So we know the cantrip 'magic stone' is able to be combined with a magic sling to get a bonus to its to hit and damage, but is it also possible to combine it with a wand of the war mage or rod of the pactkeeper to get the to hit bonuses from both, to make it a viable option for the two classes who get the cantrip but don't have extra attack?

The problem is you need to be holding the wand or rod to get its bonus, but you already need two hands to hold a sling and load it with ammo.

Does it benefit from slings? Magic stone says it is a ranged spell attack and uses your spell attack modifier, not a weapon attack so wand of the war mage should work, I think slings just increase the range.

Slings actually decrease the range. 

The wording on slings is "attack and damage rolls made with this weapon gain a plus one".

Doesn't say what kind of roll is has to be.

You only need two hands to reload. You can attack with a sling with one hand.

I don't think there's any official rulings about +1 weapons specifically but you're definitely making an attack with a sling and there's plenty of precedent in other rulings. Jeremy's ruled in favor of Sneak Attack, Sharpshooter and Archery Fighting Style when asked how they interact with Magic Stone.

Quote from Epargreboog >>

You only need two hands to reload. You can attack with a sling with one hand.

 Yes, but you only get one object interaction per turn. You load the sling, draw your wand, make the attack, and now you don't have any hands free to load the sling for your next shot.

The only way to make it work is by dropping the rod for free and picking it back up as an object interaction, which is definitely cheese and your DM may not like it. In my own games I changed picking stuff up from the ground as an action to prevent absurd combat maneuvers like those. 

Quote from InquisitiveCoder >>

In my own games I changed picking stuff up from the ground as an action to prevent absurd combat maneuvers like those. 

 That also makes disarming enemies a viable strategy.

"I use my action to disarm the enemy." "Roll, you succeed. Enemy's turn, he picks up his weapon as a free interaction and attacks you."

If it replaces an attack to disarm, why can you retrieve your weapon for free, WotC?

Quote from InquisitiveCoder >>

In my own games I changed picking stuff up from the ground as an action to prevent absurd combat maneuvers like those. 

 That's what I do in my games too. I do something similar with being prone. Getting up takes either all your movement or your action.

Quote from Epargreboog >>

Slings actually decrease the range. 

The wording on slings is "attack and damage rolls made with this weapon gain a plus one".

Doesn't say what kind of roll is has to be.

 Nani!? At least sling increases the far range.

Quote from DxJxC >>

Quote from InquisitiveCoder >>

In my own games I changed picking stuff up from the ground as an action to prevent absurd combat maneuvers like those. 

 That also makes disarming enemies a viable strategy.

"I use my action to disarm the enemy." "Roll, you succeed. Enemy's turn, he picks up his weapon as a free interaction and attacks you."

If it replaces an attack to disarm, why can you retrieve your weapon for free, WotC?

 I use my action to disarm my enemy. I use my free action to pick it up. Or stand on it.

Works both ways.

Quote from InquisitiveCoder >>

.. you only get one object interaction per turn. You load the sling, draw your wand, make the attack, and now you don't have any hands free to load the sling for your next shot.

 

 The druid and warlock only get one attack per turn anyway, so they don't really lose out. And they are holding their arcane focus for follow up spells.

Fun fact I discovered though: historically slingers would use shields, because a shield strapped to the arm would still leave the hand free enough to get a rock out of a pouch and slot it into the sling. Too bad about game mechanics.

Funny, the Artificer has an infusion to remove the ammo quality. So an Artificer could be more along the lines of that. (Not that applies with this situation cuz the infusion creates ammo for it, moment you want to use a magic ammo you regive the property lol)

Also: I believe wand of the warmage wouldn't stack with a sling. Sling + Ammo Property prevents you from holding the wand to get the + from the wand. However if you just threw it, you would (as it's considered a ranger spell attack). 

Without a house ruling on preloading it you wouldn't be able to, or some customized can't I use the wand as the stick part of the sling house ruling. .

Quote from MarioInfinity >>

 

 some customized can't I use the wand as the stick part of the sling house ruling. .

 Slings don't really work like that. A wand is too short for a staff sling.

You wouldn't need a house rule on preloading. It must be possible as they talk about in the PHB using a sling as a melee weapon and how it has to be loaded to do that.

Quote from Epargreboog >>

You wouldn't need a house rule on preloading. It must be possible as they talk about in the PHB using a sling as a melee weapon and how it has to be loaded to do that.

 They do say that, but there are still no written rules refering to loading in a context outside of the attack action, so any rules for pre-loading would be house rules.

(I'm not say can't be done, or shouldn't be done, just that there are no written rules for it.)

It would be similar to making a ranged attack where loading and attacking are the same action. Loading the sling stone or bullet would be done right before you smack somebody with it.

i have a druid that has a hat of endless rocks, that he uses for magic stone. seeing as magic stone is a cantrip, he just has to use an enlarge spell on the magic stone as it hurtles forward.  screw the sling. enlarge the stone.

Quote from RelicasWise >>

i have a druid that has a hat of endless rocks, that he uses for magic stone. seeing as magic stone is a cantrip, he just has to use an enlarge spell on the magic stone as it hurtles forward.  screw the sling. enlarge the stone.

 Not sure how you would go about doing that and casting enlarge on a flying sling stone wouldn't do much.