The sling was a feared weapon in the ancient world. I did some research on the ballistics of weapon suggest that:
1. The energy generated by current skinny academics is at the .45 acp level.
2. David most likely could push it to .357 levels.
3. If David was the athlete I think he was .44 mag levels of energy are plausible.
The results of my calculations surprised me. I did not expect those results.
Slingers were not just random shepherds who participated in small group attacks as guerrillas. The Roman legions for example had auxiliary units made up of slingers from the Baleric islands which were famous for this skill in war as were the Rhodians.
Unlike David who apparently relied on finding a small smooth river rock in the valley, professional army slingers were provided with specially manufactured ceramic or lead balls to use as projectiles.