There is no indication that there's a die roll involved. You land the railgun shot in the SCB Activating phase (IE: Between that first obvious big blue flash that runs the length of the ship's shield bubble from nose to tern and the point the when the shields start refilling), their shields go pop. As written, they are completely depleted, meaning the ship hit is knocked into the whole loop of waiting out the overloaded generator then rebuilding to 50% capacity.
Let me be clear, I'm not saying big ships should be immortal, nor that there isn't a place for a weapon that serves as a bane of sorts for SCB's, one that rewards a well timed skill-shot. I am saying giving this effect to a weapon this precise and easy to use, however, is massive overkill (45+ seconds of shields down under heavy fire from multiple targets gunning for your power plant with railguns in a big, slow or un-maneuverable ship is a virtual death sentence). Would much rather see the effect be something like "Shields blink off until the SCB completes its refill phase at which point the shields come back on at their refilled strength." Still lets you open up a window to hammer a target, one that (with a enough railguns pointed at it) still has potential to be fatal for probably any ship, but rewards the player that survives that sudden vulnerability with much restored shields that haven't been losing HP due to attrition in the interval they were down. It also keeps SCB's from becoming suicide buttons on large, slow vessels.
