[off-topic]AAH I've read Hilter's U boat war a year ago, when I borrowed it from a public library. Quite a good book and it's pretty informative
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No surprise that U boat captains use desk guns to sink targets - the torpedoes lack enough explosive charge (warhead is half of that in WWII while enemy ship size is similiar), the shortage of torpedo supply which means torpedoes had to be saved for large targets, means desk guns are necessary to sink ships, and that's why British could employ Q-ship successfully. Without luring a Captain to surface the boat and uses his gun, any hidden guns won't work