gun
A device resembling a firearm or cannon, as in its ability to project something, such as grease, under pressure or at great speed. [...] A portable firearm, such as a rifle or revolver. [1]
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. [...] A gun is a muzzle or breech-loaded projectile -firing weapon. [2]
A cannon with a long barrel and a relatively low angle of fire. [1]
Most guns are described by the type of barrel used, the means of firing, the purpose of the weapon, the caliber, or the commonly accepted name for a particular variation. [...] At one time, land based artillery tubes were called cannon and sea-based naval cannon were called guns. [2]
A weapon consisting of a metal tube from which a projectile is fired at high velocity into a relatively flat trajectory. [1]
VINTAGE H.H. HEISER HOLSTER COLT NEW SERVICE 6 IN. [3]
A less strict application of the word is to identify one artillery weapon system or non-machine gun projectile armament on aircraft. [...] In modern parlance, these weapons are breech-loaded and built primarily for long range fire with a low or almost flat ballistic arc. [2]
A discharge of a firearm or cannon as a signal or salute. [1]
The propellant may be air, an explosive solid, or an explosive liquid. [...] 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. [2]
A cannon with relatively long barrel, operating with relatively low angle of fire, and having a high muzzle velocity. [1]
Sources:
[1] gun - definition of gun by the Free Online Dictionary …
[2] Gun - Wikipedia
[3] Guns - Online Gun Auction - Guns for Sale at GunBroker.com