I find that sometimes rocks fall out because the sling is too long/heavy and I can't get it around fast enough to keep the rock in the pocket. Smaller rocks would then work better because they require less force to stay in the pocket where larger ones fall out under the same conditions. That's why I use a solid pouch sling. Killing myself over finding ammo of "just the right size" isn't worth it. The best thing about a sling is that supposedly you can use pretty much any rock you find on the ground. a solid pouch allows me a much wider range of ammo types, weights etc...(i.e. I can sling both oblong and round ammo with a solid pouch, but only semi-oblong with a split pouch.) My advice is get a solid pouch and alleviate all your problems.