hitman vs hired gun

hitman and hired gun both are nouns.

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

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

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

hitman (noun)	hired gun (noun)
a professional killer who uses a gun	a professional killer who uses a gun