WordNet (r) 3.0 (2006):

hired gun
    n 1: a professional killer who uses a gun [syn: gunman,
         gunslinger, hired gun, gun, gun for hire,
         triggerman, hit man, hitman, torpedo, shooter]

The Jargon File (version 4.4.7, 29 Dec 2003):

hired gun
 n.

    A contract programmer, as opposed to a full-time staff member. All the
    connotations of this term suggested by innumerable spaghetti Westerns are
    intentional.