assault gun vs machine rifle

assault gun and machine rifle both are nouns.

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

As nouns, machine rifle is a hypernym of assault gun; that is, machine 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
machine rifle: self-loading rifle capable of automatic fire

Other hypernyms of assault gun include automatic, automatic rifle.

assault gun (noun)	machine 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