From Halopedia, the Halo wiki
Armor-piercing rounds, colloquially known as AP rounds, are a type of ammunition designed to penetrate armor.
Armor piercing rounds are used to penetrate hardened, armored targets, such as ballistic vests, vehicle armor, concrete, tanks, and other defenses, depending on the caliber, sectional energy, and length of the round. Armor piercing ammunition can use a cap of a dense, tough metal, or a core of it, or a mix of both. Modern AP ammunition is comprised of tungsten alloys, usually then surrounded in a lead or copper jacket , which allow it high potential against protected targets. The hard, dense construction prevents expansion, shattering, and deformation, granting much increased effect against armored opponents. The problem is that they 1, don't cause great injury, since the lack of yaw and deformation even after piercing an armor panel means little terminal effect, and 2, armor may still defeat the AP round.
In Halo, the disadvantage 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 12.7×99mm Armor-Piercing and the 14.5×114mm, perform well under these conditions. However, smaller AP rounds, such as the M118 7.62×51mm and the M634 9.5×40mm, are ineffective unless used in large quantities.
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