automatic firearm vs automatic gun

automatic firearm and automatic gun both are nouns.

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

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

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

Other synonyms of automatic firearm include automatic weapon.

automatic firearm (noun)	automatic gun (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