Lagopus hit the nail on the head regarding pitting. It seems that as long as black powder was the norm most everyone understood the need for a through cleaning & did so.
With the switch to smokeless due to the small amount of residue left in the barrel many felt this need for cleaning had been eliminated & did so. The old primers did their job on the bores.
The common methods of cleaning the black powder residue has also taken care of the priming, which had always been the primary source of corrosion, though was not recognized until some time after the advent of smokeless.
Julian Hatcher has a very good write up on this in his "Notebook".
