A shotgun is a firearm typically used to fire a number of small balls, the shot, from a smoothbore tube of relatively large diameter. The energy of any one ball of shot is fairly low, making them useful primarily for hunting birds and other small game, or as close-combat weapons where the short range ensures that many of the balls of shot will hit the target.
The United States legal code (18 USC 921) defines the shotgun as "a weapon designed or redesigned, made or remade, and intended to be fired from the shoulder and designed or redesigned and made or remade to use the energy of the explosive in a fix the shotgun shell to fire through a smooth bore either a number of ball shot or a single projectile for each single pull of the trigger." United Kingdom law requires that a shotgun not be capable of holding more than three rounds; if it holds more it is classed as a firearm.
This definition, however, does not exactly match the technical use of the term, which would include the growing number of shotguns specifically designed to fire single projectiles instead of shot. Rifled slugs, which have fins or rifling on or behind the bullet designed to keep the bullet tracking straight at the target, is an example of a single projectile. Some shotguns have rifled barrels and are designed to be used with a "sabot" bullet. A sabot bullet is typically encased in two-piece plastic ring and the plastic is designed to fall away after it passes the end of the barrel, leaving the bullet to continue toward the target while twisting (from passing through the rifled barrel) to keep it's trajectory. These shotguns although they have rifled barrels still use a shotgun-style shell instead of a rifle cartridge. Hunting laws may differentiate between smooth barreled and rifled barreled guns.
Also, technically speaking, many people would likely call a fully automatic shotgun a shotgun, even though legally it would fall under a different category.
There are many types of shotguns, typically categorized by the number of barrels or the way the gun is reloaded. Since shotgun shells are much larger than bullets, it took longer for automatic or semi-automatic loading systems to be developed for them. For most of the history of the shotgun two barrels were used to allow for two shots, known as the double barreled shotgun. In this case there are several "subtypes", the over and under shotgun puts the two barrels one on top of the other, while the side-by-side shotgun puts them beside each other.
Semi-automated loading systems are also available. A small number of guns are available with a bolt action, but far more common is the pump action shotgun, in which a sliding handle, the pump, works the action to reload the single barrel. Several fully automatic actions have been developed since the 1960s, including the SPAS and NeoStead 2000.
The caliber of shotguns is measured in terms of gauge. The gauge number is determined by the number of solid spheres of a diameter equal to the inside diameter of the barrel that could be made from a pound of lead. So a 10 gauge shotgun has the inside diameter equal to that of a sphere made from one-tenth of a pound of lead. Common gauges are 12 and 20, although 8, 10, 16, 28 gauges and .410 calibre are also produced, the later measured in inches instead of gauge for historical reasons.
Despite the above mention of slugs and sabots most shotguns are used to fire "a number of ball shot". The ball shot or pellets are used to be lead but this has been replaced by bismuth, steel, tungsten-iron, tungsten-nickel-iron and even tungsten polymer loads. They are termed either birdshot or buckshot depending on the shot size. Informally birdshot pellets have a diameter smaller than 0.20 inches and buckshot larger. Pellet size is indicated by a cartridge number, for birdshot this ranges from the smallest 12 (0.05") to 2 (0.15") and then BB (0.18"), for buckshot the numbers usually start at 4 (0.24") and go down to 1, 0, 00 and finally 000 (0.36").
As the shot leaves the barrel there is a "puff" of gunpowder that can push the shot sideways, thereby spreading out the pattern of shot. To combat this problem most shotgun barrels come with a system known as a choke, a constriction near the end that directs the flow of gasses and thereby keeps the shot in a tight pattern. In increasing order of constriction: Improved (or 1/4), Modified (or 1/2), Improved Modified (or 3/4), and Full choke. "Cylinder barrels" have no constriction.
In hunting circles, the shotgun is used for bird hunting, although it is also increasingly used in deer hunting in semi-populated areas where the long-distance travel of the rifle bullet may pose too great a hazard. Many modern smooth bore shotguns using rifled slugs are extremely accurate out to 75 yards or more, while the rifled barrel shotgun with the use of sabot slugs are typically accurate to 100 yards and beyond -- well within the range of the majority of kill shots by experienced deer hunters using shotguns.
Law enforcement often use shotguns, especially for crowd and riot control where they may be loaded with non-lethal rounds such as rubber bullets or bean bags. The shotgun is also commonly used for home defense in the United States and Canada. It is particularly suitable for this purpose because it is very intimidating and will often end the assault with a single shot, if it comes to that.
"Shotgun" can also mean the passenger position next to the driver of a vehicle, from the tradition of that person defending the vehicle with a shotgun. In America, there is a tradition known as "calling shotgun", which determines who gets to sit next to the driver.
Among smokers of cannabis, shotgun can have several meanings:
Shotgun (used as a verb) also refers to a practice of drinking beer very quickly out of a can. When one shotguns a beer, he or she punches a hole in the side of the can near the bottom -- often using a key or other sharp instrument. The person then puts his or her mouth around the hole and opens the can in the usual manner. This causes the beer to be forced out of the can (and into the drinkers mouth) very rapidly.