Today I began experimenting with different types of ammo, over a river. It's great to be able to see where the shot lands due to all the little splashes. I agree that gravel doesn't travel too far at all. Experiments with small around marble sized pebbles ended up with a very wide shot area, splashes everywhere! It seems the velocity and spread is a little random. I agree that it's a good idea to wet the part of the tissues that is twisted together, it gives much more uniform openings, and it turns out that most of the bags do rupture, but only slightly (most were torn about a quarter of the way in). I think this would be a good small game hunting ammo if you can get enough small and heavy projectiles. When the little sacks were more heavily loaded they tended to break up before leaving the sling. The best load seems about 1 and a half handful per kleenex of "shot", of course you could use multiple tissues (I am using kleenex not toilet tissue, as that way I can have larger sacks). I might try a bunch of clay or concrete marbles if I have the chance.
I was thinking of a square of very soft cloth with a hole in each corner, feed a piece of string loosely through each hole and draw it together around the stone so it can easily be pulled apart, something like a simple pouch.