'Built under license from the Swedish manufacturer, the 40-mm Bofors was one of the most popular medium anti-aircraft weapons of World War II, and in a modernized form remains in production to this day. Most naval applications used a twin mount. Not originally fitted to submarines, the 40-mm was added to the arsenal when commanders argued that they needed something to fill the gap between the 20-mm and the deck gun for close-in attacks on small vessels. The single-barrel “wet” version used on submarines had seats for the aimer and trainer, who could elevate, depress, traverse, and aim the gun with hand controls. Stops were fitted to prevent traversing and/or depressing the gun to where it could hit the boat. Sights were fitted to both sides of the gun, so that either operator could aim when necessary.
Besides its rarely-used anti-aircraft role, the 40-mm was used for attacks on supply junks and other light craft that weren’t considered adequate targets for torpedoes or the main gun. (In actual practice, this was virtually the only thing it was used for, as shooting at enemy planes was highly discouraged except in harbor, where there was no possibility of submerging.)
:yeah:Later in the war, a 40-mm was usually installed on the cigarette deck at the aft end of the conning tower, where it replaced the original 20-mm mount (which, in the case of most early boats, was itself something of an afterthought). Some boats also added a second 40-mm in the forward gun position at the front of the conning tower. Ready use ammunition was kept in pressure-proof lockers near the gun positions, where it was easy to get at.