Using a headspace gauge correctly is the most reliable way to measure a rifle chamber's length. Headspace refers to the distance between the breech face and the base of the cartridge when the action is closed. Excessive headspace can cause safety issues and reduce accuracy. For example, firing unsupported brass in a rifle with too much headspace may cause the case to rupture, allowing gas to escape backward like rocket exhaust. Because safety is paramount, Forster headspace gauges are made with great precision and strict quality standards to ensure accurate chamber testing. If a rifle closes on a GO gauge, it means the chamber meets SAAMI specifications and will safely fit standard ammunition. 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