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-matériel role, used to damage or destroy enemy equipment and vehicles. However, thick Covenant armor, and the use of energy shields by Sangheili and Kig-yar, has caused it to take on an anti-personnel role in addition to its 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. These 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, at least compared to that of a weapon firing rounds that would tumble or fragment. One possible reason for the round's effectivness agains living targets is that, while the round itself won't deform, fragment or tumble while passing through its target, it would shatter and force fragments of the target's armour from the impact site into the target's vital organs.

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

 * Currently the term "APFSDS" is reserved for large Tank mounted weapons. In small arms such as the Halo sniper rifles it is called a Flechette.
 * 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, later used in Anti-Aircraft and Vehicle-Mounted roles with the KPV; it remains in use to this day.
 * Its armor piercing capability can be seen in a single bullet firing through multiple Spartans and Elites with full Energy Shields.
 * It also bears striking similarity to the 14.5 and 15.2mm APFSDS rounds developed by Steyr-Mannlicher AG & Co KG in the mid 1980's for their AMR 5075 and IWS 2000 sniper weapons.