Talk:Kunai/@comment-24.243.185.197-20130527231352/@comment-4719392-20130528184655

Basically, armor piercing is an elemental type of damage, which causes additional damage on some armored targets (150% on medium Grineers, 400% on Corpus Crewman's head, etc), but reduced damage on other (50% damage on non-ancient infested).

The Kunai is most likely a bolt-type weapon, which ignores armor.

See Weapon Types and Elemental Damage.