Headspace (firearms)

Used as a verb by firearms designers, headspacing refers to the act of stopping deeper cartridge insertion.

It can allow a cartridge to slide forward beyond the distance within which the firing pin has the ability to adequately indent the primer for reliable ignition (though extractor hooks may act as the replacement headspace determinant in this situation).

Thus, the cartridge can rupture rearward, which releases hot gases under high pressure that can damage the firearm and injure or even kill the shooter or bystanders.

[1] If a chamber's headspace is too short, the gun may be unable to go fully into battery (close and lock completely), preventing firing.

The part of the firearm called an extractor claw hooks over the rim to extract the spent case from the chamber after it has been fired.

Headspace for rimmed cartridges is the space between that forward ledge and the bolt face when the action is closed.

Bottle-necked rimless cartridges are intended to headspace on the conical shoulder between the narrow neck and the larger diameter portion of the case.

In effect this was similar to the headspacing function of a rim, but gave a long enough surface for cartridges to lie side-by-side in a magazine without risk of interference during the feed-stroke.

or CIP) uses a rimless, rimmed, magnum, pistol and revolver and rimfire cartridges classification to differentiate between hundreds of different small arms chamberings.

The view is that in the very unlikely case (according to the current quality standards) the cartridge is too long, once pressed by the bolt, the chambering force will rise too high causing rejection.

Additionally though, upon firing, the expanding case seals the breech of the firearm to prevent burning powder gas from escaping rearward.

Pressure of burning powder gasses expands the thinner forward walls of the cartridge case to firmly grip against the sides of the chamber preventing rearward motion.

Some competition reloaders leave the primer slightly proud of the case by a known amount that is taken up when the bolt is locked home.

This mechanical action on the center of the case base via the primer, as it is pushed a very small amount further into its cup, translates into an even distribution of pressure through the rim to the rim-stop.

This works with the cartridge moving backwards, stretching it to an extent until it unlocks the bolt to cycle its operation.