The Collaborative International Dictionary of English v.0.48:

staple gun \sta"ple gun`\, n.
   A device used to drive a heavy staple[8] through multiple
   objects, so as to fasten them together; it has a spring
   mechanism which stores force as a lever is pulled by the
   operator's hand, and the force is released all at once to
   drive the staple fully into the object to be fastened.
   [PJC]

WordNet (r) 3.0 (2006):

staple gun
    n 1: a hand-held machine for driving staples home [syn: staple
         gun, staplegun, tacker]