field artillery vs field gun

field artillery and field gun both are nouns.

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

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

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

field artillery (noun)	field gun (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)