field artillery vs heavy weapon

field artillery and heavy weapon both are nouns.

Word	Noun	Adjective	Verb	Adverb
field artillery	Yes	No	No	No
heavy weapon	Yes	No	No	No

As nouns, heavy weapon is a hypernym of field artillery; that is, heavy weapon 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)
heavy weapon: large but transportable armament

Other hypernyms of field artillery include artillery, gun, ordnance.

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