Armor-piercing round

Armor piercing rounds, colloquially known as AP rounds, are the type of ammo used in most small arms weapons used by the United Nations Space Command.

Armor piercing round is used to penetrate hardened armored targets such as ballistic vests, vehicle armor, concrete, tanks, and other defenses, depending on the caliber of the firearms. AP rounds are tipped with a hard metallic cap. The cap can be made from tungsten, steel, a carbide of some sort, or even depleted uranium. This allows the bullet to pierce through armor, as the projectile stays intact because the tip of the round does not deform enough to slow it down dramatically. Armor piercing ammunition is designed to dump almost all of its energy directly into the target, causing massive trauma and kinetic energy damage.

In the Halo universe
When all the AP rounds in a weapon are used up, most personnel switch to Shredder Rounds, 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 the shielding defeats the piercing qualities of the bullet. An AP round must thus rely simply 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. However, smaller AP rounds, such as those used in 20th century assault rifles, are ineffective unless used in large quantities.