Abstract: A locking device for locking into the bolt cavity of a firearm for making it non-functional, comprises a housing (1) which contains a lock cylinder (3) operable by means of a key which is introducible through a drilling protection (4) in the housing. The cylinder arm (8) of the lock cylinder (3) is non-rotatably connected to a follower (7), which has a flattening (9) which upon rotation of the follower pushes a wedge member (15) in the form of a steel ball between two blocking members (14), also in the form of steel balls, so that they are brought to extend partly from opposite sides of the housing (1). The locking device may be introduced into the magazine well of the firearm and located such that the extending blocking elements (14) will cooperate with e.g. the locking piece or receiver rails of the firearm in order to prevent the extraction of the locking device in its locked condition.