Armor-Piercing Rounds is an item in Risk of Rain 2. It increases your damage by 20% (+20% per stack) against bosses and monsters spawned by the teleporter. It reflects this by changing the color of your damage numbers, if they are enabled, to orange when you are damaging an applicable boss.
This item only affects bosses spawned by the teleporter, signified by their red health bar. It does not give you a damage bonus against bosses that spawn outside of teleporter activations in the late game (with the exception of the Alloy Worship Unit), however it will still increase your damage against Hordes of Many.