As with any good tool, the more you use, the better you get, within limitations of course.

Slingshots aren't bad for close up work, but just like a smoothbore musket, you can't stabilize the projectile, and most of my stones usually peeled off to the left of target out past twenty or thirty yards. I liked using white marbles most of all cuz they worked like tracers and gave good feedback on how well your aim and dope was. Usually I could tag the can at 10 paces after three shots.

The ones grandad used to make for me out of plywood and inner tube rubber weren't powerful enough to do much. Nowadays they have wristrockets with rubber tubing that'll turn a cat inside out with a 45 cal lead ball at 15 paces. How long the rubber lasts varies greatly, from a few weeks to a couple years, depending on use, environment, care, etc. The biggest practical slingshots I've seen were made from multiple strands of great lengths of rubber tube attached to stationary frames with buckets as the well and were used to launch water balloons at nearby frat houses during the frat wars at Washington State University. Lots of fun.