Old shells, bullets and fuses uncapped (9)

'old shells' is the definition.
The definition and answer can be both natural objects as well as being plural nouns.
Perhaps there's a link between them I don't understand?

'bullets and fuses uncapped' is the wordplay.
'bullets' becomes 'ammo' (I've seen this in another clue).
'and' says to put letters next to each other.
'fuses' becomes 'unites' (uniting is a kind of fusing).
'uncapped' means to remove the first letter.
'unites' with its initial letter taken away is 'nites'.
'ammo'+'nites'='AMMONITES'

Can you help me to learn more?