A gun is a mechanical device that fires projectiles at high velocity, using a propellant such as gun powder or compressed air. The projectile, usually designated by its diameter in inches (caliber) or millimeters, is fired through a hollow tube known as the gun's barrel. Related to muskets, the modern gun is more advanced in technology.
The term "gun" is often used as a synonym to firearm, but in military usage the term refers only to artillery that fires projectiles at high velocity, such as anti-aircraft artillery, field guns, tank guns, anti-tank guns and naval guns. A gunner is a member of the team charged with the task of operating and firing a gun. Mortars and all hand-held firearms are excluded from this definition. The exception to this is the shotgun, which is hand-held, has a smooth bore and fires a cartridge of pellets or a single projectile known as a slug.
See the related article on firearms for information on weapons informally known as guns.
The word "gun" is also applied to some more or less vaguely gun-like tools, such as staple guns and glue guns.
In a gun-type fission weapon the "gun" is part of a nuclear weapon. The "projectile" is fissile material that is fired and captured inside the device. In the case of nuclear artillery it should not be confused with the gun that fires the whole warhead.
At times, the word gun is used to describe the person holding the weapon rather than the weapon itself, as in "a hired gun".
Etymology: "gun" is found in Middle English as "gonne", and seems to come from the Germanic woman's name Gunhild or Gundhild = "war sword", applied to an early cannon.