Ashotgun is a firearm that is usually designed to be fired from the shoulder, which uses the energy of a fixed shell to fire a number of small spherical pellets called shot, or a solid projectile called a slug.

Shotguns come in a wide variety of sizes, ranging from 5.5 mm bore up to 5 cm bore, and in a range of firearm operating mechanisms, including breech loading, single-barreled, double or combination gun, pump-action, bolt-, and lever-action, semi-automatic, and even fully automatic variants.

Source: Wikipedia