Forum adverts like this one are shown to any user who is not logged in. Join us by filling out a tiny 3 field form and you will get your own, free, dakka user account which gives a good range of benefits to you: No adverts like this in the forums anymore. Times and dates in your local timezone. Full tracking of what you have read so you can skip to your first unread post, easily see what has changed since you last logged in, and easily see what is new at a glance. Email notifications for threads you want to watch closely. Being a part of the oldest wargaming community on the net. If you are already a member then feel free to login now.

Do special rules like Guide or character aura special rules which let you reroll failed hits (or rolls of 1) also let you reroll overwatch or grenade pack drops which cause a mortal wound on a 6?

Overwatch yes, it is a shooting attack.

Grenade pack no, it's not a shooting attack.

It depends I'd say, some like Cawl and the other Domini have an aura that says "You can re-roll any Hit rolls of friendly <Mars> units within 6" in the Shooting phase" while others like Azrael are worded this way: "You can re-roll failed Hit rolls for friendly Dark Angels unit within 6" of Azrael". So in the first case we have a specific restriction that you can only benefit from the re-roll in your own Shooting Phase, while the second case allow re-rolls any time. First one doesn't allow Overwatch re-rolls because:

Overwatch is resolved like a normal shooting attack (albeit one resolved in the enemy’s Charge phase) and uses all the normal rules [...]

The enemy's Charge phase is not your Shooting phase, so no re-rolls in Overwatch.

And yes this is absurd that some can do this while others can't.