Most people buy steel balls because they are more accurate and can maintain a certain size for your slingshot. Well What about pennies? Not just any penny. Any penny made before 1986 is copper and copper melts at a very high temperature. Penny's made after 1986 are zinc so they melt easily and with You can make about a thousand balls with about 20$ in penny's. It is very cheap so you will get the same effect as steel balls but less effect on your wallet.