The present invention relates to a locking element (42) to be insertable into a cartridge chamber (420) of a firearm, said locking element having a proximal end (220) and a distal end (260). The locking element comprises an outer sleeve (14) with a movable operating element (16) provided therein, wherein the sleeve (14) comprises a (first) diameter such that the sleeve (14) is snugly insertable into the cartridge chamber (420) from the cartridge chamber side, but said diameter being larger than the diameter of the barrel (6) of the firearm. The operating element (16) is provided movably within at least one blocking element (18), and the operating element (16) is formed and provided such that when the operating element (16) is moved within the sleeve (14) into the direction of the proximal end, the operating element (16) causes the at least one blocking element (18) to be urged radially outwardly and be pressed against the wall of the cartridge chamber (420) thus blocking the cartridge chamber, wherein the at least one blocking element (18) is pressed further against the wall of the cartridge chamber (420) if external pressure is applied from the distal end to the operating element (16). The locking element further comprises an actuator which is adapted for temporarily fixing the position between the operating element (16) and the outer sleeve (14).