|There is more information available on this subject at M118 7.62x51mm Full Metal Jacket Armor-Piercing on the English Wikipedia.|
The M118 7.62x51mm Full Metal Jacket Armor Piercing (FMJ-AP) is a round used by the UNSC in the MA5 Series of assault rifles as well as the M392 Designated Marksman Rifle. Originally developed in the mid-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.
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.