A gun is a common name given to an object that fires high-velocity projectiles. The projectile is fired through a hollow tube known as the gun's barrel. The projectile's caliber, or diameter, is usually designated in fractions of an inch or in millimeters. Differing from the musket, most modern guns are rifled, with a series of grooves spiraling along the barrel; exceptions include smoothbores on tanks, AFVs and some artillery.

The term "gun" is often used synonymously with firearm, but this is common only for civilian usage. In military usage, the term refers only to artillery that fires projectiles at high velocity, such as naval guns (which are never referred to as cannon) or tank guns. A gunner is a member of the team charged with the task of operating and firing a gun. By military terms, mortars and all hand-held firearms are excluded from the definition of guns. The exception to this is the shotgun, which is hand-held, has a smooth bore and fires a load of shot or a single projectile known as a slug.

The word "gun" is also applied to some more or less vaguely gun-like or gun-shaped 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".

Source : http://en.wikipedia.org