shooter vs hired gun

shooter and hired gun both are nouns.

Word	Noun	Adjective	Verb	Adverb
shooter	Yes	No	No	No
hired gun	Yes	No	No	No

As nouns, shooter and hired gun are synonyms defined as:

Other synonyms of shooter include gun, gun for hire, gunman, gunslinger, hit man, hitman, torpedo, triggerman.

shooter (noun)	hired gun (noun)
a large marble used for shooting in the game of marbles	a professional killer who uses a gun
a person who shoots (usually with respect to their ability to shoot)
a professional killer who uses a gun
(sports) a player who drives or kicks a ball at the goal (or a basketball player who shoots at the basket)
a gambler who throws dice in the game of craps