A large gun installation put considerable weight on top of a submarine and added drag. Slow surface speeds did not remove the weight problem. Diving then made waterproofing and ammunition handling difficult. Shells and propellant stored above the strength deck, behind the gun, might detonate under depth-charge attack after the submarine dived to escape an escort. Storage inside the hull required a mechanism to raise ammunition to the gun. That created another opening that had to resist water pressure and be strengthened, with another possible route for water ingress.
Notable Point · 6:28-7:59
Gun-mount weight and waterproofing
Large topside guns added drag and weight; storing their ammunition above or inside the hull created different risks when diving.