six-shooter vs side arm

six-shooter and side arm both are nouns.

Word	Noun	Adjective	Verb	Adverb
six-shooter	Yes	No	No	No
side arm	Yes	No	No	No

As nouns, side arm is a hypernym of six-shooter; that is, side arm is a word with a broader meaning than six-shooter:

six-shooter: a pistol with a revolving cylinder (usually having six chambers for bullets)
side arm: a firearm that is held and fired with one hand

Other hypernyms of six-shooter include handgun, pistol, shooting iron.

six-shooter (noun)	side arm (noun)
a pistol with a revolving cylinder (usually having six chambers for bullets)	a firearm that is held and fired with one hand