A shotgun is a firearm
A firearm is a weapon that launches one, or many, projectile at high velocity through confined burning of a propellant. This subsonic burning process is technically known as deflagration, as opposed to supersonic combustion known as a detonation. In older firearms, the propellant was typically...

 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
Lead shot
Lead shot is a collective term for small balls of lead. These were the original projectiles for muskets and early rifles, but today lead shot is fired primarily from shotguns. It is also used for a variety of other purposes...

, or a solid projectile called a slug
Shotgun slug
A shotgun slug is a heavy lead projectile, that may have pre-cut rifling, intended for use in a shotgun and often used for hunting large game. The first effective shotgun slug was introduced by Wilhelm Brenneke in 1898, and his design remains in use today...

. Shotguns come in a wide variety of sizes, ranging from 5.5 mm (.22 inch) bore
Gauge (bore diameter)
The gauge of a firearm is a unit of measurement used to express the diameter of the barrel. Gauge is determined from the weight of a solid sphere of lead that will fit the bore of the firearm, and is expressed as the multiplicative inverse of the sphere's weight as a fraction of a pound . Thus...

 up to 5 cm (2 inch) bore, and in a range of firearm operating mechanisms, including breech loading, single-barreled, double or combination gun
Combination gun
A combination gun is a break-action hunting firearm that comprises at least two barrels, a rifle barrel and a shotgun barrel, often but not always in an over and under configuration; side-by-side variations are known as cape guns. A ' refers to a combination gun that has three barrels...

, pump-action
A pump-action rifle or shotgun is one in which the handgrip can be pumped back and forth in order to eject a spent round of ammunition and to chamber a fresh one. It is much faster than a bolt-action and somewhat faster than a lever-action, as it does not require the trigger hand to be removed from...

, bolt-, and lever-action, semi-automatic, and even fully automatic variants.

A shotgun is generally a smoothbore
A smoothbore weapon is one which has a barrel without rifling. Smoothbores range from handheld firearms to powerful tank guns and large artillery mortars.-History of firearms and rifling:...

 firearm, which means that the inside of the barrel is not rifled
Rifling is the process of making helical grooves in the barrel of a gun or firearm, which imparts a spin to a projectile around its long axis...