Spells cast from the ring use normal actions and no components.
Many magic items allow the user to cast spells. The general rules for this are on p. 141 of the DMG, under the Spells subsection of Activating an Item:
Some magic items allow the user to cast a spell from the item. The spell is cast at the lowest possible spell level, doesn't expend any of the user's spell slots, and requires no components, unless the item's description says otherwise. The spell uses its normal casting time, range, and duration, and the user of the item must concentrate if the spell requires concentration.
(emphases mine).
Beyond these rules for magic items, casting a spell follows the rules in the Player's Handbook; generally, the character casting the spell takes the Cast a Spell action on their turn.
Here's what the Ring of Spell Storing says about casting the spells in it (DMG, p. 192:
While wearing this ring, you can cast any spell stored in it. The spell uses the slot level, spell save DC, spell attack bonus, and spellcasting ability of the original caster, but is otherwise treated as if you cast the spell.
Let's look at two examples of stored spells being cast:
- Fireball stored in the ring at 3rd level by a 10th Level School of Evocation wizard with a save DC of 16
- Magic Circle stored in the ring at 5th level by a 11th level Cleric with a save DC of 17.
Fireball, cast from the ring by a 3rd level Sorcerer (save DC 14):
- Will require one action to cast (same as the spell)
- Will do 8D6 damage; the original casting wizard's Empowered Evocation feature will not apply since it is not one of the characteristics that the ring lists as being 'stored'.
- Can be used with Quickened Spell by the casting sorcerer, since that falls under being "treated as if you cast the spell".
- Will have a save DC of 16, from the original caster.
Magic Circle, cast from the ring by a 19th level Fighter:
- Will require one minute to cast (same as the spell).
- Will not require any chalk or other material components, even though that is normally required.
- Will have a duration of three hours (base duration plus two extra hours for using a 5th level slot).
- The fighter has no spellcasting ability, and thus no spell save DC, but the Magic Circle will have a save DC of 17.
The spell needs a save DC; the DC of the Sorcerer who triggers it is used
In the case you offer, fireball, the magical effects require that anyone within the fireball's radius make a saving throw.
What is a saving throw made against? The spell's DC.
Spellcasting Ability
Bards, paladins, sorcerers, and warlocks use Charisma as their spellcasting ability, which helps determine the saving throw DCs of spells they cast. (Basic Rules p. 62)
Saving Throw
The Difficulty Class for a saving throw is determined by the effect that causes it. For example, the DC for a saving throw allowed by a spell is determined by the caster’s spellcasting ability and proficiency bonus. (Basic Rules p. 62)
How do you set the DC for a spell from a wild magic surge?
Use the DC of whomever triggered the surge. The Wild Magic Surge rules text (PHB p. 103) does not offer a separate DC rubric for that particular magical effect, so it is tied to the sorcerer who is the cause of that magical effect.
Is that stated explicitly anywhere in the Rules? Not as far as I can find.
Does it make sense? Yes.
Why? D&D 5e design principles were to make the game simpler, not more complex. If you accept that the "you cast a spell" rules text is a pointer to how the DC for a spell effect from a Wild Magic surge is determined (so that the DM does not have one more thing to rule on or figure out) it meets the simplicity standard.
Try the common sense test
Should a wild magic surge from a 19th level sorcerer be more potent than from a 1st level sorcerer? (Difference in DC being +6 to +2 based on proficiency alone) and thus harder to save against?
Yes, common sense would suggest that this is the case.
(If I can find a Crawford tweet on this, I'll add it).
From the Sage Advice Compendium:
Does a sorcerer’s Wild Magic Surge effect replace the effect of the spell that triggered it, or do both effects happen? The spell and the Wild Magic Surge effect both happen.
This supports the point that the two are directly tied to the Sorcerer who cast the original spell that triggered the wild magic surge. It shows a linkage.
Further linkage between the Sorcerer and the wild magic spell effect
A wizard multiclasses into Wild Magic sorcerer. Do spells cast from their spellbook trigger Wild Magic Surge if they are on the sorcerer spell list or do they have to gain them from Sorcerer to trigger? From the multiclassing rules: “Each spell you know and prepare is associated with one of your classes” (PH, 164). This rule means only the spells gained from levels in sorcerer trigger Wild Magic Surge.
Direct linkage between sorcerer, sorcerer spells, and Wild Magic Surge.
Bottom Line
Your sorcerer casts a sorcerer spell, a wild magic effect (in this case a fireball spell) goes off at the same time thanks to the roll on the table, so the effect is that your sorcerer cast both spells thanks to the wonders of wild magic.
Best Answer
Based on the wording there, I would be inclined to say that it cannot wild surge when cast from storage unless the wild mage is also the one casting it from storage.
This doesn't mention wild magic as one of the things that's retained from the original caster, so if someone other than the wild mage used the spell there would be no surge. If the wild mage is the one to use the stored spell, since it is treated as them casting the spell for everything not explicitly listed, I'd say there's a chance for a surge.
As for storing the spell, it says that the spell has no effect, but the description of the Wild Magic Surge feature says
This strongly suggests that the wild magic effect is not part of the spell itself, as it happens after your cast the spell. In particular, no wild surge effect modifies the spell that triggered it. So it seems fair to say that storing the spell can trigger a surge.