horse pistol vs side arm

horse pistol and side arm both are nouns.

Word	Noun	Adjective	Verb	Adverb
horse pistol	Yes	No	No	No
side arm	Yes	No	No	No

As nouns, side arm is a hypernym of horse pistol; that is, side arm is a word with a broader meaning than horse pistol:

horse pistol: a large pistol (usually in a holster) formerly carried by horsemen
side arm: a firearm that is held and fired with one hand

Other hypernyms of horse pistol include handgun, pistol, shooting iron.

horse pistol (noun)	side arm (noun)
a large pistol (usually in a holster) formerly carried by horsemen	a firearm that is held and fired with one hand