automatic gun vs automatic firearm

automatic gun and automatic firearm both are nouns.

Word	Noun	Adjective	Verb	Adverb
automatic gun	Yes	No	No	No
automatic firearm	Yes	No	No	No

As nouns, automatic gun and automatic firearm are synonyms defined as:

automatic gun and automatic firearm: a firearm that reloads itself and keeps firing until the trigger is released

Other synonyms of automatic gun include automatic weapon.

automatic gun (noun)	automatic firearm (noun)
a firearm that reloads itself and keeps firing until the trigger is released	a firearm that reloads itself and keeps firing until the trigger is released