Yeah no, the sling was considered a very very deadly weapon back then. That’s the thing. And that’s not what bible school would want to portray. But the sling as a weapon was pretty much the Magnum Revolver of those days.

I don’t know about ancient duel rules to say whether bringing a sling was permitted. The take is more along the lines of “David wasn’t an underdog. If anything, David was the clear favourite to win because of his weapon”. Because a sling at the time was a highly effective and deadly weapon which was still regularly used for centuries after the supposed events of that biblical story because of its effectiveness.