7.62x51mm

7.62x51mm NATO is a round used by the UNSC in many of their weapons. This round was originally developed by the NATO organization in the 20th Century. This NATO round's main purpose was for fully automatic reliability and heavy power. Its size makes it a battle rifle round in modern times; though humans have decided that larger rounds are better; and in Halo 2 the Battle Rifle round is 9.5mm.

Background
As it is used with the MA5 rifles, which are one of the most common armaments in the Army and Marine Corps, it is probably the most commonly fired and used 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 armored and shielded enemies.

Trivia

 * This round was developed by the modern day, real life organization NATO, which stands for North American Treaty Organization.