ammo vs arms

ammo and arms both are nouns.

Word	Noun	Adjective	Verb	Adverb
ammo	Yes	No	No	No
arms	Yes	No	No	No

As nouns, arms is a hypernym of ammo; that is, arms is a word with a broader meaning than ammo:

Other hypernyms of ammo include implements of war, munition, weaponry, weapons system.

ammo (noun)	arms (noun)
projectiles to be fired from a gun	weapons considered collectively
	the official symbols of a family, state, etc.