For the pre WW1 shotgun, the steels then were unpredictable due to the poor process controls of the period and the basic lack of understanding of metallurgy. We know that Springfield Armory did not have temperature gages in the forge rooms, probably did not have temperature gages anywhere heat was applied to metal. Instead, they relied on human eyeballs to judge heat by the color of the metal. I expect that commercial manufacturers were similarly behind in process technology. Instead of precisely being heat treated, older steel varies a lot in heat treatment, the worst case is if it was over heated and turned into a brittle part.
Plus, they were using plain carbon steel in firearms. Alloy steels, which totally outclass plain carbon steels in all structural applications, except cost, were not being used back then, except by Winchester. I have looked at Springfield Class C steel, which is typical of the era, that stuff is so low grade that it is used for rebar and rail road ties. The fatigue life of plain carbon steels is about a half to a third that of modern alloy steels.
So, if you shoot your shot gun, I would recommend only light target loads, nothing hot or magnum, as the firearm has already had one lifetime of use, and the uncertainty of the steel quality.