From Halopedia, the Halo wiki
|There is more information available on this subject at 12.7×99mm Armor-Piercing on the English Wikipedia.
This round, also known as the .50 BMG, was first developed in the early 20th century for use in machine guns and heavy automatic weapons. It was an upscale of the .30-06 cartridge common to rifles of the time. The cartridge itself has been made in many variants. The rounds intended for machine guns are linked using metallic links. It was later refined for use in sniper rifle systems, setting several distance records for sniper kills. In this role, it was mainly used as an anti-matériel round.
It is effective against both vehicles and infantry, regardless of shielding. Its armor-piercing nature is the reason for its effectiveness against vehicles; armor piercing rounds are designed to shoot through armor to damage the occupants of the vehicle. While unshielded infantry are easily killed, energy shielding defeats most of the armor-piercing benefits of this round. However, it is still effective due to its large size and high kinetic energy. Only the SRS99 series of sniper rifle uses a larger round.
This round should not be confused with the M225 SAP-HE rounds that are used with the M6 Series pistol. The pistol rounds are smaller, with less powder and thus lower muzzle velocity and kinetic energy. They also operate on a different system, mixing both armor-piercing and high-explosive properties. This round is not high-explosive.