WordNet sense 1 (a person who shoots a gun (as regards their ability)):

gunman

WordNet sense 2 (a professional killer who uses a gun):

gunslinger, shooter, torpedo, hitman, triggerman, gun for hire, hired gun

WordNet sense 3 (a hand-operated pump that resembles a revolver; forces grease into parts of a machine):

grease-gun

WordNet sense 4 (large but transportable armament):

artillery, ordnance, heavy weapon

WordNet sense 5 (a pedal that controls the throttle valve):

accelerator, gas, throttle, accelerator pedal, gas pedal