Cal 22. This is one of the US Cartridge test guns and is marked on the side of the receiver "USC CO #422-12-12-08". Gun retains about 50% blue turning a plum color. Correct front sight. Rear bbl sight is missing. Stock has numerous scars and dings which was typical on all these cartridge test guns. Correct checkered shotgun style buttplate. Number one configuration bbl measures 22". I think it was originally 24" and has been re-chambered and set back into the frame which they frequently did since they shot the hell out of these cartridge test guns. A scarce unique gun.