One shot goes in. Four or five come out. If you're lucky.If you're in a mod that has passthrough weapons with a low enough shot speed to be internally reflected or script meddling has made it nondirectional you can get projectile multiplication limited only by the damage the station takes when the shots fail to reflect.
Unlike ship armor segments and shields, stations do not lose reflectivity as they take damage so if you have internal reflection the station dies in one shot and you get omnidirectional death.
Reflection from events can be 100% (eg. related to a damage threshold) and produce shot multiplication limited only by the game crashing.
I'm pretty sure the Katana Starcannon didn't produce shot multiplication cascades against Dwarg Fortresses or Behemoths in previous versions so I think this is new with whatever change to passthrough makes it apply to shields.