Here's what I posted on TFL.
I have been messing around with slings on a regular basis for about twenty years now. I haven't tried lead bullets, but I'd like to. But I can pitch a well-shaped rock 3-400 yds with a sling that's got 42" lines. The sling imparts a great deal of rotation to the projectile, so with the football profile ammo, this becomes a low-velocity 1 1/4 inch caliber rifle. When I malf the throw, the rocks hum like a richocheting bullet, and corkscrew like mad, with severely curtailed range. When they leave correctly, it's with a quite hiss, and they vanish amazingly fast.
Some points:
Accuracy at max range is about a 25' group. Good for barrage fire, certainly, without a whole lot of training, and a cheap, simple weapon.
Close range accuracy is more like 2' at 25 yds. Direct fire is a little trickier than barrage fire. It's easier to be accurate with less power in the throw, but that defeats the purpose.
Close range impacts are vicious. I've knocked chunks off of granite boulders. A sling stone wouldn't make a small hole, it would crush a skull.
My 42" sling I call "the varmint gun". The big one is called "the war sling". It's lines are about 36" long. The war sling was built to handle tennis ball-size rocks, and has worked successfully with stones the size of a cantaloupe. Tennis ball rocks are worth about 100 yds. Cantaloupers are worth about 40 feet, and take two hands. Keeping rocks that size in the sling until release is an issue, though. Another problem is that the war sling is very hard on the muscles of my shoulder and upper arm. If I'm not careful, it'll leave me stiff for days. The little slings are capable of this also, so warming up and stretching are definitely neccessary, just ask any baseball pitcher!
Long range slinging is WORK. A ten minute warm up, and an hour of pitching rocks will work up a good sweat and have you panting. Something about heaving these things as hard as you can when striving for range, I'm sure.
Need some excersize? Can't go shooting? Go throw some rocks, it's lots of fun. You need a lot of space, though, and a good supply of ammo. Old quarries make good spots.
Technique note: Don't whirl the sling around your head a bunch of times before releasing. Timing the release is almost impossible, and it winds the lines up so they drag on each other and change the release to much later. This has a high velocity rock going behind you, or worse, it FAILS to release, winds around your body and smacks you one! OUCH! (Thus speaks the voice of experience. Had that bruise for days.)
More silly stuff from the H_R_G Useless Information File. Enjoy!