massed fire vs firing

massed fire and firing both are nouns.

Word	Noun	Adjective	Verb	Adverb
massed fire	Yes	No	No	No
firing	Yes	No	No	No

As nouns, firing is a hypernym of massed fire; that is, firing is a word with a broader meaning than massed fire:

massed fire: fire from two or more weapons directed at a single target or area (as fire by batteries of two or more warships)
firing: the act of firing weapons or artillery at an enemy

Other hypernyms of massed fire include fire.

massed fire (noun)	firing (noun)
fire from two or more weapons directed at a single target or area (as fire by batteries of two or more warships)	the act of setting something on fire
	the termination of someone's employment (leaving them free to depart)
	the act of discharging a gun
	the act of firing weapons or artillery at an enemy