Girl With A Gun

Autocannon are automatic guns designed primarily to fire shells and are mounted on a vehicle or other mount. Machine guns are similar, but usually designed to fire simple projectiles. In some calibers and some usages, these two definitions overlap.
A related military use of the word is in describing gun-type fission weapon. In this instance, the "gun" is part of a nuclear weapon and contains an explosively propelled sub-critical slug of fissile material within a barrel to be fired into a second sub-critical mass in order to initiate the fission reaction. Potentially confused with this usage are small nuclear devices capable of being fired by artillery or recoilless rifle. In civilian use, a related item used in agriculture is a captive bolt gun. Such captive piston guns are often used to humanely stun farm animals for slaughter.