Why gun-armed cruiser submarines appealed in the 1920s

The 1920s gun-armed cruiser submarine looked attractive on paper, although the concept arguably came before its time. The French Surcouf exemplified the large-gun approach, and Britain, France and other nations considered the idea. Shells occupied less space than torpedoes and cost less, making a gun attractive aboard a vessel with limited internal volume. In theory, the submarine could appear unexpectedly, attack merchant shipping and bombard land targets. The long-range commerce-raiding requirement made sense; the difficulty was turning the large-gun concept into a workable boat.