assault gun vs automatic rifle

assault gun and automatic rifle both are nouns.

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

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

assault gun: any of the automatic rifles or semiautomatic rifles with large magazines designed for military use
automatic rifle: self-loading rifle capable of automatic fire

Other hypernyms of assault gun include automatic, machine rifle.

assault gun (noun)	automatic rifle (noun)
an armored vehicle with the chassis of a tank (but no turret) and a large gun; used as an antitank weapon and to support infantry	self-loading rifle capable of automatic fire
any of the automatic rifles or semiautomatic rifles with large magazines designed for military use