I build pumpkin catapults. Got interested in ancient slings as a result since a trebuchet is just a big mechanical arm with an ancient sling attached.
I've built a few slings to experiment with and I'm very impressed with how powerful and accurate they are. I can easliy see how a head shot with a 1/4 lb rock would kill.
Ammo is everywhere, they fit in your pocket, kids can even use them to kill giants.