[1] : WordNet (r) 2.0

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

[2] : Jargon File (4.3.1, 29 Jun 2001)

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.

Gun >> G Words
Gun, definition of term: Gun
gun_pag1.html

Gun Control >> G Words
Gun Control, definition of term: Gun Control
gun+control_pag1.html

Gun Barrel >> G Words
Gun Barrel, definition of term: Gun Barrel
gun+barrel_pag1.html

Bb Gun >> B Words
Bb Gun, definition of term: Bb Gun
bb+gun_pag1.html

Field Gun >> F Words
Field Gun, definition of term: Field Gun
field+gun_pag1.html

Gatling Gun >> G Words
Gatling Gun, definition of term: Gatling Gun
gatling+gun_pag1.html

Gun Barrel City >> G Words
Gun Barrel City, definition of term: Gun Barrel City
gun+barrel+city_pag1.html