Pressure limits for American 2 3/4" and 3" 12ga loads are the same (11,500 psi), so pressure would not be the issue in potential damage from shooting 3" shells in a gun with original 2 3/4" chambers. Shot charge and pellet size might be an issue, although as noted above it does not pertain only to steel. Possible to bulge barrels with very heavy loads of large lead shot. The scoring of barrels from steel would have been more likely to happen back before heavier and more effective wads were used in steel shot loads.
