14.5×114mm

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.

Background
Possibly similar to an ammunition type employed by the Soviet Union during the twentieth century, the round is employed in an anti-material role, used to damage or destroy enemy equipment and vehicles. However, thick Covenant armor, and the use of energy shields by Elites and Jackals, has caused it to take on an anti-personnel role in addition to it's anti-armor roles.

The 14.5mm sniper round has less effect on living targets due to the ammunition type- APFSDS, or "Armor-Piercing, Fin-Stabilized, Discarding-Sabot" rounds, which are designed for penetration and NOT terminal performance, this type of bullet does not fragment nor does it tumble, which would cause maximum damage to a target but instead punches through with extreme velocity, so the actual wound it creates is rather unimpressive, compared to that of a weapon firing rounds that would tumble or fragment. APFSDS rounds were originally intended for use in armor vehicle main weapons (IE 90mm-120mm smoothbore guns) to defeat armored vehicles due to the increasingly ineffectiveness of HEAT and HESH shells against modern armors, though many other large caliber weapons have seen use with it.

Trivia

 * The 14.5x114mm round is actually a Russian heavy machine gun round that was developed for use in anti-tank rifles like the PTRD and PTRS in World War II, and it remains in use to this day..