The Breacher shotguns work on inverse Shatterhead Colony Missile rules, and borrow wording from that weapon. What it means is no matter how many Accuracy you throw at the Breacher, the shotguns (any roll) can never benefit from positive Accuracy. You can use Accuracy to cancel out Difficulty, but no attack roll with that weapon can benefit from Accuracy beyond that. I probably won't be changing the wording on this given, as noted, the same wording for the intended effect exists elsewhere in the game.