Some thought rocks quite effective indeed...
http://www.christiananswers.net/q-abr/abr-slingsforkids.html
what do you think the NYT would say if Israeli’s threw rocks back?
"Surprisingly, a good slinger hurled a stone as far and accurately as a good archer. Roman military texts recommended archery target practice at about 200 yards (183 meters). Slingers are known to hurl their projectiles even farther, as much as 440 yards (402 meters) (a quarter of a mile / 0.40 km).
As for accuracy, one ancient writer noted that the best slingers "would wound not merely the heads of their enemies, but any part of the face at which they might have aimed." Experiments demonstrate that missiles leave a sling in excess of 60 miles per hour (97 km/h).
One Roman writer noted that opponents in leather armor were in far greater danger from sling missiles than arrows. Even if the stone did not penetrate the armor, it was capable of inflicting a fatal internal injury.
Unarmored bodies were easily penetrated by sling stones. An ancient medical book included instructions for removing lead and stone sling missiles from the bodies of wounded soldiers."
So much for the NYT's evaluation of this innocent “hobby” as a game and describes it as “an act of defiance and a rite of passage”