While stationed in Greenland I got rather good with a sling. There were lots of rocks and lots of time. We had compititions and some of the guys got very skilled at both targeting and power.
I still carry a sling much of the time when in rocky country. I doubt that I retain enough skill to use it for hunting or protection however I still enjoy playing with it.
Mine is two long strings with a woven basket, all of nylon line. One string is a bit longer than the other and has a loop which I put over my wrist. The other, a bit shorter has a large stopper knot at the end.
The whole trick is to be consistant. Much like golf. Keep everything as uniform as possible. Don't worry about range or power. Just try to hit the same point over and over. Throw a rock, then a second. Try to hit the first one. do it again and again.
Try both underarm and overarm throwing. They both are useful. Don't try for power, it will come later. Use your whole arm and when you release, let go of everything. The loop on your wrist will retain the sling. Follow through just like golf, keep your eye on the rock.
The length of the strings and the size of the pocket are variables depending on what you are doing. Lobbing a large rock underhand like a morter takes a different sling than a powerful overhand (really for me a kind of sidearm throw). Just make one and get started. Lots of fun and good eye hand coordination training as well. I found it helped a lot in my "practical" pistol shooting.
Nomad.
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...........From Nomad.........Been "on the road" since '97