Think of it this way, a .357 Magnum produces 535 foot lbs., It is quite likely that David generated 600-650 foot lbs. Goliath died on his feet.
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Yeah, it’s pretty amazing what a simple sling can do. The greeks and romans cast lead bullets, and engraved them with pithy quips like “catch”, and “take that”, or lightning bolts.
Wiki has the record for a rock thrown via sling being 1434 feet.
Freegards