7.62x51mm

M118 7.62x51mm Full Metal Jacket Armor-Piercing is an ammunition used in their MA5 Series of assault rifles as well as the M392 Designated Marksman Rifle. It was originally developed by the NATO organization in the late 20th Century. It is used due to its reliability and heavy power. As it is used with the MA5 rifles, which are one of the most common armaments in the Marine Corps, it is probably the most commonly fired and used round in the UNSC ground forces. It can be regularly found in packs littered around battlefields.

Overview
The round is effective against infantry, especially against unarmored or unshielded opponents. Energy shields drastically reduce the effectiveness of this round, as it does not have enough kinetic energy to significantly drain the shields. The round is also largely ineffective against vehicles, as it is not significantly armor-piercing. Massed fire from a squad of Marines, however, is enough to drain shields and destroy or disable most vehicles short of tanks due to the sheer number of rounds fired. Thus, this is primarily an infantryman's round for use against other infantry.

Background
The 7.62x51mm NATO was originally developed by NATO, a military alliance between national governments, in the middle of the 20th century. It was developed to be reliable when used in fully automatic weapons, to have a large punch, and to standardize service rifle calibers between governments, in order to improve logistics. It replaced a variety of smaller ammunition and largely achieved the goals of development.

It is a full-power rifle round in the 20th century, and is fairly common in machine guns, battle rifles, and sniper rifles, but not in lightweight assault rifles due to recoil and the size of 7.62x51mm NATO weapons; they are usually larger and heavier to accommodate the longer, more powerful cartridge. Its ineffectiveness in individual infantry weapons led to the adoption of the 5.56x45mm NATO round, due to better controllability and the ability to fit it into small, lightweight assault rifles.

It is not known why the 5.56x45mm NATO round disappears in the Halo universe, but it could be possible that weapons technology advanced so much as to fit enough recoil control systems on a firearm to allow this powerful round be used as an infantry rifle round (the same could apply to the M6 series). It also seems that in the Halo universe, the 7.62x51mm round is no longer considered a full-power rifle cartridge, explaining why weapons that use the round such as the MA5 series are considered assault rifles when they would be considered battle rifles by today's standards.

Trivia

 * This round was developed by the modern day, real life organization NATO.
 * In Halo: Combat Evolved, this round is labeled "M118 7.62x51mm FMJ-AP." FMJ stands for Full Metal Jacket, or having an outer coat of metal that completely encloses the bullet, and AP stands for Armor Piercing.