14.5×114mm
From Halopedia, the Halo wiki
Template:FactOrFiction The 14.5 x 114mm bullet is used by the United Nations Space Command's Sniper Rifle System 99C-S2 AM and Sniper Rifle System 99D-S2 AM. It was originally developed by the Soviet Union during the Twentieth Century. It was originally designed to pierce the armor of a tank, but the heavy vehichles of the Twenty-Sixth Centruy can easily withstand a strike from this round. It is one of the oldest rounds in the UNSC's armament as its use can be traced back to World War II.
