IMO, this is probably the best non-lethal (for an animal that size) suggestion so far. I grew up using slingshots like that. They're surprisingly powerful and accurate. They're probably even better now. And they're a ton of fun! If nothing else, the little buggers will know they're being shot at.

The OP liked the idea of a starters pistol, so maybe consider an air horn. It would be less expensive to try. The problem with noise makers like that with nuisance animals is that if they become accustomed to them, they'll learn that they're just noise makers and will learn to ignore them.