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 distinguised 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 for directing the projectile as well as other purposes—an expansion chamber for propellant, stabilizing the projectile's trajectory, aiming, etc.—and assumes a generally, flat trajectory for the projectile.

The generic form of a trigger-initiated, hand-held, and hand-directed tool with an extending bore has additionally been applied to implements resembling guns in either form or concept. Examples of this application include items such as staple guns, nail guns, and glue guns. Occasionally, this tendency is ironically reversed, such as the case of the American M3 submachine gun which carries the nickname "Grease Gun".

The projectile may be a simple, single-piece item like a bullet, a casing containing a payload like a shotshell or explosive shell, or complex projectile like a sub-caliber projectile and sabot. The propellant may be air, an explosive solid, or an explosive liquid. Some variations like the Gyrojet and certain other types combine the projectile and propellant into a single item.

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.

Barrel types include rifled—a series of spiraled grooves or angles within the barrel—when the projectile requires an induced spin to stabilize it and smoothbore when the projectile is stabilized by other means or is undesired or unnecessary. Typically, interior barrel diameter and the associated projectile size is a means to identify gun variations. Barrel diameter is reported in several ways. The more conventional measure is reporting the interior diameter of the barrel in decimal fractions of the inch or in millimeters. Some guns—such as shotguns—report the weapon's gauge or—as in some British ordnance—the weight of the weapon's usual projectile.

Terminology

In military use, the term "gun" refers primarily to direct fire weapons that capitalize on their velocity for penetration or range. In modern parlance, these weapons are breech-loaded and built primarily for long range fire with a low or almost flat ballistic arc. A variation is the howitzer or gun-howitzer designed to offer the ability to fire both low or high-angle ballistic arcs. In this use, example guns include naval guns. A less strict application of the word is to identify one artillery weapon system or non-machine gun projectile armament on aircraft.