Deck guns conserved expensive torpedoes when a submarine found a genuinely isolated and poorly armed merchant ship. By 1941, however, Allied and German boats were increasingly focused on torpedo attack because convoys carried escorts both around and inside the formation. Surfacing to use a gun exposed the submarine to faster-firing weapons, radar, and reinforcement. German suspicion that an apparently lonely merchant might be a disguised Q-ship made the opportunity even less attractive. The tactical environment, rather than the mere availability of guns, pushed submarines back toward stealth and underwater attack.
Notable Point · 1:38:15-1:39:04
Escorts and Q-ships narrowed the deck gun's role
The exchange explains why deck guns became weapons for isolated merchant targets rather than the principal armament of wartime submarines.