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P.S.: Before plastic collars were perfected for steel shot, there was also the possibility of the harder steel scoring or scarring the softer steel the barrels may have been made of.
And the efficacy of the plastic collars protecting barrels against scoring depends upon the shells being properly loaded and the collar/wad working properly. The reloading manuals will go on at length on how and why to be exceedingly careful loading steel, to avoid both it getting between the wad and barrel and ahead of the wad fingers/cup.
Add to that risk (and the barrel-bulging bridging risk) the need for additional velocity to provide killing energy to the shot because the steel loses velocity and energy much more quickly than lead, and you get additional pressure which the vintage double may, or may not, be able to handle.
Responding to the post immediately upthread: The steel will hit the choke at a much higher velocity, exacerbating the bulge risk that way, too. I don't think one would be able to segregate the risks of pressure, velocity and bulging in any meaningful way other then acknowledging they are there. They seem to be the kind of risks which grow non-linearly and exacerbate each other.
