gun for hire vs hired gun

gun for hire and hired gun both are nouns.

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

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

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

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