My parents (well-intentioned liberals) would never let me near guns when I was a kid (perhaps explaining my adult arsenal). But we lived in the boonies, so I got to be pretty good with a sling . . . it was fun and effective, but it needs a big area for practicing. I recall that lug nuts over a piece of nylon cord (one end of which was tied around my wrist) wound up being my eventual favorite combo. I suspect I couldn't hit a thing today with one all these years later, but I consistently was able to hit coke-can-sized rocks at over ten yards. (Consistently hitting a snake at 50 yards is a long ways better than I ever got.) The sling had POWER, too: I accidentally blew a hole in a barn wall when I missed a bird at somewhere between 75 and 100 feet.
You can certainly TAKE a sling anywhere, but can you USE it anywhere? I suppose my concerns about the sling for defense are that it takes a fair amount of time and space to use. The self-defense situations I've encountered generally had me in a confined area or needing to respond more quickly than I'd be able to with a sling: shaking out the sling, digging a rock out of my pocket, winding up and letting loose.
Hucking rocks at something needs a specialized skill that I don't have. My little brother was an excellent pitcher - college scholarship. He could throw a perfectly-aimed ball with enough "English" on it to do some harm. I would not have wanted him to nail me with a rock when he was in his prime (and he actually did nail some kid with a rock in a fight and wound up getting juvie charges against him - later dropped when cops sorted it all out). I'm a whole lot stronger than Mark, but there's no way I could deliver a ball or a rock on target the way he could. Different muscle groups and a whole lot more training on his part. I think I'd rip my shoulder out trying.
I don't know. Everyone's been in a rock fight when he was a kid. I feel that a lot of effective self-defense is having the ability to effectively threaten harm to actually inflict harm. I would just be afraid that if one wound up with a rock, a mugger or robber would be A) amused and B) ready to show you now to bring a rock to a gun fight. In the event of a surprise attack against a robber at a 7-11, I'd rather bash him over the head with the can of soup (I'd be more likely to hit to the side of the base of the skull, actually) than try to hit him with it from across the store. From across the store, if all I had was a can of soup, I'd be on my hands and knees, backing away.
But I like this thread a lot! Makes me want to find someplace to play with a sling again.