automatic vs automatic gun

automatic and automatic gun both are nouns.

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

As nouns, automatic gun is a hypernym of automatic; that is, automatic gun is a word with a broader meaning than automatic:

automatic: a pistol that will keep firing until the ammunition is gone or the trigger is released
automatic gun: a firearm that reloads itself and keeps firing until the trigger is released

Other hypernyms of automatic include automatic firearm, automatic weapon, handgun, pistol, shooting iron, side arm.

automatic (noun)	automatic gun (noun)
self-loading rifle capable of automatic fire	a firearm that reloads itself and keeps firing until the trigger is released
a pistol that will keep firing until the ammunition is gone or the trigger is released

automatic (adjective)	automatic gun (adjective)
operating with minimal human intervention; independent of external control
without volition or conscious control
resembling the unthinking functioning of a machine