deckgun + early war + west coast of uk = complete slaughter
seriously get a type viib hang out near the southeast coast of ireland and you will rack up the kills. theres tons of unescorted single ship traffic here even big merchies on occasion if you sink every vessel you encounter via deckgun (even tiny boats) the tonnage adds up quick
when hunting merchies via deckgun its best to run with decks awash and approach from behind wait until your within about 3000m and open fire. make sure you sink em as fast as possible (waterline hits can sink even a large merchie in 10-15 shells) the longer they stay afloat the more time they have to call for help and when your in the enemys backyard it doesnt take long for help to arrive
with this method ive easily racked up 10+ ships sunk per patrol without the use of a single torpedo
now most kaleuns will agree that this is highly unhistorical but if you want easy tonnage this is a good way to do without exploiting the ai via a devious harbor raiding trick