OUCH!
≈230-ish joules is basically the energy of a .32 ACP bullet btw (some loadings of it anyway, a single cartridge can vary a lot in its different variants ). the ≈190-ish joules on the lighter stone is comparable to some .22LR loads
also, from 9:50
the visor was 8mm polycarbonate, and that smashed... now, polycarbonate is, basically, is often used in place of bulletproof glass, it's used for riot shields...
Spain has fun history with the sling. The most feared slingers in the Mediterranean ancient world were the Balearic slingers, who are said to not be given meals by their families if they didn't knock the target off with the sling whole practising as children.
Additionally, there are reports of Spanish soldiers of the Republican army during the Spanish civil war who were shepherds and therefore knew how to use a sling, and were using a sling to toss LIVE GRENADES at the fascists.