The elephant in the room is that shotguns actually designed to handle steel shot cartridges are all very recently manufactured using modern steel alloys, so the cartridges can be loaded fairly hot on the safe assumption there will never be a structural failure.

When we open the chokes on a vintage gun to remove the potential for ring-bulging of the forcing-cones at the muzzles, we do nothing to strengthen the gun's breech, lumps or chambers for modern proof pressures.

To err on the safe side, modern steel-pellet factory-loads should only be used in heavy duck or pigeon guns that pass scrutiny for strength, like the Greener Empire in my earlier post.

Simon Reinhold should have made that point more emphatically in his video IMHO.