They found it quite effecitve for throwing grenades. Would have been some time in mid-May 1940. In any case, my apologies if this is a bit vague, but this is from memory. I do recall that there was a Scientific American article about them some years ago. There is anecdotal evidence that Slingers have been able to penetrate armor, shatter swords, and let's not forget The Roman General Vegetius indicated that the standard for a slinger was the ability to hit a man sized target regularly at 600' the whole David and Goliath story.
Any uneducated shepherd can learn to use one in an afternoon.
No examples of slings have survived. I don't have the formulas with me, but essentially there are four variables in Slinging. The length of the sling, how fast you can move the sling, the mass of the bullet, and the diameter of the bullet. The first two really govern the velocity of the weapon, The center two govern the momentum of the bullet. The third governs the impact.