The Collaborative International Dictionary of English v.0.48:

Spray gun \Spray" gun`\ (spr[=a]"g[u^]n`), n.
   a container for liquid, fitted with a conduit that can be
   supplied with compressed air, desiged to allowing the
   contents to be sprayed as an aerosol. Such devices are used
   to spray paints and various other liquids.
   [PJC]

WordNet (r) 3.0 (2006):

spray gun
    n 1: an applicator resembling a gun for applying liquid
         substances (as paint) in the form of a spray