Allow me to clarify something here.  This substation is basically an antechamber BEFORE the door leading into the armory, so that guards can have a defensive position to keep intruders out as they come through the hallway choke point.  Because the X in here are not observing military tactics, the door is just an obstacle, not a position meant to kill you.  The point is that it's not the door which has artillery behind it.  It has X behind it, and THEN there is the artillery door.  So, you CAN blast the substation gate.