Boulder Salvo — ruling 1

For some cards, surge represents only an alternative cost, a discount that applies if you or a teammate has cast another spell this turn. Other cards, like Reckless Bushwhacker, have additional abilities or effects if you paid the surge cost to cast the spell.

Boulder Salvo — ruling 2

The other spell that you or a teammate cast can be one that’s resolved, one that was countered, or (for instants with surge) one that’s still on the stack.

Boulder Salvo — ruling 3

If an instant or sorcery spell cast for its surge cost is copied, the copy is also considered to have had its surge cost paid.

Boulder Salvo — ruling 4

Casting a spell for its surge cost doesn’t change its mana cost or its mana value.

Boulder Salvo : règles, fonctionnement et interactions (MTG)

Boulder 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

Surge (You may cast this spell for its surge cost if you or a teammate has cast another spell this turn.) Boulder Salvo deals 4 damage to target creature.

How it works : Boulder Salvo

Original English text — Wizards of the Coast / Scryfall

#1January 22, 2016· Wizards of the Coast
For some cards, surge represents only an alternative cost, a discount that applies if you or a teammate has cast another spell this turn. Other cards, like Reckless Bushwhacker, have additional abilities or effects if you paid the surge cost to cast the spell.
No other card is named in this ruling.
#2January 22, 2016· Wizards of the Coast
The other spell that you or a teammate cast can be one that’s resolved, one that was countered, or (for instants with surge) one that’s still on the stack.
No other card is named in this ruling.
#3January 22, 2016· Wizards of the Coast
If an instant or sorcery spell cast for its surge cost is copied, the copy is also considered to have had its surge cost paid.
No other card is named in this ruling.
#4January 22, 2016· Wizards of the Coast
Casting a spell for its surge cost doesn’t change its mana cost or its mana value.
No other card is named in this ruling.