Characters with at least Basic in sling can do something (use a touchstone? flint stone? any gray stone?) to break rocks into slingshot, which weigh 1 and have comparable to-hit and damage to flint stones. Slingshot may be weapon-class instead of rock-class. The higher the sling skill, the more shot that one can get out of a single rock. There should also be rare silver rocks, which can be broken into silver shot.