field gun vs field artillery

field gun and field artillery both are nouns.

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

As nouns, field gun and field artillery are synonyms defined as:

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

field gun (noun)	field artillery (noun)
movable artillery (other than antiaircraft) used by armies in the field (especially for direct support of front-line troops)	movable artillery (other than antiaircraft) used by armies in the field (especially for direct support of front-line troops)