I'd guess either the nipple or the hammer face. The rifle lock usually has a bit more force than a pistol lock, but the extra power shouldn't be a problem. The hammer face might not be "square" and could be hitting the nipple face one one side instead of evenly distributing the force, but I would think it would damage the soft copper cap before it broke a nipple???? If it were my gun, I'd get a couple quality nipples and try again.