20mm Diameter Ball Mold (for slingshot ammo)

I love slingshots. But the challenge is having uniform ammo to shoot. Marbles are cheap but they leave broken glass everywhere. Steel shot (ball bearings) is not cheap, but with a magnet you can retrieve your ammo. But I'd rather not. So what I need is some biodegradable disposable ammo that is sufficiently dense enough to take down a jackrabbit. So eventually my plan is to make a metal mold which can be heated. Then I was going to take my PLA rafts and powder them in a blender, then mix