Does Opponent move Sneaky Deck hand to start showdown with no other units present, and the other player responds with Cannon Barrage—does it go off?
No, your Cannon Barrage will not deal damage to the Sneaky Deckhand.
Here is the breakdown of how this interaction works based on the rules for showdowns and the chain:
Sequence of Events
- Move Initiates Showdown: The opponent moves Sneaky Deckhand to an empty battlefield. This action creates a contested battlefield and opens a Non-Combat Showdown (Rules 446.1, 344.2).
- State and Priority: Per Rule 342, the showdown creates an "Open State" where players can play spells. The moving player gains the first focus.
- Opponent Passes Priority: Assuming the opponent (the moving player) passes their first priority without playing a card, you gain focus/priority to play an action or reaction (FAQ #4290).
- Playing Cannon Barrage: While Cannon Barrage is a legal play during this showdown, it does not affect the Sneaky Deckhand.
- Chain Resolution: The chain resolves in Last-In, First-Out (LIFO) order. Because Sneaky Deckhand is not "in combat" (as combat only occurs when a battlefield is occupied by opposing units, per Rule 741.3 and FAQ #1596), the damage effect does not apply.
- Aftermath: After the chain resolves, the game returns to an open state, and players may continue to take actions or pass focus until both players pass in sequence, at which point the showdown concludes (Rule 347.3.a).
Important Clarifications
- Damage and Might: Per FAQ #5165 and FAQ #1193, damage dealt to a unit does not reduce its might. However, if damage dealt by a card equals or exceeds the might of the unit, the unit will be killed.
- Combat vs. Non-Combat: Per FAQ #7200, a showdown occurs regardless of whether an enemy unit is present. Because your opponent moved a unit to a contested battlefield, the showdown is valid, and your reaction timing is fully supported by the "Open State" rules for showdowns (Rule 342).
Note: I could not fully verify this ruling against the retrieved rules.