Considering that I have personally witnessed a shepherd from Trás-os-Montes [that's northern Portugal for anyone who doesn't know, which I assume is 99% of the people here], using a simple stone to completely pulverize a sheep skull mounted on a pole a few meters away, I'd say they do a decent amount of damage.

And accurate too, the guy could consistently place the shot in the vicinity of the target so that if it was the size of the person he'd always hit a part of it. Don't dismiss the weapon just because it's a simple thing.