How surfaced U-boats used deck guns inside convoys

How often did Second World War submarines use deck guns?

Often enough to shape convoy tactics; some U-boats surfaced inside formations until corvettes began waiting for them there.

Some Type VII U-boats are described as surfacing inside convoys and firing their deck guns at merchant ships. British defenders initially struggled to recognize that a submarine would deliberately operate in the middle of the formation. Wargaming by members of the Women's Royal Naval Service is credited with identifying the tactic. Putting a Flower-class corvette among the merchant ships then gave the convoy an escort positioned to confront the intruder. This is an explanation of a particular use of deck guns, without a numerical estimate of how often submarines fired them.