Headspace

Headspace of a .45 ACP cartridge, headspacing off the case mouth

In firearms terms, headspace refers to the distance between the bolt face and chamber necessary for reliable functioning of the weapon, or as a verb, the mechanism by which the correct positioning is achieved.

The headspace is the distance measured from the part of the chamber that stops forward motion of the cartridge, usually called the datum line, to the face of the bolt. Different cartridges have their datum lines in different positions in relation to the cartridge. For example, 5.56 NATO ammunition headspaces off the shoulder of the cartridge, whereas .303 British headspaces off the rim of the cartridge.

Headspace is measured with a set of headspace gauges. Each caliber gun typically has a "go" gauge, a "no-go" gauge, and, especially for current or former military calibers, a "field" gauge. On non-military surplus (that is, commercial) firearms, the bolt must lock on the "go" gauge, must not lock on the "no-go" gauge, and must not lock on the field gauge.

For military surplus firearms, there is a bit more latitude in the "no-go" gauge, as it may indicate correctly for commercial firearms, but may be overly-tight in tolerance for checking military-surplus guns designed to operate safely with a wider range of ammunition tolerances. For a military surplus gun that has a bolt lock on the "no-go" gauge, but that will not lock on the "field" gauge, the firearm may be perfectly safe firing military ammunition with heavier wall-thickness brass, but may have a tendency to overstress brass on commercial ammunition. At the very least, a military surplus firearm that locks on the "no-go" gauge and not on the "field" gauge will be difficult to reload for, as the brass will likely not last for more than one or two reloadings before cracking during re-sizing. An alternative approach for shooting a military-surplus gun is simply not to re-load commercial brass cartridges for it.

Any military-surplus gun that does lock on a "field" gauge, however, is unsafe to fire, and must be checked by a trained gunsmith.

Guns that fail to lock on the "go" gauge may simply need cleaning, especially at the bolt face, as build-up may occur on this surface and cause problems in chambering a round without stressing the brass.

There can be other types of gauges that measure exactly how over or undersize a chamber might be.

Headspace is important for the proper functioning of the firearm. If the headspace is too short, ammunition that is in specification may not chamber correctly. If headspace is too large, the cartridge case may rupture, possibly damaging the weapon and or possibly injuring the shooter. Some military-surplus firearms are designed to safely handle a case rupturing, whereas other military-surplus firearms may not be able to safely handle such an event. When in doubt, always seek a trained gunsmith to check headspace before firing any gun, but especially before firing any used gun.

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