The issue is that when the barrel constricts at the choke it compresses the shot slightly. Lead and bismuth being soft metals this isn't a huge issue as the shot will deform before the barrel/choke but with harder metals like steel the shot won't compress. Most likely will just open up what was a mod or full into an open choke but in worst cases cause dangerous pressure spikes and blow a barrel.
I believe the difference between steel rated chokes and lead only has more to do with the taper of the choke and less about barrel strength. Lead only chokes taper in quite abruptly while steel have more of a gentle taper. The abrupt taper is cheaper to make and was popular with cheaper manufacturers before the mandate of steel. The question is was Ithaca a company that made things the right way or the cheap way? Probably did it right but bismuth sidesteps the question altogether.Ithaca apparently advertised their roto forged barrels were strong enough for steel shot and warrantied them. That’s cool. Makes me want to invest in an Ohio M37. Ad was taken from an old Field And Stream book in 1978 so 5 years after my M37.
