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. In general, bottleneck rifle cartridges headspace on their case shoulders; rimmed cartridges headspace on the forward surfaces of their case rims; and rimless pistol cartridges headspace on their case mouths. The case belts on belted cartridges were originally added to allow headspacing on the belt's forward surface, But in practice, this is often vestigial, and rifles chambered for belted cartridges may well headspace them on their shoulders and still be within CIP or SAAMI dimensional limits. However, belted cartridges and their corresponding chambers at their nominal CIP or SAAMI dimensions (maximum cartridge in minimum chamber) will headspace on the belt. When the 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 U.S. firearms and ammunition industry. Excessive head clearance is undesirable for several reasons. 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). In chamber designs that don't fully support the case head, excess headspace can allow a case to expand excessively, which can thin or crack open the brass. 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. If a chamber's headspace is too short, the gun may be unable to go fully into battery (close and lock completely), preventing firing.
Cartridge types
All small arms cartridges have a flange at the base of the case called a rim. 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. The cartridge is said to be of a rimmed type when the rim is of larger diameter than the remainder of the case. Most early cartridges were rimmed, and the rim-stop ledge at the rear of the chamber prevents the cartridge from moving forward. Headspace for rimmed cartridges is the space between that forward ledge and the bolt face when the action is closed. The cartridge is said to be rimless if the extractor groove is machined into the head of the case so the rim is the same diameter as the adjacent part of the case. Most modern automatic firearms use rimless cartridges. Straight-walled rimless cartridges often headspace on the forward lip of the cartridge as shown in the diagram at the top of this article. This necessitates careful attention to consistent case length. Bottle-necked rimless cartridges are intended to headspace on the conical shoulder between the narrow neck and the larger diameter portion of the case. Some large rimless magnum cartridges have a belt formed above the extractor groove. This belt is of slightly larger diameter than the adjacent case, so the cartridge can headspace from the forward edge of the belt closest to the bullet. The original purpose of the belt was to give accurate headspacing for cartridges with shallow shoulder angles, where longitudinal precision of seating using such a shoulder presents difficulties. 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. The adoption of the belt as headspacing feature on rounds such as the .300 H&H Magnum and its derivatives ultimately started a fashion that resulted in most later magnum rifle cartridges featuring belts. Some more or less straight cartridges have no bottleneck on which to headspace. Such cylindrical shaped cartridge cases use the case mouth (the forward end of a cartridge case) as a forward positioned flange used for headspacing.
C.I.P. ammunition approval procedure and headspace The standards organization Commission Internationale Permanente pour l'Epreuve des Armes à Feu Portatives (Permanent International Commission for Firearms Testing - commonly abbreviated as C.I.P. or CIP) uses a rimless, rimmed, magnum, pistol and revolver and rimfire cartridges classification to differentiate between hundreds of different small arms chamberings. The dimensional checking of the ammunition, as described in the C.I.P. Decisions, Texts and Tables, is left to the manufacturers themselves. During C.I.P. ammunition approval headspace is not checked, even on rimless cartridges with taper. 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. If it is too short, firing will fail also causing rejection.
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