Plastic, whiffle golf balls - they are the worst in my opinion. What have you thrown that was a really, really bad ammunition for your sling?

I will admit I have enough experience slinging whiffle golf balls to consider myself very knowledgeable on how horrible they are. They are just TOO light!  Granted, they don’t damage the target and can be slung indoors, so they are not totally useless. And with practice you can actually hit your target-here is proof:

https://youtu.be/qY_FEHcEGi8?is=NGgbNls-sGC0hBdE

Jagged quartz rocks - they'll rip up slings like it's nobody's business  :)

Acorns don't work very well.  They're usually too small and don't fly very far.