+Claytonian JP posted about slings over on G+ today. He was wondering if making the damage die exploding (like firearms in 2E AD&D), where a maximum on the damage roll would allow a second die to be rolled for more damage, would unbalance the sling.
I think that's not a bad idea for the weapon. Scrap Princess tried to do some of the math involved, and concluded that it would make it statistically about the same as the d6 damage done by bows/crossbows. And it would be a whole lot cooler.
Of course, a few people on the thread thought he meant slings firing exploding ammunition like grenades. That could be cool, too.
Anyway, sticking with Claytonian's original idea, I might allow this, with the proviso that only manufactured sling bullets get exploding dice. If you run out and restock with rocks from the local stream, they only do a standard 1d4 damage.