While fooling around I built a small sling for marbles. The lines are about 12 inches and the pocket is made from light weight leather. I found the glass  marbles at a local dollar store @ a dollar per 100. Atleast the ammo is cheap. I'd thought about getting some cheapo superballs but that seemed a bit suicidal 
Pretty much it's just a mini sling. I'm not sure what the range is due to the fact that the marbles are hard to keep an eye on, those little 1/2" puppies sure do move fast.  I'm able to hit "Bob" at about 100 feet with no problems. Btw "Bob" is a target made from a heavy plastic pop up rifle target a 2x4 and some 3/4" pvc pipe. I call it "Bob" because when the wind blows it moves around alot, much like a drunk boxer. .. At some point I'll try and post some pics from my p.o.s. digital camara.