300px|thumb|right|A pistol cartridge which headspaces off the case mouth, e.g. [[.45 ACP]]

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. 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.

Theory

A firearm cartridge case serves multiple functions. The most obvious role is to provide a container for the powder, bullet, and primer. Additionally though, upon firing, the expanding case seals the breech of the firearm to prevent burning powder gas from escaping rearward. Most cartridge cases are made of brass and expand under pressure to form a closed gas tight fit with the sides of the chamber, and then contract slightly so that the case remains loose and can be removed. Centerfire cartridge cases are thicker near the base to prevent expansion into openings between the rear end of the barrel and the action where the cartridge case is not fully enclosed or supported by the chamber.

When a firearm has more headspace than the cartridge design anticipated, the closing of the bolt or impact of the firing pin may move the cartridge forward to leave space between the chamber face of the action and the base of the cartridge. 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. The thicker base of the cartridge case (or sometimes the primer) may then be forced back into available space at the rear of the chamber, causing the case to stretch. Where rearward movement exceeds cartridge design, the primer may rupture or the base of the cartridge case may begin to separate from the forward walls. Either type of failure may release high pressure gas and case fragments from the action of the firearm. These may cause burns, cuts, or eye injury to the shooter or persons standing nearby.

See also

  • Glossary of firearms terms

References

  • Cruffler.com article from October 1999
  • Forster Products article
  • How to measure headspace PDF paper of Armalite.com
  • C.I.P. HOMOLOGATION List of TDCC - Tab I - Rimless cartridges
  • C.I.P. HOMOLOGATION List of TDCC - Tab II - Rimmed cartridges
  • C.I.P. HOMOLOGATION List of TDCC - Tab III - Magnum cartridges
  • C.I.P. HOMOLOGATION List of TDCC - Tab IV - Pistol and revolver cartridges
  • C.I.P. HOMOLOGATION List of TDCC - Tab V - Rimfire cartridges - Crusher
  • C.I.P. HOMOLOGATION List of TDCC - Tab V - Rimfire cartridges - Transducer
  • C.I.P. HOMOLOGATION List of TDCC - Tab VI - Cartridges for industrial use
  • C.I.P. HOMOLOGATION List of TDCC - Tab VII - Shot cartridges
  • C.I.P. HOMOLOGATION List of TDCC - Tab VIII - Cartridges for alarm weapons
  • C.I.P. HOMOLOGATION List of TDCC - Tab IX - Dust shot cartridges
  • C.I.P. HOMOLOGATION List of TDCC - Tab X - Cartridges for other weapons
  • Ask Ian: What is Headspace? (And Why It Matters)