As far as I can tell, you can use the stones with extra attack.

As for using a sling, there's no additional sling damage or Dex mod. Though, I think it does use the sling's range. But mostly using the sling is for abilities that require the use of a weapon, like sneak attack.

better than fire bolt

Can you use it in Dual Wielding?

1.Round:
Bonus Action: Cast Magic Stone (on 3 Pebbles)
Atack: Throw 1
2.Round:
Atack Action: Throw the 2. Pebble
Bonus Action: Throw the 3. Pebble

And would you make which each pebble 1d6+ Spellcastin Modifier Damage?

Ohh
And can you throw 2 or 3 pebbles with one Action?
Because i can, not very precise but  i will hit a human in 15 feet distance.
And farer away i will also not hit a target with one pebble...

It's good because it's a bonus action

Why doesnt this spell get any upgrade like other Cantrips? 

Every other Cantrip gets 2 dice at Level 5 and 3 at level 11 and 4 at level 17... 

Why not allow this Cantrip to throw multiple stones at higher levels? 

Personally I love Magic Stone for low levels of play. Because there are not a lot of ranged attack options for a Druid at tier 1 (levels 1 through 4). Since my druids tend to not be so good a taking a hit, being able to lob magical rocks from 60 feet away is clutch. 

Course one day I'd love to play a small Harengon druid and get a DM to agree to let me cast magic stone on my pellets. Magic poop attack!!

Used this on my Artificer until I got my Subclass.

Level 1-5, I would actually say this is the strongest damage Cantrip in the game, just by the value of that flat +Spellcasting Mod damage. 

If you start with 16 in your Spellcasting Attribute thats a +3, giving you an average damage of 7 per hit. Thats on par with Spare the dying D12 damage average. You might only have a maximum damage Potential of 9, but in return you can never do less than 4 damage.

After Level 5, the Spell really goes down because it doesn't scale, and since throwing the stone is a Spell attack, you cant use Extra Attack with it either., but all 3 Casters who get it can switch out Cantrips at certain points.  

So, as a starting cantrip, especially if you aint using your Bonus action for anything, its a very solid pick. Just dont stick with it past Level 5. 

Extra Attack actually doesn't seem to limit you to weapon attacks. The attack action only limits you to "a melee or ranged attack".

Something like Flame Blade doesn't work because it requires a special "Flame Blade" action to do its thing. Magic Stone just requires you make "a attack" through any means necessary, probably via the attack action.

Makes sense too cause the spell seems to be basically the ranged version of Shillelagh.

edit: added links

I just checked it out, and you seem to be right. 

In that case, I have to make a correction and say for Casters who get extra attack, it is still a decent cantrip after Level 5, if you dont need your bonus action for anything else. 

Though from the Casters who get the spell naturally, this would not really be interesting for anyone... At Level 5, an Artificer who would get extra attacks has his special weapons, and the Warlock would have Eldritch Blast + Agonising Blast as a better option... 

If you have multi-attack, can you throw two?

Answered my question, thanks.  Adding to the sling damage would be sweet, but, fair that it doesn't work that way.

What people sleep on with this spell is that you can cast the spell then hand off the pebbles to other people (or since it’s a touch spell, via a familiar). 

Bonus action: Make your pebbles, then have three hirelings (3 silver a day) make your attacks from a distance. (1d6+mod)X3 as a bonus action is pretty sweet! Especially if you fork out the cash to protect your hirelings! (Or when you have 3/4 level slots, use Tiny Servant)

Yes - as you're taking the attack action. It means it's actually pretty useful for monks. Take the Magic Initiate (Druid) feat at Level 4 (or 1 if Variant Human etc). Pick Magic Stone and any other cantrip (Guidance is always helpful, and Thorn Whip is just pure Mortal Kombat "Get over here!"), and one 1st Level Druid spell (Faerie Fire, Thunderwave, Ice Knife, Fog Cloud... there's some great options!). 

Now when you hit Level 5 you can throw two magic pebbles, dealing 1d6 + your high WIS mod per stone with a range of 60 ft. Not bad at all, and beats throwing darts/knives as the damage is magical. 

Jeremy Crawford seems to agree r.e. extra attack: https://twitter.com/jeremyecrawford/status/752595441477312512?lang=en

Use it with tiny servent. Use a lvl 4 slot to get 3 servant and use them to throw the rock.

Why does this not increases by 1d6 when you reach 5th level (2d6), 11th level (3d6), and 17th level (4d6)?

Quote from ArtificeMeal >>

now does the attacker get to add their proficiency bonus to the attack roll if they are just throwning it?

anyways be a beast master ranger, get a baboon companion, get druidic warrior, get it to throw the stones with its disposable limbs and get that sweet proficiency bonus to hit and damage

 These words are a great source of amusement to me.

Quote from Darrius8 >>

Why does this not increases by 1d6 when you reach 5th level (2d6), 11th level (3d6), and 17th level (4d6)?

 Because there are three stones. 12d6 + 3(modifier) as a bonus action and costing no spell slot? No way.

what if you hold it to there back with a sling would it be a sneak attack.

Okay but why is it transmutation?

Quick thoughts.

- This is more like a Javelin than a short bow (closer range, one-handed, damage, etc). With this thinking, you can equip a shield for more AC or an off-hand weapon for a bonus attack and still throw these. Assuming that a pebble is a light/ throwing weapon.

 - Being able to throw it AND use it in a weapon does open some interesting synergizes with the spell. Adding bonus damage as a weapon attack, like Archery fighting style, Ensnaring Strike  and the like is very unique for a ranged attack spell.

- Think Gambit from the Xmen. Instead of playing cards, it small stone. (actually... that might be a good concept for a character, just fluff stones for cards....)

- Effectively it is a spell that makes magic ammo. Its that simple. If you throw it, use your magic bonus to hit (see Hexblade on how magic can be used as a weapon bonus:  because a Smile can kill!) and goes 60ft . If in a sling, you use the weapon bonus and the weapon's range (because your arm is stronger, better, then magic).  Either way the pebble does a magic 1d6+ your magic bonus.

- This spell is really easy to see why it was made as it is useable by the Martials and Casters alike. Your Rogue's spell casting bonus likely is  not be that high but your Dex might be; use that. If your are a Caster, odds are your physical attributes will be lower. Use the magic ones. Sure the Druid might be doing 1d6+4 while the Rogues doing a 1d6+0, but they can hit a target at 120ft and might be adding addition damage (Sneak attack) as well.

My questions are if your use it as an improvised sling melee weapon, does it do magic damage? (likey just for 1 attack) AND can my Artificer make a rubber band gun to shoot all three as a burst fire? Laser site optional.