six-gun vs shooting iron

six-gun and shooting iron both are nouns.

Word	Noun	Adjective	Verb	Adverb
six-gun	Yes	No	No	No
shooting iron	Yes	No	No	No

As nouns, shooting iron is a hypernym of six-gun; that is, shooting iron is a word with a broader meaning than six-gun:

six-gun: a pistol with a revolving cylinder (usually having six chambers for bullets)
shooting iron: a firearm that is held and fired with one hand

Other hypernyms of six-gun include handgun, pistol, side arm.

six-gun (noun)	shooting iron (noun)
a pistol with a revolving cylinder (usually having six chambers for bullets)	a firearm that is held and fired with one hand