Shotgun, a shoulder weapon used primarily for hunting birds and small game. Shotguns are also used in hunting deer, in the sports of skeet shooting and trapshooting and, to some extent, in police work. A shotgun has a smooth-bore barrel from which small metal pellets, called shot, are usually fired. (For deer hunting, a lead slug is used in place of shot.) A cartridge, or shell, holds the shot (or slug) prior to shooting.

Shotguns are made in both single-barrel (in single-shot and repeating types) and double-barrel models. The size of the bore of a shotgun barrel is stated as its gauge. Gauge has a historical origin. It was defined as the number of round balls with the same diameter as the bore that could be molded from one pound of lead. Common shotgun sizes include (in order of descending size) 12 gauge (.725 inch), 16 gauge (.665 inch), and 20 gauge (.615 inch). Shotgun sizes in countries using the metric system generally are referred to by caliber. The calibers corresponding to 12, 16, and 20 gauge are 18.2 mm, 16.8 mm, and 15.7 mm.

Most shotguns have some choke, or narrowing of the bore near the muzzle. The choke affects the rate at which the shot spread after they leave the barrel. Many double-barrel shotguns have a different choke in each barrel, giving a choice for short or long range. Some single-barrel models have adjustable choke.

The shotgun developed from the blunderbuss, a large-bore gun with a bell-shaped muzzle, popular in the 18th century.