Aluminum makes sense, 1/3rd the density of steel.
It’s light so that it can achieve higher velocity for the same energy. And the delivered impact energy is equal to the mass times the velocity squared.
>> It’s light so that it can achieve higher velocity for the same energy. And the delivered impact energy is equal to the mass times the velocity squared. <<
A pound of feathers weights the same as a pound of bricks. If it has one third the weight when launched, so it goes three times faster, than it impacts with one third the mass. IOW, 3 * 1/3 = 1. But the pound of bricks hurts your foot more, because the denser object creates less wind resistance.
1/2 MVsquared