I have spent over a year trying to get my 870s working. I am not an 870 hater. I love the 870. I do, however, have a very low opinion of Remington as a company. Every time I go to the trap and skeet club, I see newbies slamming the buttstock of their 870s on the ground to extract rounds.
My Wingmaster actually had the worst case of these tool marks. It was one of the ones that I had the chamber recut. It still malfunctioned. Now, it works fine.
It is not the chamber, it is not the preservative. It is the tool marks + steel based shells.