.390

10mm (.390)-caliber is an ammunition used in the 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.

The SPARTAN-IIs used this ammunition in their MA2Bs when they abducted Colonel Robert Watts.

Trivia
This round size may have been specifically created for the MA2B - its modern counterpart is an obscure Soviet assault rifle round.

List of appearances

 * Halo: The Fall of Reach
 * Halo: First Strike
 * Halo: Ghosts of Onyx