7.62x51mm
From Halopedia, the Halo wiki
File:NATO 7 62x51.jpg
A picture of the 7.62x51mm, with a ruler for scale.
7.62x51mm NATO is a round used by the MA5C Assault Rifle and MA5B Assault Rifles, among many other weapons. This round was originally developed by the North Atlantic Treaty Organization 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.
