7.62x51mm

7.62x51mm NATO is a round used by the United Nations Space Command in many of their weapons. This round was originally developed by the North Atlantic Treaty Organization in the 20th Century.

Background
As it is used with the MA5 series rifles, which are one of the most common armaments in the Army and 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.