automatic gun vs automatic weapon

automatic gun and automatic weapon both are nouns.

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

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

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

Other synonyms of automatic gun include automatic firearm.

automatic gun (noun)	automatic weapon (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