Forster Rimmed and Belted Headspace Gage GO length for 300 Mag and all belted Mags. "Headspace" is the distance between the face of the breech and the base of the cartridge when the action is closed and excessive headspace can be dangerous as well as impair accuracy. For instance unsupported brass fired in a rifle action with excessive headspace can rupture allowing gas to blow rearward like a rocket exhaust. Because your safety is on the line Forster Headspace Gages are made with painstaking care and exacting manufacturing standards to ensure accurate testing of your rifle?s chamber. If a rifle closes on a GO gage the rifle chamber will accept any ammunition that is made within S.A.A.M.I. (industry standard) specifications. If a rifle closes on a NO GO gage the rifle chamber might have excessive headspace. Most gunsmiths chamber a rifle?s headspace between the GO and NO GO dimensions. If a rifle closes on a FIELD gage the rifle chamber is dangerously close to or already over the S.A.A.M.I. specified maximum chamber size. Specifications for Forster Rimmed and Belted Headspace Gage: Manufacturer: Forster Fabric/Material: Brass Color: Black Quantity: 1 Package Contents: Forster Rimmed and Belted Headspace Gage Forster BG300MG: Rimmed and Belted Headspace Gage GO length for 300 Mag and all belted Mags