Sling shot? Dag, I have not thought about that in, less see, about 30, I tellin!<img src=icon_smile_big.gif border=0 align=middle>
I don't know about the tech stuff now a day. But, gallon milk jugs, lots of rocks(free) kinda simular, and have at it. Pull back and let it rip. Practice will take care of it. Work you way down to a tomato paste can and you're ready.
Good luck, Be Careful
FirstBow