Proper utilization of a headspace gauge is the most reliable method for assessing the length of a rifle chamber. "headspace" refers to the distance between the breech face and the base of the cartridge when the action is closed. Excessive headspace can pose safety risks and negatively affect accuracy. For example, unsupported brass fired in a rifle with excessive headspace may rupture, causing gas to escape rearward in a manner similar to rocket exhaust. Given the critical importance of safety, Forster headspace gauges are manufactured with meticulous care and stringent standards to ensure precise testing of your rifle's chamber. If a rifle closes on a GO gauge, it indicates that the rifle chamber can accommodate any ammunition produced within SAAMI (industry standard) specifications. Extensive range of rimless gauges in large calibers Engineered from premium-grade, hardened steel and meticulously precision-ground to meet stringent ANSI and SAAMI specifications The headspace tolerance is precisely ground to an exacting specification of +/-0.00015 inches Each headspace gauge is meticulously inspected using NIST-certified measurement instruments