Abstract: The inbore sub-caliber device has spaced apart front and rear adapter plate members, each of which has an opening formed therethrough. A plurality of spaced apart rods extend between the front and rear adapter plate members to locate the openings in the front and rear adapter plate members in alignment. Support structure is coupled to the rear adapter plate. A gun comprising an elongated barrel and a rear receiver member is secured to the support structure. The barrel is supported in alignment with the openings in the front and rear plate members with at least a portion of the receiver located rearward of the rear plate member and the front end of the barrel located forward of the rear plate member to allow a bullet when fired to pass from the front end of the barrel by way of the opening of the front plate member.