There appear to be two plausible interpretations.
- The magic stone’s effect amplifies and thereby replaces the regular damage of the stone hurled by the sling.
- The sling is hurling the stone at high velocity causing regular damage and the spells effect is that it causes it’s damage (additional).
What damage dice are rolled when a creature is hit by a magic stone that was slung by a sling, and why?
Example: A Druid named Agar has +3 to his spellcasting ability modifier, he carries a sling and has already cast magic stones on a few pebble carried in his pocket. In the distance, a goblin slowly crests a hill and comes into view. Agar readies his sling as the goblin advances from about 100 feet away. Agar slings a magic stone toward the goblin. Before the goblin can react, he is struck by the stone.
Does the Goblin take 1d6+3 damage?
Does the Goblin take 1d4+1d6+3 damage?
You touch one to three pebbles and imbue them with magic. You or someone else can make a ranged spell attack with one of the pebbles by throwing it or hurling it with a sling. If thrown, it has a range of 60 feet. If someone else attacks with the pebble, that attacker adds your spellcasting ability modifier, not the attacker’s, to the attack roll. On a hit, the target takes bludgeoning damage equal to 1d6 + your spellcasting ability modifier. Hit or miss, the spell then ends on the stone. If you cast this spell again, the spell ends early on any pebbles still affected by it.
Sling, damage 1d4 blugeoning (PH, 149)