radius has a visual impact only if efectivenes is set to 5 and above. Thats why I set all shells to half the stock value (only EF, I left radius untounched). youŽll need to put a lot of rounds to sink it.
Yesterday I catched a large freighter in the middle of a storm at night (I couldnŽt see a thing so I stalked it by sonar only
) and put 3 torpedos on it. Then I manned the deck gun and put around 20 shells on it (my reload time is set to 30 seconds). The ship was badly damaged but still refused to go down so I finished the job with another torp.