headspace gauge

In firearms, headspace is the distance measured from a closed chamber's breech face to the chamber feature that limits the insertion depth of a cartridge placed in it. Used as a verb by firearms designers, headspacing refers to the act of stopping deeper cartridge insertion. The exact part of the cartridge that seats against the limiting chamber feature differs among cartridge and gun designs. Bottleneck rifle cartridges headspace on their case shoulders; rimmed cartridges headspace on the forward surfaces of their case rims; belted cartridges headspace on the forward surfaces of their case belts; rimless pistol cartridges headspace on their case mouths.
When headspace is larger than the cartridge case, there is front-to-back cartridge wiggle room when the breech of the gun is closed. This extra space is called head clearance. Many, including some major manufacturers, confuse head clearance with headspace in their literature. This commonplace error is explained in the glossary of firearms terms maintained by the Sporting Arms and Ammunition Manufacturers Institute (SAAMI) which is the ASTM standards organization for the domestic firearms and ammunition industry. Excessive head clearance is undesirable for several reasons. It can allow a cartridge's primer to slide beyond the reach of the firing pin to adequately indent the primer to achieve reliable ignition (though extractor hooks may act as the replacement headspace determinant in this situation). In chamber designs that don't fully support the case head, it can allow a case to expand excessively, which can thin or even burst the brass. Thus the cartridge can rupture rearward, which risks releasing hot gases under high pressure that can damage the firearm and injure or even kill the shooter or bystanders.If a chamber's headspace is too short, the gun may be unable to go fully into battery (close and lock completely), preventing firing.

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