In the German Navy the size of deck gun (surface action) was based on class of submarine. 10.5CM gun mounted on type IA and type IX (except IXD) The 8.8CM gun was mounted on type VII's. Size of the boat overall, dimensions of the deck casing, and storage space for gun ammunition all play a part in the decision of what size deck gun to put on a submarine. Submarines are notoriously poor gun platforms. In any kind of sea except flat calm it becomes a major challenge for the gun pointer and trainer to keep the gun on target. You also have a number of crew members on deck serving the gun as well as an ammunition handling party below decks creating lots of confusion there.

Picking the wrong target for a surface gun action can ruin your whole day. Thats what happened to USN S-44 of Paramushiro in the Kurile Islands in september 1943. Using a radar contact, the skipper ordered battle surface gun action on what he thought was a small merchant vessel. It turned out to be a destroyer. The skipper couldn't get the gun crew below and the boat buttoned up fast enough to avoid being hit by return shell fire from the destroyer before he could dive. S-44 was sunk, there were two survivors. A classic example of what can go wrong during submarine surface gun actions.