Abstract: A dummy cartridge is placed in the firing chamber of a revolver. A machine screw in the front of the cartridge is then turned to extend forward partly into the barrel, preventing both firing of the gun and rotation of the cylinder. The screw is turned by a key inserted through the barrel from the muzzle. The key can be stored separately from the gun, so the gun owner, knowing its location, can quickly unlock the gun, but an intruder cannot use the gun. All chambers in the revolver cylinder except one can be loaded with live ammunition, so the gun can quickly be used for self-defense after unlocking. However, if the gun is locked without live ammunition, none can be added to any chamber until the gun is unlocked, which is ideal for firearm display applications. The dummy cartridge has means to prevent its turning in the chamber, so the key will turn the screw instead of the cartridge. The key fits closely within the barrel to center the key tip and align it with the screw head for immediate coupling.