ordnance vs field gun

ordnance and field gun both are nouns.

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

As nouns, field gun is a hyponym of ordnance; that is, field gun is a word with a more specific, narrower meaning than ordnance:

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

Other hyponyms of ordnance include cannon, field artillery, four-pounder, depth charge thrower.

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