Volcanic Salvo — ruling 1

The cost reduction applies only to generic mana in the cost of Volcanic Salvo. It can't reduce the {R}{R} requirement.

Volcanic Salvo — ruling 2

If a creature's power is somehow less than 0, it subtracts from the total power of your other creatures. If the total power of your creatures is 0 or less, the cost remains {10}{R}{R}.

Volcanic Salvo — ruling 3

You can't target the same creature or planeswalker twice with Volcanic Salvo to deal 12 damage to it.

Volcanic Salvo — ruling 4

Once you begin to cast Volcanic Salvo, players can't take other actions until you've finished casting it. Notably, they can't try to change the total power of your creatures.

Volcanic Salvo — ruling 5

The total cost to cast Volcanic Salvo is locked in before you pay that cost. For example, if you control three 2/2 creatures, including one you can sacrifice to add {C}, the total cost is {4}{R}{R}. Then you can sacrifice the creature when you activate mana abilities just before paying the cost.

Volcanic Salvo — ruling 6

To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions (such as that of Volcanic Salvo). The mana value of the spell remains unchanged, no matter what the total cost to cast it was.

Volcanic Salvo : règles, fonctionnement et interactions (MTG)

Volcanic Salvo is one of those cards players keep asking about. Here is, in plain words, what its official text says, the rulings published by Wizards of the Coast, and the combos players actually run.

Official card text

This spell costs less to cast, where X is the total power of creatures you control. Volcanic Salvo deals 6 damage to each of up to two target creatures and/or planeswalkers.

How it works : Volcanic Salvo

Original English text — Wizards of the Coast / Scryfall

#1June 23, 2020· Wizards of the Coast
The cost reduction applies only to generic mana in the cost of Volcanic Salvo. It can't reduce the requirement.
No other card is named in this ruling.
#2June 23, 2020· Wizards of the Coast
If a creature's power is somehow less than 0, it subtracts from the total power of your other creatures. If the total power of your creatures is 0 or less, the cost remains .
No other card is named in this ruling.
#3June 23, 2020· Wizards of the Coast
You can't target the same creature or planeswalker twice with Volcanic Salvo to deal 12 damage to it.
No other card is named in this ruling.
#4June 23, 2020· Wizards of the Coast
Once you begin to cast Volcanic Salvo, players can't take other actions until you've finished casting it. Notably, they can't try to change the total power of your creatures.
No other card is named in this ruling.
#5June 23, 2020· Wizards of the Coast
The total cost to cast Volcanic Salvo is locked in before you pay that cost. For example, if you control three 2/2 creatures, including one you can sacrifice to add , the total cost is . Then you can sacrifice the creature when you activate mana abilities just before paying the cost.
No other card is named in this ruling.
#6June 23, 2020· Wizards of the Coast
To determine the total cost of a spell, start with the mana cost or alternative cost you're paying, add any cost increases, then apply any cost reductions (such as that of Volcanic Salvo). The mana value of the spell remains unchanged, no matter what the total cost to cast it was.
No other card is named in this ruling.