This video popped up in my recommended (I wonder what that says about my choice of videos...) and if you look past him calling it a slingshot, the enthusiastic presenting style and the fact he's obviously an amateur, I think this is quite a good example of what a sling would do against a shield. It's obviously not that high powered a shot but it still leaves quite a mark. The guy in the video doesn't seem all that impressed but I think he's more concerned with the penetrating power which of course a stone isn't so good at. What a stone is good at though is hitting it very hard and knocking the person back. I was surprised he didn't mention  the fact that the sling was the only weapon to knock the shield over. I imagine that this would be very useful in a battle with slingers being able to disrupt front lines and generally be a nuisance. I would love to see what an accomplished slinger using a lead glande could do to it and it's making me want to head out with the giant metal frying pan I have

https://www.youtube.com/watch?v=VS3pWwlqp8c

Oh I'm very familiar with Timpa's videos So much so I'm worried that I live in the same country as him... But yeah Timpa's videos are much better but it's always interesting to see what novices think about the sling. Mostly they underestimate it...