I had the first of the bi-annual parent-teacher conferences at my school yesterday evening and when there was a little lull and no parents waiting in front of my door, I pulled out my slingshot and used some of those small chalk stubs that are always in the chalk tray under the blackboard for ammo.
From my teacher's desk I shot a tidy little group at seven meters against the far wall of the classroom and I was very impressed by the result: Not only did the stubs fly quite straight at that distance, on impact they blew up into dozens of little shards and a very impressive dust cloud. The point of impact was marked by a little cone of chalk powder that stuck to the surface, with jets of chalk dust radiating several inches from the center cone outwards. It looked awesome, like a tiny meteorite had struck the wall and eploded
If you are looking for relatively cheap, light ammo that is very unlikely to ricochet and creates a quite spectacular effect, you ought to give those little pieces of chalk a try. They apparently work best when broken into little pieces that are as "round" or symmetrical respectively as possible.