Odd that you mentioned that today. I was just about to look through the mod section to learn how to tweak the gunfiles myself after haveing that experience yesterday night:
I had a medium old frighter which I had already hit with 3 (!!!) MK10 torpedoes. It listed heavily but still refused to sink. No way I could finish her off with the deck gun.
Not one scratch after 50 rounds (!!!) most of them close to the waterline or (as intended) slightly below. No matter what ammo I used the hull wasn't breached.
And reloading times of almost 30sec per round seems a bit slow to me too.
I'm using RFB 031408 and NSM 4.0.