And a normal sling too. Please? As a martial proficiency given to clerics who don't want to go a route that allows bows.
Also make the basic dmg ammo a stack that replenishes slowly like a wand charge does, to indicate finding proper sized pebbles.
If nothing else it would be useful for pegging breakables.
Just a thought... use the rune arm mechanic of hitting alt to start twirling and then give a to-hit bonus the longer you wait to release the sling.