A firearm is a device, often designed to be used as a weapon, which projects either single or multiple projectiles at high velocity through a controlled explosion. The firing is achieved by the gases produced through rapid, confined burning of a propellant. This process of rapid burning is technically known as deflagration. In older firearms, this propellant was typically black powder, but modern firearms use smokeless powder, cordite, or other propellants. Most modern firearms (with the notable exception of smoothbore shotguns) have rifled barrels to impart spin to the projectile for improved flight stability.

There exist a wide variety of firearms with different intended uses. Many firearms are employed as weapons, but many types of firearms exist that are not used as weapons, but as tools.

Background

In the Middle Ages the term "firearm" was used in English to denote the arm in which the match was held that was used to light the touch hole on the hand cannon. The term was a variation on the contemporary terms of bow arm and drawing arm still used in archery. Due to the effects of firing the ordnance (barrel) at the time, the gunner had to be located somewhat behind the weapon, steadying brace with the other hand, hence the name "hand gun" became synonymous with the "fire arm". Although the modern term 'gun' is often used as a synonym for firearm, in specialist or military use it has a restricted sense referring only to an artillery piece with a relatively high muzzle velocity, such as a field gun, a tank gun, or a gun used in the delivery of naval gunfire. Artillery guns are much larger than these firearms, mounted on a movable carriage, having bores of up to 18 inches (46 cm) and possibly weighing many tons. Strictly speaking, such weapons are not firearms.

Hand-held firearms, like rifles, carbines, pistols and other small firearms are rarely called "guns" in the restricted sense among specialists. Machine guns fire small caliber ammunition (generally 14.5 mm or smaller), and many machine guns are crew served infantry support weapons, requiring the services of more than one crewman, just like artillery guns. Generally, an automatic firearm designed for a single user is referred to as an automatic rifle. Other terms, including "firearm" itself, have been defined in specialized ad hoc ways by various legislation.

In recent centuries, firearms have become the predominant weapons used by mankind. Modern warfare since the late Renaissance has relied upon firearms, with wide-ranging effects on military history and history in general. This created a whole new kind of battle, which molded modern-era armies.

For handguns and long guns, the projectile is a bullet, or in historical hand cannons, a shot. The shot was initially made from lead already used as ammunition for the slings, and ironically begun with ballistic shape of a modern bullet, but was rapidly replaced by the cast iron ball. The projectile is fired by the burning of the propellant, but in small arms rarely contains explosives itself as such ammunition is banned by the Hague Convention. The use of expanding (e.g. hollow-point) small-arms ammunition in warfare is also banned by the Convention for similar reason (it aggravates the severity of wounds from small-arms fire). For modern artillery the projectile is a shell, which almost always contains explosives, and explosives were also common in older artillery pieces as well.