horse pistol vs handgun

horse pistol and handgun both are nouns.

Word	Noun	Adjective	Verb	Adverb
horse pistol	Yes	No	No	No
handgun	Yes	No	No	No

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

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

Other hypernyms of horse pistol include pistol, shooting iron, side arm.

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