Rimless Case
Definition
Cartridge case geometry where the extractor groove is machined into the head and the rim diameter equals the body diameter; dominant in modern centre-fire semi-automatic pistol and rifle ammunition including 9x19mm, .45 ACP, and 5.56x45mm NATO.
- Extractor groove
- Machined into the case head
- Rim vs body diameter
- Equal
- Examples
- 9x19mm, .45 ACP, 5.56x45mm NATO
- Dominant in
- Modern centre-fire semi-automatic ammunition
Common questions
Why is a rimless case design preferred for semi-automatic firearms?+
A rimless case headspaces on the case mouth or shoulder rather than the rim, and its uniform diameter feeds more reliably through a box magazine and semi-automatic action than a rimmed case, which can catch on adjacent rounds during feeding.
How does a rimless case differ from a semi-rimmed or belted case?+
A semi-rimmed case has a rim slightly larger than the body for partial headspacing, and a belted case has a raised band ahead of the extractor groove for high-pressure cartridges, whereas a true rimless case relies entirely on the case mouth or shoulder.
Related terms
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- Belted Case
- Cartridge case with a raised band (belt) ahead of the extractor groove that controls headspace; found in large-calibre magnum cartridges including .300...
- Berdan Primer Pocket
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- Boxer Primer Pocket
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- CIP
- Commission Internationale Permanente pour l'Epreuve des Armes a Feu Portatives; the European proof body that standardises chamber and cartridge specifications for European...
- Extruded (Stick) Propellant
- Cylindrical rod-shaped propellant grain with single or multiple perforation; dominant in medium- and slow-burning rifle propellants for 5.56mm, 7.62mm NATO, and larger...
- Gilding Metal
- 90 percent copper and 10 percent zinc alloy used for bullet jackets in most commercial FMJ and jacketed pistol ammunition; softer than...
- Headspace
- The dimensional distance from the bolt face to the datum line in the chamber at which the cartridge case head rests; excessive...
- Obturation
- The gas-sealing function of the cartridge case during firing, in which the case wall expands outward under propellant pressure to seal the...
- SAAMI
- Sporting Arms and Ammunition Manufacturers' Institute; the US voluntary standards body that publishes chamber and cartridge drawings and maximum average pressure (MAP)...
- Slamfire
- Unintended cartridge ignition during bolt closure, without a discrete trigger pull. Caused by free-floating firing pin inertia or primer over-sensitivity. Produces automatic-like...
- Squib
- A cartridge where primer fires but propellant combustion is partial or absent, leaving the projectile lodged in the barrel; the case shows...