The strategic concern behind the gun limits was the First World War model of large cruising submarines built to attack trade. German designs threatened the British Empire, and Britain experimented with the heavily armed M class as well. A torpedo carries its own propulsion machinery and fuel, making each shot expensive in weight and endurance. A deck gun lets one reusable weapon fire many shells against merchants, auxiliaries, or troopships that do not require a torpedo to sink. Article 7 addressed economical long-range commerce raiders as well as submarine dimensions.
Notable Point · 25:22-27:55
Why gun-armed submarines threatened commerce
Large cruising submarines could use reusable deck guns against merchant traffic, conserving expensive torpedoes for warships.