I have a first year model 12 that suffered from that condition. I'm thinking that it might be unwise to assume every gun that has this condition has the exact steel or the same exact problem, even though it all looks similar. This gun had spent many years as a farmers "barn gun" used mainly for shooting "barn ducks" or pigeons that gathered around a silo. The gun had seen little, if any, maintenance or care for many years. I'm thinking 50-60 years.
At any rate, shooting the model 12 with typical, modern ammunition, perhaps 500-1000 rounds, IF I recall correctly, made it disappear. The bore went from looking like frosted glass to looking like a mirror.
I was told by an elderly gent that it would clean up with use. He was right, this time.
Save that one experience, I can offer little information.
Best,
Ted
