Go do the nightfall and try to put 3 rounds in an ultra knight and tell me he doesn't shield after the 2nd shot every time. Shields after 1st shot if you're going real slow. What I meant is that when their arc shield is full, the 1st round doesn't penetration to their actual health so it does not cause them to stumble. Knights don't shield when they stumble. Try shooting a knight with say, vision of confluence, said knight should throw up a shield before death. Now, try headshotting a knight with a sniper rifle. Knight will stumble until dead basically, no shield.