Polymer-cased ammunition is a plastic rimfire cartridge.
A plastic cylinder makes up the body of the cartridge. Starting from the back of the interior of the cartridge: The primer material. A priming method for rimfire cartridges is disclosed in which a propellant solution is disposed adjacent a centrifugally located primer material in a rimfire cartridge. The solvent is then evaporated to leave a propellant film near the primer material in order to provide slower, more uniform ignition to thereby allow use of propellant powders which would otherwise be too fast or too sensitive. Then comes the main charge of smokeless powder. Next is the projectile/projectiles. This can be a variety of things, from multiple shot (bird shot), a single-bullet projectile, or a saboted flechette, and then, if necessary, the sabot/sabots. Sabots are a piece of plastic that fits around the projectile. The plastic fits snugly against the rifling of the barrel and thus makes the cartridge more accurate if the projectile does not fit snugly against the rifling.
The Steyr ACR is the most notable user of polymer cased ammunition. It is a bullpup, rimfire rifle, that shoots a 5.56 X 45 mm flechette cartridge. The muzzle velocity is 4,757 ft/s. The flechette design allows it to retain much of its velocity and kinetic energy as it travels through the air; making it shoot very flat. This allows the rifle to be "sighted in" at 50 yards and still maintain close to the same point of impact out to a range of 300 yards or farther. The Steyr IWS 2000 is a closely related anti-material rifle. It shoots a 15.2mm saboted, fin-stabilized, armor-piercing flechette made of tungsten carbide or depleted uranium.