ammo vs weapons system

ammo and weapons system both are nouns.

Word	Noun	Adjective	Verb	Adverb
ammo	Yes	No	No	No
weapons system	Yes	No	No	No

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

Other hypernyms of ammo include arms, implements of war, munition, weaponry.

ammo (noun)	weapons system (noun)
projectiles to be fired from a gun	weapons considered collectively