A
gun is a muzzle or breech-loaded
projectile-firing
weapon. There are various definitions depending on the nation and branch of service. A "gun" may be distinguished from other
firearms in being a
crew-served weapon such as a
howitzer or
mortar, as opposed to a small arm like a
rifle or
pistol, but there are exceptions, such as the
U.S. Air Force's
GUU5/P. At one time, land-based
artillery tubes were called
cannon and sea-based naval cannon were called guns. The term "gun" evolved into a
generic term for any tube-launched projectile-firing weapon used by
sailors, including
boarding parties and
marines.