A firearm lock and a firearm locking method including a key-operated lock, which is lockingly engageable with an interior portion of a firing chamber of a firearm. The lock comprises: a cylinder lock body including a plug rotatable therewithin; an inner multiply grooved generally circular cylindrical portion fixed to or integrally formed with a first end of the lock body; and an outer circular cylindrical portion correspondingly multiply grooved generally circular cylindrical portion, fixed to or integrally formed with the plug for rotation therewith between a locked orientation, wherein grooves on the inner cylindrical portion are arranged out of phase with corresponding grooves on the outer cylindrical portion, and an unlocked orientation, wherein the grooves on the inner and outer cylindrical portions are mutually aligned.