1) If it is an ignored damage types, which include * Armor Piercing (i.e.: Snipetron, Despair, Piercing Hit) * Physics Impact (i.e.: Boltor, Kunai, Paris) * Poison (i.e.: Torid, Acrid, Mire) * Serrated Blade (i.e.: Most (but not all) melee charge attacks as well as Hikou, Flux Rifle, Spectra and Lanka) 2) If you are firing at an unprotected body part.