7.62x51mm
From Halopedia, the Halo wiki
The 7.62X51 NATO is the round used by the assault rifle in Halo: Combat Evolved and Halo 3. This round was originally developed by the North Atlantic Treaty Orgainzation in the 20th Century. As it is used with the assault rifle, the most common weapon in the Marine Corps, it is probably the most fired round in the Halo universe.
