Look closely and you will observe the rifling grooves  on the inside of the deck gun barrel.

OPERATION RISING SUN

This image includes the forward  heavy deck gun and an open hatch. Several hatches were observed to be open on I-52 and it is believed that they were intentionally left open due the extreme conditions of being under attack. The outer hatches lead to another hatch which closes on the pressure hull thereby allowing more time to prepare for the emergency dive.

 

The deck gun broke through the outer hull and is resting on the pressure hull