So it's a magic shortbow, but I don't see the purpose of picking up this spell over other attack cantrips or just casing a shortbow. I guess it's cheaper and more handy option, but it costs a whole cantrip.

I also figured it would do magic damage for resistance, like shillelagh but it's just magically propelled.

If I'm reading this correctly it says "throwing it OR hurling it with a sling". Does that make the sling optional? Saying both of those feels redundant and "throwing" is a mechanical thing already in the game.

It makes a "ranged spell attack" so its magic damage. You can choose to throw it with a 60ft range or use a sling to make it go further but with less accuracy.(30/120)

One of the main benifits is its a ranged attack that you can give to another creature, but said creature throws it with your modifiers. You could give it to someone who couldnt usualy do any sort of ranged damage. A peasent throwing rocks vs a peasent throwing MAGIC rocks...

Or go warlock give the pebbles to his/her familiar and get them pelting your enemies from above with them. Its a bonus action to cast so it doesnt eat to much action economy either.

A fairly under rated spell in my opinion.

That's pretty much the exact answer I was looking for- thanks! 
I didn't consider the use with a familiar- especially with a non-combat familiar like the sprite this could essentially give you an extra 3 shots with a shortbow for the cost of a bonus action.
It doesn't seem /optimal/ but I can definitely see it's utility. It's just a hard choice to pick this over the other cantrips that generally provide either more damage or more utility. It's a "team play" spell.

Damage on this is pretty good since you can add your spell mod to it. So it has a minimum of 3-5 (depending on your stats) and a max of like 9-11(again depending). I'd prefer this over something like a firebolt. 

So would you throw it as part of the cantrip or would that be a separate action? I think I know the answer but I just wanna double check.

It's a bonus action to cast.  So you could cast it as a bonus action, then take your action as a attack.

What rule book addition is this from? It's not from the PHB is it??

To the point: It's phenomenal! Add a +2 sling to this and you are looking at some serious business.

Aaaand it says in the spell description where it's from - nvm. Thanks!

So your turn would go something like this: Bonus Action, cast Magic Stone to make sling ammo magic. Action, attack with sling loaded with Magic Stone ammo. As opposed to: Bonus Action, cast Magic Stone on pebbles and throw a pebble as an attack. Action, throw another Magic Stone pebble as an attack.

wait so this cantrip doesn't add a dice when you get higher level????

I thinks its something mainly for low level play but tactical as you now basically created 3 spells that are 1d6 + spell casting modifier.essentially a 3d6+spellcasting for a cantrip

I have a small idea for u (now that pistols r legal in AL) could u cast magic stone on the ball of the ammo before shooting it to add 1d6 plus spell mod?

for those who ask how to get the spell and prof in pistol i am fighter with magic Initiate feat.

I think it's just more of a flavor kind of thing. say you're in a bar and you have a set of dice, you can cast the spell on the dice and throw them at random people or at the people who just beat you at a game of dice.

Do you think this will work well in conjunction with the Catapult spell? and if so how do you think my GM would react?

Alas, pebbles tend to be less than a pound in weight.

yeah, i absolutely love this spell.

 This is a very underrated cantrip, especially in the right hands. One thing that is regularly overlooked is that you can choose your delivery method. You can throw it and apply your spellcasting modifier, or you can use a sling and use your attack modifier.

Regardless of the delivery method, you're doing the same damage as a shortbow at 1d6 + your spellcasting modifier (likely at least +2).

Here's the big thing folks overlook though - since you can choose to use a sling for these, it opens up the potential to add sneak attack damage since you're using a weapon attack rather than a spell attack. Now you're looking at a ranged attack that does 2d6 + spellcasting mod. You're basically running with a maul that has a 30-foot reach, and it's magical damage so it works against constructs.

This is in my top 3 favorite cantrips.

this seems like a perfect spell for artificer, why doesnt artificer have this?

I do NOT think it would work with being shot off with Catapult, the only damage you would get would be the 3d8 from Catapult. The reason being in Magic Stone, it SPECIFICALLY states that you HAVE to attack with the pebble. If you don't attack with the pebble, I don't think its damage would go off. I empathize with you because that's not broken or busted at all, and that's an AWESOME combo!!! But speaking literally, the rules say that it wouldn't work that way. I also hate that you don't ALSO add the d4 from using a sling as well... The d6 from Magic Stone is the only damage you would get, because it almost serves as a replacement effect. 

So when you throw it with a sling. Do you use both the damage listed for the sling AND the damage from the enchanted stone or just the damage from the stone?