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Headspace is measured in different ways on different calibers. For a rimmed cartridge such as the 7.62x54R (where R stands for “rimmed”) headspace is the distance between the bolt face and the face of the chamber where the loaded cartridge rests. In a rimless cartridge such as a .308 the headspace is measured to the shoulder of the cartridge. This dimension is important for proper and safe firearm function regardless of the type of cartridge being checked. Too little headspace will not allow the bolt to fully close rendering the weapon unable to fire. Too much headspace can cause excess chamber pressure, as a result of the cartridge not being held firmly in the chamber on firing. This can be particularly dangerous given the chamber pressures generated by centerfire rifle cartridges..
Headspace gauges generally come in 3 sizes:
GO: measures the minimum acceptable headspace. This size is most often used when re-barreling or re-chambering a firearm.
NO-GO: This gauge is used to check for excessive headspace. If a firearm closes on the NO-GO gauge it is an indication that the weapon MAY not be safe to fire. Reloading ammunition for a firearm that fails NO-GO can result in unsafe loads due in part to the expended brass being elongated as a result of not being firmly chambered. Most military surplus firearms that pass NO-GO (the bolt will not fully close on the gauge) are considered safe to fire with either surplus ammunition, or with modern loads that may or may not be intended for reloading.
FIELD: A firearm failing the NO-GO spec can be tested on the slightly more forgiving FIELD gauge. Military chamber specifications are generally looser than commercial firearms, giving them a bit of tolerance for adverse conditions such as dirty chambers, weather extremes, etc. A firearm passing FIELD spec (not closing fully on the gauge) is generally considered safe to fire the ammunition it was designed to fire, i.e. military surplus ammunition of the designated caliber.
These are to be considered GENERAL GUIDELINES and are in no way intended to be an assurance of the safety of a particular firearm. If you have doubts about a firearm’s safety and usability you should have it checked by a professional gunsmith.
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