This one piece gauge for checking factory and re-loaded ammunition to ensure fit in firearms is crucial for the at home re-loader and gunsmith. This particular unit will allow you to check 9mm Luger, .40 Smith & Wesson, .45 ACP, and .38 Super cartridges in one easy convenient block. The block is made from aircraft grade 6061-T6 aluminum and features a nice anodized matte black finish. This block is used to compare cartridges against SAAMI chamber dimensions. Simply drop a loaded round into the gauge. If the round enters the gauge easily and fits flush with the top of the block the round will fit in the guns chamber.