I expect some readers have been pretty dubious about Hugo's seemingly rapid acclimatization to the modern world. Well, I think the way he descibes it may explain a little. When one finds oneself in an utterly foreign environment, one two reactions are possible. One is simple panic, giving in to what amounts to a fear that's both irrational and functionally useless of what's different from what one's accustomed to; the other, far more useful, is acceptance and at least an attempt at adjustment. Too often I've seen people who give in to their trepidation and succumb to the former, but that isn't going to be very helpful; the latter is the only thing a reasonable person can do, and I prefer my characters to act as reasonable people.
I think I've described a sling pretty accurately. I remember messing about with one when I was a boy of perhaps 10-11. I never got very good with it, that takes more persistence than I was willing to invest at that age, but I could at least make a stone fly in the right general direction at considerable speed. By nature it's a one-shot weapon, but with practice one can reload "on the fly," as it were, without completely stopping the rotating motion. The most efficient method would be to drop the stone, with either hand, into the still-moving pouch, timing the drop to the motion. That's what I've postulated.
Thanks for the many positive comments, especially about the cave-dwelling. My sister, to whom I gave this to read shortly after I finished it, objected vociferously to that. She wants the cavemen of her own imagining hunkered down in the caves where she feels they belong. If there aren't any caves in the immediate area, she doesn't want prehistoric people living there, either.