machine gun

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A gun that fires rapidly and continuously as long as the trigger is held back.

from The Century Dictionary.

• noun A gun which, by means of a variously contrived mechanism, delivers a continuous fire of projectiles.

from the GNU version of the Collaborative International Dictionary of English.

• noun A fully automatic rapid-firing rifle, which continues to fire bullets repeatedly as long as the trigger is depressed; lighter versions may be carried in the hands, and heavier versions may be mounted on a tripod, vehicle, or other mount. The lighweight versions are sometimes called a submachine gun.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun A type of firearm that continuously fires bullets in rapid succession by a single action of the trigger.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• verb shoot with a machine gun
• noun a rapidly firing automatic gun (often mounted)