I believe once I was outta torpedos and came across a cruise ship bound for England.
Why I decided to shoot at it til it sunk I have no idea; took myself right outta the game. Vowed never to do that again...
Edit:
Oh wait! I see now that you said "flak"; in that case haven't attempted anything bigger than a tugboat or trawler. Somehow I think some others responding may have also missed the "flak" requirement. An ore carrier? Really? with your machine guns? This is insane... Why did I read flak and think deck gun?