Gun readiness required daily attention to moving parts and checks that barrels were clear. Crews also checked ready-use ammunition and the commander's required mixture of explosive and incendiary shells. Deck guns could throw shells about 14,000 metres, farther than lookouts could see from the conning tower. Aiming from a pitching and rolling submarine was much harder than the nominal range suggested. The aimer received a target and fired when it crossed the sights, rather than firing at the instant an officer gave an order.
Notable Point · 1:31:10-1:34:22
A U-boat gun's range exceeded practical aiming conditions
Maintenance, low observation height and the submarine's motion limited the practical use of a deck gun.