hired gun vs gun for hire

hired gun and gun for hire both are nouns.

Word	Noun	Adjective	Verb	Adverb
hired gun	Yes	No	No	No
gun for hire	Yes	No	No	No

As nouns, hired gun and gun for hire are synonyms defined as:

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

hired gun (noun)	gun for hire (noun)
a professional killer who uses a gun	a professional killer who uses a gun