Abstract: A device for insertion in the barrel or chamber of a firearm, e.g. a shotgun to prevent unauthorized use employs a key-operated lock (7) which is rotated to cause, through a screw-thread, axial movement of an operating member (25) with tapered surfaces which causes expansion of another component (21) having convergent surfaces, so as to engage tightly against the walls of the chamber. The key is used both to unlock the lock and to rotate the operating member. If an attempt is made to drive the device out from the barrel end there can be teeth (40) which spread apart to bite into the walls of the chamber.