The Collaborative International Dictionary of English v.0.48:

Maxim gun \Max"im gun`\
   A kind of machine gun; -- named after its inventor, Hiram S.
   Maxim.
   [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

Maxim gun
    n 1: an obsolete water-cooled machine gun having a single barrel