The Collaborative International Dictionary of English v.0.48:

machine-gun \machine-gun\, machine gun \machine gun\a.
   Occurring in rapid succession, like the firing of a machine
   gun; as, Tom was a persuasive speaker, with a smooth deep
   voice, polysyllabic vocabulary, and a machine-gun
   articulation that overwhelmed listeners.

   Syn: rapid-fire.
        [PJC]