One of the big design innovations/improvements of the Type XXI was the streamlined hull. This enabled it to go faster underwater than on the surface.
Adding a gun would be counter to that.
But, as was noted, by that late in the war, even the allied rowboats are toting guns.
In 1942 or so I sometimes encounter lone armed merchants at night, and I'll engage them from 7000meters or so. Usually I can gun them down before they can hit me. Usually.
After 1942, good luck with any surface attack. I usually retire my qualified deck gunners at that point and take on new petty officers and train them for AAA.
Steve
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