shotgun vs small-arm

shotgun and small-arm both are nouns.

Word	Noun	Adjective	Verb	Adverb
shotgun	Yes	No	No	No
small-arm	Yes	No	No	No

As nouns, small-arm is a hypernym of shotgun; that is, small-arm is a word with a broader meaning than shotgun:

shotgun: firearm that is a double-barreled smoothbore shoulder weapon for firing shot at short ranges
small-arm: a portable gun

Other hypernyms of shotgun include firearm, piece.

shotgun (noun)	small-arm (noun)
firearm that is a double-barreled smoothbore shoulder weapon for firing shot at short ranges	a portable gun