In modern parlance, a gun is a projectile weapon using a hollow, tubular barrel 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.

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