I know. Slingshot ammo isn’t the most expensive. Nevertheless, unless you have access to free or cheap heaps of ballbearings, after hours of practice it may turn out you’ve parted with a significant amount of money. Money that could’ve been saved easily.
The idea is very simple and it isn’t mine. I first saw it on Far North Bushcraft and Survival’s YouTube channel. All you need is a large piece or, better still, two pieces of fabric like an old blanket. Hang them on a few yards of cord stretched between two trees behind your target making sure there is a fair bit of the fabric spread before the backstop to prevent the balls from rolling down on the ground. It’s easier to find and pick them up this way.
The target doesn’t have to be paper. It can be a metal can, a fruit… Whatever works for you. Bear in mind though, a reactive target will make the balls bounce off of it and you will lose most of the once that hit.