Abstract: A firearm includes a locking device for receiving a removable key, and a latch element movable by an inserted key either to a locking position or to a releasing position. The locking device is located such that the latch element, when in the locking position, interferes with the operation of an essential operative component of the firearm to disable the firearm from firing a cartridge, and when in the releasing position permits the essential operative component to operate properly and thereby to enable the firearm to be used for firing a cartridge.