A cartridge, also known as a round, is a type of pre-assembled firearm ammunition packaging a projectile (bullet, shot, or slug), a propellant substance (smokeless powder, black powder substitute, or black powder) and an ignition device (primer) within a metallic, paper, or plastic case that is precisely made to fit within the barrel chamber of a breechloading gun, for convenient transportation and handling during shooting. Although in popular usage the term "bullet" is often used to refer to a complete cartridge, the correct usage only refers to the projectile.
Cartridges can be categorized by the type of primer. This can be accomplished by igniting a small charge of an impact-sensitive explosive compound or by an electric-sensitive chemical mixture that is located: at the center of the case head (centerfire); or inside the rim (rimfire); or inside the walls on the fold of the case base that is shaped like a cup (cupfire); or in a sideways projection that is shaped like a pin (pinfire) or a lip (lipfire); or in a small bulge shaped like a nipple at the case base (teatfire). Only small-caliber rimfire cartridges and centerfire cartridges have survived into the modern day.
Military and commercial producers continue to pursue the goal of caseless ammunition. Some artillery ammunition uses the same cartridge concept as found in small arms. In other cases, the artillery shell is separate from the propellant charge.
A cartridge without a projectile is called a blank; one that is completely inert (contains no active primer and no propellant) is called a dummy; one that failed to ignite and shoot off the projectile is called a dud; and one that ignited but failed to sufficiently push the projectile out of the barrel is called a squib.
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