10mm (.390)-caliber ammunition is an ammo size used by the UNSC MA2B assault rifles. It is a significantly larger round than the 7.62x51 NATO round used in the MA5B (7.62x51mm = 0.308 caliber). The .390 comes in at least two varieties: the standard Armor-Piercing round and "Shredder" rounds made from depleted Uranium.
Trivia
- This round size may have been specifically created for the MA2B - its modern counterpart is an obscure Soviet assault rifle round.
- Most human weapons in the Halo series use current ammunition sizes, this round is among the several that don't.
- The Spartan-II's used this round size in their MA2Bs when they abducted Colonel Robert Watts.