Thanks for the replies. That Captain's log is really helpful. He used guns on a steamer as he had no torpedo ready. Sounds like he hit it hard with just 6 rounds. So guns were effective. But for 2 other ships in the area he was going back to torpedoes, only they were too fast and daylight was approaching.
My impression from reading his entire log is that guns were used mostly for mop up. His log is from late 1941 to early 1942 and he was operating off the U.S. coast. I noticed he made a number of references of trying to attack from range so he would have time to evade destroyers. Good stuff.
So before the escorts get armed Guns it is I guess
Though I think I will forgo the gunner qualification. Thanks to the uboat aces site I learned how difficult it was to man a gun on deck. With rounds being passed up from below and the gunner crew having to be tied on a line so as not to get swept overboard.