I gotta go with daboarder here. The fact that the d6 role can result in more actual wounds than models hit would lean towards each MODEL rolls a dice.
D6 - 1
Vehicle or Building - Clipped: The target suffers a penetrating hit.
Non-vehicle - Lucky Escape: The model is unharmed.
D6 - 2-5
Vehicle or Building - Solid Hit: A Super-heavy vehicle loses D3+1 Hull Points. Other
vehicles suffer an Explodes! result from the Vehicle Damage table. Buildings suffer a
Detonation! result from the Building Damage table.
Non-vehicle - Seriously Wounded: The model loses D3+1 Wounds.
D6 - 6
Vehicle or Building - Devastating Hit: As for Solid Hit, above, except a Super-heavy
vehicle loses D6+6 Hull Points instead.
Non-vehicle - Deathblow: The model loses D6+6 Wounds.
Let's say you hit a termagaunt pile and get 6 hits. Then you roll 6d6, all resulting in a 2+. That would mean you get 12 minimum wounds from that, instead of just 6 gaunts.
I'm pretty sure it's a model by model basis, otherwise you could get twice/three times more wounds from these.
It's the ultimate shot, more or less. It just eradicates that circle.