I think you're mistaking effective range for absolute range. Muskets have an absolute range of up to about what you can get from the best bows (many time what you can get from sling-shots), but their accuracy is pretty poor out beyond about 30-40 yards, which is why barrage fire was used in the first place.I guess you could sling-shot it? Most early guns didn't have much range or accuracy, so a barrage of sling-shotted obsidian landing in the middle of you would probably be as effective as a round of musket fire
Could you incorporate obsidian slivers into a cloth-wrapped shrapnel shot?