field artillery vs ordnance

field artillery and ordnance both are nouns.

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

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

field artillery: 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 artillery include artillery, gun, heavy weapon.

field artillery (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