field gun vs heavy weapon

field gun and heavy weapon both are nouns.

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

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

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

field gun (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