12.7×99mm Armor-Piercing
From Halopedia, the Halo wiki
The 12.7x99mm AP, also known as the .50 Browning Machine Gun' .50 BMG and the .50 caliber' is a cartridge that first entered service in the early 20th Century. The round is based on a greatly scaled-up .30-06 cartridge. The cartridge itself has been made in many variants: multiple generations of regular ball, tracer, armor piercing, incendiary, and saboted sub-caliber rounds. The rounds intended for machine guns are linked using metallic links.
A wide variety of ammunition is available, and the availability of match-grade ammunition has increased the usefulness of .50 caliber rifles by allowing more accurate fire than lower quality rounds.
The M41-Light Anti-Aircraft Gun utilizes the 12.7x99mm Armor Piercing Rounds. The M6 series uses the similar 12.7x40mm .50 caliber semi-armor-piercing high-explosive rounds.
