Armor-Piercing Rounds

Increases all damage you deal to Bosses by 20% and each stack beyond the first increases that damage by 20%.

Unlocking

 * This item is unlocked by completing the Challenge Advancement, which is to beat the first stage unlocks it.

This item only increases damage against teleporter bosses making it much less useful than it may seem at higher levels since many of the enemies that spawn are bosses yet are still unaffected.