Two Sponson Mounted Gravis Heavy Bolter Batteries:

When attacking with a weapon that has this special rule, the controlling player may re-roll all failed To Hit rolls.

Twin-linked Blast Weapons
If the Scatter dice does not roll a Hit, you can choose to re-roll the dice when making a Shooting Attack with a Twin-linked Blast weapon. If you choose to do so, you must re-roll both the 2D6 and the Scatter dice.

Twin-linked Template Weapons
Twin-linked Template weapons are fired just like a single weapon, but must re-roll failed To Wound rolls and Armour Penetration rolls.