I believe all factory ammo whether lead or steel is high pressure close to SAAMI maximum and it is done to insure reliability in gas operated autoloading shotguns.
I am not engineer but I also believe the thick plastic wads used in steel shells is to prevent scoring the barrel. I don't think it fully protects from the potential of choke bulging/rib loosening as the steel pellets are not going to give as much as lead as they shift when entering the constriction.
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