From Halopedia, the Halo wiki
10mm (.390)-caliber is a round used in the MA2B assault rifles. It is a significantly larger round than the 7.62×51 NATO round used in the MA5B (7.62×51mm = 0.308 caliber). The .390 comes in at least two varieties: the standard armor piercing round and "shredder" rounds made from depleted Uranium.
This round size may have been specifically created for the MA2B - its modern counterpart is an obscure Soviet assault rifle round.
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