Armor piercing round

There is more information available on this subject at Armor piercing round on the English Wikipedia.

Armor piercing rounds, colloquially known as AP rounds, are a type of ammunition utilized by the small arms weapons of the United Nations Space Command.

AP rounds are used to penetrate through a variety of protective armors ranging from the ordinary to the exotic, such as ballistic vests, reactive armor and Covenant harnesses. The effectiveness of AP rounds depends on the caliber of the round as well as the muzzle velocity of the firearm used. AP rounds are tipped with a hard metallic tip, made from tungsten, steel, different types of carbide, or even depleted uranium. These high density metals penetrate by virtue of sheer toughness and are often coated in a harder metal to prevent desintegration. The hardened tip allows the bullet to pierce through armor, as the tip of the round does not deform as easily upon impact, keeping the rest of the round intact as it penetrates the target. Conversely, normal rounds would deform upon impact into a mushroom-shaped slug of metal, absorbing a significant portion of the shock of impact and thus greatly diminishing its capability to penetrate armor.

In the Halo universe

When all the AP rounds have been expended, most personnel switch to Shredder rounds to accomplish the same effect (albeit at reduced efficiency), as the SPARTANs of Blue Team did on a mission in the Lambda Serpentis system. The SRS99C-S2 AM Sniper Rifle's bullets are armor piercing as well. The downside of AP rounds is that they perform poorly against energy shields, as they negate the bullet's armor piercing capabilities. An AP round must thus rely solely upon its kinetic energy to deplete the shields before it can damage the shielded enemy. Large rounds, such as the 12.7x99mm Armor-Piercing, perform well under these conditions as they drain more energy from energy shields. Conversely, smaller AP rounds are relatively ineffective unless fired in large quantities.