Lyman Pro Ammo Checkers are precision machined from corrosion resistant stainless steel to minimum SAAMI chamber tolerances. All Pro Ammo Checkers are clearly marked for caliber. Simply drop your loaded cartridge into the gauge and if it fits, it will chamber in your gun. A "must" final check for competitive shooters or anyone that wants to ensure smooth, trouble free chambering of reloaded ammo. Mfg: Lyman