So I was out on the trails today showing a few of my friends how to chuck rocks. I spotted some wreckage out in the woods and it happened to be the shell of an old Huey helicopter (how it ended up there, I cannot even guess) and a crummy little POS car that had been driven down into the woods and left. All the windows were busted and it was in rough shape. So...
I got some rocks, and slung them into the front quarter panel and door. They left some serious dents and made some serious noise! Now I can officially say I have made dents in a car with my sling.

well, unfortunately the only rocks around are sandstone kind of stuff. Some of them are heavy enough to fly far, but when they hit, they just explode. The deepest dent I got was maybe 1/4 inch. But it was a goo 3 inches wide. Needless to say, it would make any car owner very angry to have a rock slung at their car.