Lucky shot, also any Iowa class battleship would blow Bismarck and her sister ship out of the water in short order. Iowa class battleships saw lot's of action in WW2 and not one was ever seriously damaged.
You are right because the Iowa's 16 inchers had a much higher muzzle velocity plus a heavier AP shell than the Brits had on their battlewagons.
The Iowas saw a lot action but unfortunately never had a chance to meet another enemy battleship in combat. Halsey blew his chance at Leyte Gulf when he fell for the Jap deception. Had he fought Kurita at Samar he had the Iowa, New Jersey, Alabama, Massachusetts, Indiana and Washington plus 12 cruisers and 50 tin cans vs Yamato, Nagato, Haruna and Kongo 6 cruisers and 25 tin cans. It would have been the day the Jap navy ceased to exist.