I agree that the harder Zinc will be better for casting into the copper or brass pipes.
But why not just cast the balls out of Zinc Wheel Weights.
It is a little more work to cast balls out of Zinc.
But I have cast Boolits out of Zinc for various reasons.
Zinc is far less toxic than lead.
It could get expensive.
But you could buy some Brass Rod and cut it into short sections the same length as the width of the rod.
I am sure that tumbling those brass sections will round over the edges and eventually make them into almost a round ball.