Ammunition (commonly known as Ammo, or rounds of ammo) typically for firearms refers to the combination of a projectile, explosive propellant, and primer for a firearm. The ammunition for small arm is referred to in one of three ways, as caliber, gauge, or in millimeters. Shotguns are always referred to in gauges (except the .410). Pistols, and rifles can be referred to in either millimeters, or in caliber (100th of an inch).

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