The Collaborative International Dictionary of English v.0.48:

Six-shooter \Six"-shoot`er\, n.
   A pistol or other firearm which can be fired six times
   without reloading especially, a six-chambered revolver.
   [Colloq. U.S.]
   [1913 Webster]

WordNet (r) 3.0 (2006):

six-shooter
    n 1: a pistol with a revolving cylinder (usually having six
         chambers for bullets) [syn: revolver, six-gun, six-
         shooter]