gun - 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.