The steel shot that bulged the A5 was #2s, in a 2 3/4 inch loading, that may or may not have been magnums, that were popular in the era. Wisdom (at the time) stated that if you used a number 4 for ducks, steel 2s were the choice.
Lead buckshot has bulged plenty of thin barreled doubles in the past 100 years.
Also steel shot has come a long ways, especially wads, since 1980. Models 12s, have not, however. I'd get a less valuable piece for steel.
Best,
Ted