First definition here: http://www.thefreedictionary.com/gun
1. A weapon consisting of a metal tube from which a projectile is fired at high velocity into a relatively flat trajectory.
Torpedoes are self propelled. Gun projectiles are not (unless labelled "rocket assisted ordinance" and even then the rockets only come on near the target).
That's what makes something a projectile; lack of it's own propulsion (I think).