14.5×114mm
From Halopedia, the Halo wiki
File:Sabot separating.gif
Sabot separating
The 14.5x114mm Armor-Piercing Fin-Stabilized Discarding-Sabot bullet is used by the United Nations Space Command's Sniper Rifle System 99C-S2 AM and Sniper Rifle System 99D-S2 AMs. Possibly similar to rounds employed by the Soviet Union during the twentieth century, the rounds is employed in an anti-materiel role, used to damage or destroy enemy equipment and vehicles. Thick Covenant armor, and the use of energy shields by Elites and Jackals, has seen it take on an anti-personnel role instead.
