The Collaborative International Dictionary of English v.0.48:

machine gun \machine gun\ n.
   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.
   [PJC] machine-gun

WordNet (r) 3.0 (2006):

submachine gun
    n 1: machine gun that is a portable automatic firearm