I've seen far more French guns with pitted bores than without. I think it's a combination of the metallurgy, corrosive primers, and the relatively damp climate of rural France. I suspect, also, that the mercury salts in corrosive primers worked their way into the steel and stuck around longer than one cleaning.
I know with my French gun, liberated from a Norman barn in the summer of '44 and well-maintained since such that it has bright bores, that if I take it out on a damp day I had better clean it on returning to the house (as soon as it warms up) because I will see some red rust coming out on the patch if I wait even a day. And that's using contemporary shells.
