Minecraft wiki entry for Piercing, associated text: Arrows shot from a Piercing crossbow can pass through the number of entities equal to the enchantment level (for a total of level + 1 entities damaged). The shot arrows can still be retrieved after piercing a mob, allowing for infinite use of all arrow types, including spectral arrows and tipped arrows. Piercing ignores shields even when they are blocking. Though, arrows will not target mobs, meaning they will just go through a line of mobs.