Headspace is measured differently depending on whether the firearm’s caliber uses rimmed, belted or rimless cartridges.

Choosing the Correct Gage: According to SAAMI (Sporting Arms and Ammunition Manufacturers Institute), who sets product standards for firearms and ammunition, headspace is “The distance from the face of the closed breech of a firearm to the surface in the chamber on which the cartridge case seats. Some shooters are under the misconception that headspace is fixed over the life of the firearm. Headspace can increase after repeated use of ammunition that causes too much pressure. Handloaders should routinely check rifle chamber headspace every thousand rounds. Headspace is measured differently depending on whether the firearm’s caliber uses rimmed, belted or rimless cartridges.

Forster offers three lengths of headspace gages per rifle caliber. In order from the shortest to longest, they are: GO, NO-GO and FIELD:

* GO: Corresponds to the minimum chamber dimensions. If a rifle closes on a GO gage, the chamber will accept ammunition that is made to SAAMI’s maximum specifications. The GO gage is essential for checking a newly-reamed chamber in order to ensure a tight, accurate and safe chamber that will accept SAAMI maximum ammo. Although the GO gage is necessary for a gunsmith or armorer, it usually has fewer applications for the collector or surplus firearms purchaser.

* NO-GO: Corresponds to the maximum headspace recommended for gunsmiths
chambering new firearms. This is NOT a SAAMI-maximum measurement. If a rifle closes on a NO-GO gage, it may still be within SAAMI specifications or it may have excessive headspace. To determine if there is excessive headspace, the chamber should then be checked with a FIELD gage. The NO-GO gage is essential for checking a newly-reamed chamber in order to ensure a tight and accurate chamber.

* FIELD: Corresponds to the longest safe headspace. If a rifle closes on a FIELD
gage, its chamber is dangerously close to, or longer than, SAAMI’s specified maximum chamber size. If chamber headspace is excessive, the gun should be taken out of service until it has been inspected and repaired by a competent gunsmith. FIELD gages are slightly shorter than the SAAMI maximum in order to give a small safety margin.