Big Sandy Shoot, Machine Gun Paradise, United States

In military parlance, 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 USAF's GUU5/P. At one time, land based artillery tubes were called cannon and sea-based naval cannon were called guns. The term "gun" morphed into a generic term for any tube launched projectile firing weapon used by sailors including boarding parties and marines.
In modern parlance, a gun is a projectile weapon using a hollow, tubular barrel with a closed end—the breech—as the means of directing the projectile (as well as other purposes, for example stabilizing the projectile's trajectory, aiming, as an expansion chamber for propellant, etc), and firing in a generally flat trajectory.