Just my 2 cents. Caps are copper which is soft enough to absorb some of the energy from the hammer. All percussion caps release energy at a speed regulated by the blow that sets them off. 777 powder is harder to ignite because it requires a certain amount of energy with the flame, not like black which only requires the flame temperature. Think of it like this if you light dynamite with a match it only burn if you light it with explosive force like with a primer with the minimum amount of force required it explodes.