I just made a sling for the monk character in my game....
I call it the Halfling War Sling.
First off it's a masterwork weapon so it's +1 to hit.
Second, it is designed so that the force of the users strength is added to the damage. This one was tailor made for the half-dragon(gold)/halfling monk who was instrumental in defending the town and the island from an army of evil invaders. His STR bonus is +5 so this "mighty" sling is +5 to damage.
It was as made by the master weaponeer in the town where player's characters live as a token of the towns appreciation of his selfless duty in defending the town.
It costs about the same as a masterwork "mighty" bow of similar bonus I say about 1000gp.
I am certain that there will be the usual responses to this item...it's simply not allowed in the rules...see page...huma, humma, humma, and on and on. While, I concede that this may well be the case, whats allowed by the core rules means very little to me in terms of an argument for or against such an item.
I simply think its cool to have unique items not allowed by the rules and I think my players like them too. It's a one of a kind weapon designed for a specific individual by a master craftsman and given as a gift.
The players can't just outright buy one....so like the rules argument, the balance argumnent means just as
little to my decision making process. As DM I can easily re-balance my game with the types of challenges and monsters I use.
Anyway, the botom line is, if you want to use a sling that allows a STR bonus to damage go for it...if not don't...it's your game, play as you like.
Rules are meant to be broken, it just feels right to me.....