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.[1] Shotguns are always referred to in gauges (except the .410). Pistols/Submachine guns/Personal defense weapons, Rifles/Carbines and Machine guns/Autocannons can be referred to in either millimeters, or in caliber (100th of an inch).