field gun vs ordnance

field gun and ordnance both are nouns.

Word	Noun	Adjective	Verb	Adverb
field gun	Yes	No	No	No
ordnance	Yes	No	No	No

As nouns, ordnance is a hypernym of field gun; that is, ordnance is a word with a broader meaning than field gun:

field gun: movable artillery (other than antiaircraft) used by armies in the field (especially for direct support of front-line troops)
ordnance: large but transportable armament

Other hypernyms of field gun include artillery, gun, heavy weapon.

field gun (noun)	ordnance (noun)
movable artillery (other than antiaircraft) used by armies in the field (especially for direct support of front-line troops)	military supplies
	large but transportable armament