Rifle armor-piercing ammunition generally carries its hardened penetrator within a copper or cupronickel jacket, similar to the jacket which would surround lead in a conventional projectile. Upon impact on a hard target, the copper case is destroyed, but the penetrator continues its motion and penetrates the target.

Why are armor piercing rounds illegal?

The Gun Control Act of 1968 banned any armor piercing ammo that could be fired in handguns as a way to protect police officers.

Do armor piercing rounds work on Mithrix?

Armor-Piercing Rounds - Your attacks deal 20% more damage to bosses. This is a solid item already, but for fighting Mithrix it has the bonus benefit of giving Mithrix no extra damage against you when he steals them, as you are not classified as a boss.