Polymer-cased ammunition (PCA) is firearm ammunition (cartridge) with casings made from synthetic polymer instead of the typical metallic casing. PCA is commonly seen among shotgun shells, and is considered a new alternative for long guns and handguns that potentially reduces ammunition production cost and weight.
A shotgun cartridge, shotshell, or shell is a type of rimmed, cylindrical (straight-walled) ammunition used specifically in shotguns. It is typically loaded with numerous small, spherical sub-projectiles called shot. Shotguns typically use a smoothbore barrel with a tapered constriction at the muzzle to regulate the extent of scattering.
Some cartridges contain a single solid projectile known as a slug (sometimes fired through a rifled slug barrel). The casing usually consists of a paper or plastic tube with a metallic base containing the primer. The shot charge is typically contained by wadding inside the case. The caliber of the cartridge is known as its gauge.
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 or percussion cap) 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.
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.