Also the XX1 U-Boat the guns seam only to be 20mm. I have an old Historical document stating that the guns used on XX1 were 25mm and were Armer piercing rounds were used. This would have had serious impacked on any Cargo ship. Aircraft would not stand a chance with that sort of Ammo. The only issue would be hitting it. If you fired that at the hull of a Cargo ship.you would have lots of holes and it would sink.