A shotgun is generally a smoothbore firearm, which means that the inside of the barrel is not rifled. Preceding smoothbore firearms, such as the musket, were widely used by armies in the 18th century. The direct ancestor to the shotgun, the blunderbuss, was also used in a similar variety of roles from self defence to riot control. It was often used by cavalry troops due to its generally shorter length and ease of use, as well as by coachmen for its substantial power. However, in the 19th century, these weapons were largely replaced on the battlefield with breechloading rifled firearms, which were more accurate over longer ranges. The military value of shotguns was rediscovered in the First World War, when American forces used 12-gauge pump action shotguns in close-quarters trench fighting to great effect. Since then, it has been used in a variety of roles in civilian, law enforcement, and military applications.
The shot pellets from a shotgun spread upon leaving the barrel, and the power of the burning charge is divided among the pellets, which means that the energy of any one ball of shot is fairly low. In a hunting context, this makes shotguns useful primarily for hunting birds and other small game. However, in a military or law enforcement context, the large number of projectiles makes the shotgun useful as a close quarters combat weapon or a defensive weapon. Shotguns are also used for target shooting sports such as skeet, trap, and sporting clays. These involve shooting clay disks, known as clay pigeons, thrown in various ways.
The typical use of a shotgun is against small and/or fast moving targets, often taken while in the air. The spreading of the shot allows the user to point the shotgun close to the target, rather than having to aim precisely as in the case of a single projectile. The disadvantages of shot are limited range and limited penetration of the shot, which is why shotguns are used at short ranges, and typically against smaller targets. Larger shot size, up to the extreme case of the single projectile slug load, results in increased penetration, but at the expense of fewer projectiles and lower probability of hitting the target.
Aside from the most common use against small, fast moving targets, the shotgun has several advantages when used against still targets. First, it has enormous stopping power at short range, more than nearly all handguns and many rifles. Though many believe the shotgun is a great firearm for inexperienced shooters, the truth is, at close range, the spread of shot is not very large at all, and competency in aiming is still required. A typical self-defense load of buckshot contains 8-27 large lead pellets, resulting in many wound tracks in the target. Also, unlike a fully jacketed rifle bullet, each pellet of shot is less likely to penetrate walls and hit bystanders. It is favored by law enforcement for its low penetration and high stopping power.