heavy weapon vs ordnance

heavy weapon and ordnance both are nouns.

Word	Noun	Adjective	Verb	Adverb
heavy weapon	Yes	No	No	No
ordnance	Yes	No	No	No

As nouns, heavy weapon and ordnance are synonyms defined as:

Other synonyms of heavy weapon include artillery, gun.

heavy weapon (noun)	ordnance (noun)
large but transportable armament	military supplies
	large but transportable armament