Calibers and Cartridges: How to Read Ammunition Names
By Weapons.com Editorial Team · Published
What a cartridge name tells you, why '45 Auto' and '45 ACP' are the same thing, and the one rule that matters most: match the headstamp to the barrel.
Ammunition names can look like a jumble of numbers, decimals and brand names. Once you know the pattern, they're easier to read, and reading them correctly is a safety issue.
Caliber vs. cartridge
Caliber refers to the bore or bullet diameter, written in inches (like 45 or 308) or millimeters (like 9mm). A cartridge name identifies one specific, standardized round. Two cartridges can share a caliber number and still be different and not interchangeable, which is why the full name matters.
Who standardizes the names
In the U.S., SAAMI (the Sporting Arms and Ammunition Manufacturers' Institute) publishes American National Standards for cartridges and chambers. SAAMI was created in 1926 at the request of the U.S. government.
SAAMI's FAQ explains that under its standard, each cartridge has two recognized names: a full version and an abbreviated version. For example, 45 Automatic is abbreviated 45 Auto. You'll also see informal names like "45 ACP" on boxes and in conversation.
SAAMI also notes that in standard names, no decimal point appears before the caliber number: "308 Win" rather than ".308 Win." Metric names keep decimals where needed, as in 6.8mm Remington SPC.
Shotguns use gauge
Shotgun shells are usually labeled by gauge (like 12 gauge or 20 gauge) rather than caliber. As with cartridges, use only the gauge the shotgun is marked for.
The rule that matters most: match the marks
SAAMI's guidance is to make sure the cartridge designation on the ammunition's headstamp (the markings on the base of the case) matches the designation marked on the firearm's barrel. For example, 9mm Luger ammunition goes in a firearm marked 9mm Luger, and a 12 gauge shell goes in a shotgun marked 12 gauge. If a chamber is designed for more than one cartridge, check the owner's manual. The NRA's rules say the same thing: don't shoot unless you know you have the proper ammunition.
Ammunition should chamber easily. SAAMI warns never to fire a cartridge that needs abnormal force to close the bolt or breech.
Next steps
Once you know your cartridge, compare ammo prices, and read how to store and handle ammunition.