Pooch, I'd say more likely corrosive primers than anything else. You don't see it nearly as often on Brit guns, especially not nicer boxlock or sidelock ejector guns. But that may be because the Brits had a periodic maintenance habit with their guns--sending them off to the maker or a good gunsmith after the season for a good going over, which would have included a bit of polishing and honing if necessary. I also think the French had a higher percentage of more basic guns than the Brits--more like us in this country, or what the Brits might have called "keeper's guns"--that only received attention from a smith if something broke.
