I have to respectfully disagree with you Bigbulls.

The difference in markings is because steel shot doesn't deform like lead shot and tends to pattern much tighter than lead.

A choke bored to shoot a modified pattern with lead shot, will shoot a an improved modified or full choke pattern with steel. An I.C. lead choke will shoot roughly a modified pattern with steel. You do not want to use a Full choke made for lead shot with steel- it won't harm anything, but the patterns will likely be terrible.

If steel shot bulged barrels with chokes made for lead, there would be hundreds of thousands of older (pre-screw-in-choke) shotguns out there with bulged barrels from shooting steel shot through them, and this isn't the case.