Forster .30 Carbine SAAMI Dimensioned Go Length Rimless Headspace Gauge

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