If you look at the deck where the gun is placed, (I'm talking the model deck here) you'll see a couple of hatch covers on the side of the gun that is closest to the tower. It was underneath this hatch that a ready supply of shells was kept in a small enclosure that was seperate to the pressure hull, or if it wasn't seperate then there was no access to it from inside the P hull. If these shells were exhausted then more were brought up from below as described in the book outlining Wolfgang Luth's U boat career.