Imperial Calibre Notation
Definition
Calibre designation using decimal fractions of an inch, often with a name suffix indicating the designer or intended firearm (e.g. .45 ACP, .38 Special, .357 Magnum); standard in US commercial production.
- Basis
- Decimal fraction of an inch
- Example
- .45 ACP, .38 Special, .357 Magnum
- Common region
- United States commercial production
- Contrast
- Metric calibre notation, e.g. 9mm
Common questions
Why do some imperial calibre names not match the actual bullet diameter?+
Historical naming conventions and marketing by the original designer often diverge from the true measured diameter, so .38 Special and .357 Magnum rounds actually fire bullets of nearly identical diameter despite the different names.
How does the name suffix help firearms identification?+
The suffix, such as ACP for Automatic Colt Pistol or Special and Magnum for case variants, narrows down the likely firearm chambering and cartridge family, supporting a match between recovered ammunition components and a specific weapon type.
Related terms
- CIP
- Commission Internationale Permanente pour l'Epreuve des Armes a Feu Portatives; the European proof body that standardises chamber and cartridge specifications for European...
- GRC Database
- General Rifling Characteristics database maintained by the FBI Laboratory's Firearms and Toolmarks Unit; includes an ammunition component section that references headstamp codes...
- IOF
- Indian Ordnance Factory headstamp code (produced at Khadki / Kirkee, Pune); the primary Indian government producer of 9x19mm, .38 Special, and 7.62x51mm...
- LC
- Lake City Army Ammunition Plant headstamp code; the primary US military ammunition producer for small-arms calibres including 5.56x45mm, 7.62x51mm, and .50 BMG.
- Metric Calibre Notation
- Calibre designation using bullet diameter in millimetres multiplied by case length in millimetres (e.g. 9x19mm, 7.62x39mm, 5.56x45mm); standard in NATO and European...
- NABIS
- National Ballistics Intelligence Service; the UK police intelligence service that maintains a national headstamp and firearm reference collection and coordinates ballistic intelligence...
- NATO Cross
- A circle with cross symbol stamped on military cartridge headstamps confirming the round meets the relevant NATO STANAG specification; found on production...
- Primer Sealant
- Coloured lacquer applied at the primer-to-case-head junction on military cartridges to prevent moisture ingress; colour varies by manufacturer and is not a...
- RG
- Radway Green headstamp code (UK); BAE Systems' Royal Ordnance Factories facility and primary UK military small-arms ammunition producer.
- S+B
- Sellier and Bellot headstamp code (Czech Republic, now part of CBC Group); a major European commercial and military-contract producer exported globally including...
- SAAMI
- Sporting Arms and Ammunition Manufacturers' Institute; the US voluntary standards body that publishes chamber and cartridge drawings and maximum average pressure (MAP)...