Your safety and performance are on the line. Improper headspace may be dangerous to the shooter as well as an impairment to accuracy. The proper use of Headspace Gages is the most reliable way to test the length of a rifle chamber.
According to the Sporting Arms and Ammunition Manufacturers’ Institute (SAAMI), which 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. One should routinely check rifle chamber headspace every thousand rounds. If chamber headspace is excessive, the gun should be taken out of service until it has been inspected and repaired by a competent gunsmith.
Headspace is measured differently, depending on whether the firearm’s caliber uses rimmed, belted, or rimless cartridges.
Rimmed and Belted Cartridges
Headspace = distance between the bolt face and the top of the rim (chamber face) when the bolt is closed.
Headspace = distance between the bolt face and a datum line (determined by SAAMI) where the front of the cartridge rests on its shoulder when the bolt is closed.
In popular speech, “headspace” is used to describe three related measurements:
Forster offers three lengths of headspace gages for most rifle calibers, in order from shortest to longest: