7.62x51mm
From Halopedia, the Halo wiki
Overview
The 7.62X51 NATO is the round used by the MA5C Assault Rifle and MA5B Assault Rifle, among many other weapons. This round was originally developed by the North Atlantic Treaty Orgainzation in the 20th Century. As it is used with the MA5 series rifles, which are one of the most common armaments in the Marine Corps, it is probably the most fired round in the UNSC ground forces.
In Combat
The round is quite effective in combat, as its mass makes it effective against flesh, however the round performs poorly against heavy armor and energy shielding.
