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When a player shoots haywire cannons at a T8 vehicle (S4 but on 4+ wound roll inflicts a mortal wound) do they need to reroll everything but the 6s, as the 4s are technically fails? Can they choose to reroll some but not all of the dice?

See here: https://www.dakkadakka.com/dakkaforum/posts/list/764871.page

To surmise:
Yes, reroll anything but the 6's, because it's not causing a wound, and the mortal wounds will only trigger after the to-wound roll is finished. Until then, no wounds are going through.
EDIT: Sorry, it's a MAY for the Doom Roll. So you don't have to, but you can!

Does doom say may or must?

That's your answer.

Is it an all or nothing choice? Can you decide to just reroll the 1,2,3s? And keep the 4s and 5s?

Riggs wrote:
Is it an all or nothing choice? Can you decide to just reroll the 1,2,3s? And keep the 4s and 5s?

Remember in these situations that Fast Rolling is just a work-around. Technically, you are rolling your attacks one at a time. So you roll one to-hit roll, the one to-wound roll, then choose whether or not to reroll it, then they make saves, etc. You do that each time. Fast rolling just gets you to roll clumps together, but you should always treat it as one-at-a-time. As such, you'll be able to reroll just the 1's, 2's, and 3's, because if you did it one at a time it'd be a "may" choice each time.

 Yarium wrote:

Riggs wrote:
Is it an all or nothing choice? Can you decide to just reroll the 1,2,3s? And keep the 4s and 5s?

Remember in these situations that Fast Rolling is just a work-around. Technically, you are rolling your attacks one at a time. So you roll one to-hit roll, the one to-wound roll, then choose whether or not to reroll it, then they make saves, etc. You do that each time. Fast rolling just gets you to roll clumps together, but you should always treat it as one-at-a-time. As such, you'll be able to reroll just the 1's, 2's, and 3's, because if you did it one at a time it'd be a "may" choice each time.

This is indeed the correct answer.

As a general rule, re-roll effects are all worded to be optional with, off the top of my head, two exceptions (Seraphim Shield of Faith invulnerable saves and Seeker's failed Charges). In any case, if a re-roll is mandatory it will be clear from the wording of the rule.