Some aspects of the invention are related to a locking device for a firearm and a method of controlling such a locking device. The locking device may include a locking cartridge adapted to be inserted into a firing chamber of the firearm, the locking cartridge may include a cartridge-like housing and a locking mechanism located in the cartridge-like housing. In some embodiments, when the locking mechanism is in an unlocked state, the locking mechanism may be extractable from the firearm's firing chamber, by cocking the firearm and when the locking mechanism is in a locked state, cocking the firearm may fasten the locking mechanism in the firearm's firing chamber. The locking device may further include an actuator adapted to change the state of the locking mechanism between the locked state and the unlocked state and a controller, in active communication with the actuator, configured to operate the actuator according to a signal received from an input device. In some embodiments, the locking cartridge, the actuator and the controller are extricable from the firearm together as a single unit.