My six or seven year old Browning Citori came with choke tubes marked differently for steel and lead. This is because steel shoots tighter then lead out of the same choke as I believe we all agree. When Browning supplies a choke marked "Lead-Modified / Steel-Imp. Modified" with their shotgun, I would think it would be safe to shoot steel shot thru it for the life of the gun. A lead full choke tube should not however be used with steel shot as it is too tight a constriction and can cause barrel bulge. As I understand it, that is the only restriction on using steel shot in a modern shotgun.