The Collaborative International Dictionary of English v.0.48:

paint gun \paint" gun`\ (p[=a]nt" gun`), n.
   1. A device designed to rapidly cover a surface with paint by
      ejecting a spray of paint from a reservoir onto the
      surface by means of compressed air or other special
      mechanism. Also called a paint sprayer. Use of a paint
      gun is an efficient method to rapidly paint large surface
      areas.
      [PJC]

   2. A device resembling a rifle, designed to shoot a small
      quantity of paint at an opponent as part of a game often
      conducted at specially-designed indoor locations, or
      outdoors. Points are awarded to players who hit their
      target; success in hitting one's target is made obvious by
      the appearance of a spot of paint on the player who is
      hit.
      [PJC]