There is another difficult to identify problem of the Remington 870. Sometimes, it is very difficult to to pull the forend after the shot. You need to pull it several times to extract the fired shotshell from the chamber. Polishing a chamber helps but it still can happen.
Recoil lug has a lot of tooling marks and all they make ejection of a fired shotshell harder. Also, you can even hear the sound of a rim going over those marks as round is fed into a chamber. You need to polish them to make feeding and extraction smoother. Also, I noticed that I have round marks on recoil lug on my older barrel, they are normal signs of wear, you just need to polish them regularly. I think that rim of the shotshell is held by those bumps.
I used dremel and sand disc to remove tooling marks and then polished that part with wheel and polishing compound. Here is how it should like to have smooth extraction of fired shotshells:
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