.390-caliber ammunition is an ammo size used by the UNSC MA2B Assault Rifles. It is a significantly larger round than the M118 7.62x51mm round used in the MA5B (7.62x51mm = 0.308 caliber). The .390 comes in at least two varieties: the standard armor piercing rounds and shredder rounds made from depleted uranium.
- This round size may have been specifically created for the MA2B Assault rifles - 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.
- Spartan-II Blue Team used this round size in their MA2Bs when they apprehended Colonel Robert Watts.