The gun was seen as a alternative to torpedos of the day which were considered not to be as effective as a gun in some situations.
The Surcouf would have an interesting sub for the pacific, with little use of convoy tactics and limited escort co-ordination by the Japanese. American sub commanders had been crying out for years for bigger guns (5' instead of the 3' and 4' that were mounted) and more of them (some U.S. subs were fired with two 5' guns and rangefinder toward the end of the war and proved effective against what shipping remained)
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