just bought the youngster an 870 Express, had a really enjoyable first dove hunt. Had a few hard to extract shell(we used all AA), I was looking to order a non-MIM extractor and polish chamber with steel wool.
My question is, is there a chance that I could do any damage(make unsafe) by over polishing? or polishing the wrong area?
It only takes a small amount of polishing to get the job you want done. My Express would stick with certain ammo until I polished the chamber. I didn't spend more than 30 seconds doing it. I've never had a problem since and that's been about 10 years ago now.
It's a good idea to replace the extractor BTW. That and the follower spring are pretty much the only different parts between the Express and the Police that can be upgraded. I replaced that spring and the follower in mine.
An armorer for a large police department posted about how you can essentially make the Express almost as good as the Police and for a lot less money. He does it for his police department. The parts are not as polished on an Express and there are some other lesser parts on the Express but they work just fine. The spring and the follower matter if you want to use heavy loads like slugs and buckshot. You get a more positive operation of the action. And it doesn't cost much to change them.
The ejector is also MIM but there's no way to change it and it doesn't seem to matter anyway. The extractor will wear down quickly on the Express so changing it is key.
The other differences are a better dog spring and a different trigger group. The Express uses plastic but they seem to function just as well. Mine has held up through many rounds fired over 10 years including some very heavy loads.