Forgot to mention: in the kinetic energy equation, the energy increases at half the rate of mass but with the square of the velocity (RPM). That means if you double the mass with tungsten, you only get a 50% energy increase, but if you use a larger & faster motor to double the RPM, you get four times more energy. High power motors are not cheap, but if you want something as impressively destructive as possible, make the weapon spin faster.