CA (again) –experimentation-

[Norman]

Soooo........ If I were in a scenario that had slings, I could pick up "glean" the "rock" and throw it and kill someone?

Historicaly, slings were a very powerful weapon used against fully armoured combatants. Flung from a sling the rock could nicely go through a helmet (at least a bronze helmet).
I don't think you can throw that hard.

[Bob H]

Nice slinging site - <a href="http://slinging.org/">Slinging.org</a>

<a href="http://www.youtube.com/watch?v=WADQStF-Rq4&feature=PlayList&p=5883BE10391EDF2B&playnext=1&playnext_from=PL&index=1">Slinging Byzantine Style (short video)</a>

If you're too close to another fighter to do that, you probably have crowded him to much for him to use his sword well anyway.

And as for them not being accurate:
<a href="http://www.youtube.com/watch?v=Ye89vHc3Xw0">Youtube: Slinging a clay ball at a deer skull</a>
<a href="http://www.youtube.com/watch?v=Kf3qLLEYniE">Youtube: slinging ... deer ... part 2</a>

He's close, and that's an old bleached skull. But he's just slinging clay balls, imagine what a rock or hunk of lead would do.