gun down vs gun

gun down is not a noun while gun is a noun.

Word	Noun	Adjective	Verb	Adverb
gun down	No	No	Yes	No
gun	Yes	No	Yes	No

gun down (noun)	gun (noun)
	the discharge of a firearm as signal or as a salute in military ceremonies
	a weapon that discharges a missile at high velocity (especially from a metal tube or barrel)
	large but transportable armament
	a hand-operated pump that resembles a pistol; forces grease into parts of a machine
	a pedal that controls the throttle valve
	a professional killer who uses a gun
	a person who shoots a gun (as regards their ability)

gun down (verb)	gun (verb)
strike down or shoot down	shoot with a gun