assault gun

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

• noun An assault weapon.
• noun A turretless armored vehicle having the chassis of a tank but mounting a larger gun, used in antitank combat and infantry support.

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

• noun an armored vehicle with the chassis of a tank (but no turret) and a large gun; used as an antitank weapon and to support infantry
• noun any of the automatic rifles or semiautomatic rifles with large magazines designed for military use