From Halopedia, the Halo wiki
The 7.62×51mm cartridge, sometimes shortened to 7.62mm, is a heavy ammunition size in which the rifles and machine guns of the United Nations Space Command are commonly chambered. A great variation of ammunition types for this cartridge are employed.
This cartridge shares its dimensions with a form of rifle ammunition developed in the 20th century, employed commonly across the nations of the North Atlantic Treaty Organization under the designation 7.62 NATO. The UNSC utilize it in an enormous variety of weapons, including:
M118 FMJ-AP (full metal jacket, armor-piercing) rounds are the standard-issue ammo type employed by the UNSC's assault rifles, the M392 and M395 DMRs, the M739 LMG, and the AIE-486H HMG. The projectile's armor-piercing capabilities stem from its specially hardened construction. UNSC infantry combat armor is capable of preventing lethal penetration at close range from a limited burst at the cost of bruising and significant discomfort, though infantry and even light vehicles or aircraft such as un-shielded Covenant Banshees are susceptible to fatal damage under sustained, accurate fire. Energy shields drastically reduce the ammunition's stopping power, as a great deal of concentrated fire from multiple weapons - or an extended period of continuous automatic fire from a single weapon - is needed to overwhelm them. The rounds are, however, capable of taking down unprotected targets quickly and cleanly.
Shredder rounds are an alternate 7.62×51mm ammunition type occasionally employed with the MA5B assault rifle. The projectiles they fire are designed to splinter on impact, and are highly effective against lightly-armored infantry like Covenant grunt soldiers and building materials like marble tiles and wood. Ammunition of this type was used by the SPARTAN-IIs of Blue Team to engage a horde of Unggoy in the Battle of Jericho VII. Another instance of their use by Blue Team was at Cote d'Azur, when engaging a pair of hunters in a museum. After finding their AP rounds were ineffective against the hunters' armor, John-117 ordered the SPARTAN-IIs to use shredder rounds to fire at the floor beneath them, causing the hunters to fall to the level below.
7.62×51mm hollow-point ammunition is a variant type of round which is optimal for use against unprotected biological targets, also employed occasionally in the MA5B. A situationally useful attribute of this ammo type is that it is unlikely to cause unwanted to damage to potentially vulnerable structures like equipment cabinets, viewports and starship hulls, making them a useful tool when taking a ship. Magazines containing this ammunition are recognizably marked with red paint.
7.62mm HEDP (high-explosive dual-purpose) ammunition is a variant of the 7.62×51mm round which is loaded in the machine guns of AV-49 Wasps in the service of the Office of Naval Intelligence security teams. The projectiles detonate on impact.
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