The only way to accurately measure and test the length of the chamber of the rifle. Essential when chambering a new barrel to ensure a tight, accurate, safe chamber. Whenever you buy a used gun, always check the headspace before firing. Gauges are etched with "Go," "No-Go," or "Field" and caliber/ gauge name for easy identification. Both Forster and Clymer gauges are made to SAAMI specs. Mfg: Clymer SPECS: Machined steel.