: A plug is inserted into and withdrawn from the muzzle end of a gun barrel by means of an external grip to store the gun in a safe condition with a dummy cartridge in its firing chamber. A retainer formation within the dummy cartridge is engageable by an elastically deformable lock device fixed to the inner end of the plug so that the plug may be respectively locked to and released from the dummy cartridge by opposite insertion and extraction forces of different lock engaging and lock releasing magnitudes to prevent loading and unintended firing of the firearm.