n (military) A cannon with relatively long barrel, operating with relatively low angle of fire, and having a high muzzle velocity.JP 1-02. Department of Defense Dictionary of Military and Associated Terms, 8 November 2010 (As Amended Through 15 March 2012). Page 142. ([//www.dtic.mil/doctrine/dod_dictionary/ Searchable online version])
n (military) A cannon with a 6-inch/155mm minimum nominalbore diameter and tube length 30 calibers or more. See also: howitzer; mortar.
n Any device or tool that projects a substance in a superficially similar fashion to a firearm, e.g., nail gun, squirt gun, spray gun, grease gun.
n A device or tool shaped like a pistol and operated in similar fashion by pulling a trigger with the index finger, e.g., rivet gun, screw gun, price-label gun.
n (surfing) A long surfboard designed for surfing big waves (not the same as a longboard, a gun has a pointed nose and is generally a little narrower).
n (context|cellular automata) A pattern that "fires" out other patterns.