Ammo noun – Projectiles to be fired from a gun. Cartridge and ammo are semantically related. in small missile topic. In some cases you can use "Cartridge" instead a noun "Ammo", when it comes to topics like projectiles for weaponry.
Cartridge noun – Ammunition consisting of a cylindrical casing containing an explosive charge and a bullet; fired from a rifle or handgun. Ammo and cartridge are semantically related. You can use "Ammo" instead a noun "Cartridge".
The obvious exception is the Devastator of the Deathwatch, since their heavy bolters chew up ammo of such individual cartridge size that they more or less require an ammunition backpack.
Modern ADS uses both standard issue ammo for AK-74 (in the air) and new underwater cartridge that looks like the same 5.45x39 — but its bullet continues all the way to the bottom.
Heavy artillery ammo, nuclear or not, is never like a one-piece, round-nosed, giant rifle cartridge, it comes in either two parts or more before loading.