Hi everybody, Is it normal for the top part of the receiver (inside) where the bolt rides against it to have pits? I understand wear marks there but there are three places where I can see pits where metal was either removed or compressed back into a slope. Thanks!

That does NOT sound normal to me. Does it look like the metal was machined out, or a defect in the metal?

If I was to guess, I would say the locking block caused it as the bolt moved back and forth during normal operation (as if the metal on the receiver was softer than the locking bolt). The two "dimples" closer to the barrel are very small, but the one closer to the buttstock is a little bigger and it looks like it is located where the locking block would be as if the action was completely open. These "dimples" are in a straight line where normal wear would be. The shotgun functions normally but it concerns me if the pits would cause extra wear on the locking block causing it to fail eventually.

I have seen some like what you are describing. It doesn't sound like this should be normal, but the ones I have seen like that didn't seem to have any functional issues. I have never seen one fail at the locking block.

Remington's website states the 870 has a 2 year limited warranty. My 870 is older than that so that's out of the question. Should I try to smooth it out or should I leave it alone?

Leave it alone.

Monitor it for change.

Since you have no idea if those "wear marks" were there brand spanking new, you have no idea they haven't been there for years now, from the beginning as flaws in the original steel billet.

Pull your trigger group out and run your action very slowly, first top up. Feel/note where there's more friction vs where the "pits"/marks are. If you do same with receiver upside down, it should be more pronounced, particularly if you press down on the bottom of the bolt plate (like when you're running over the hammer). It shouldn't be much on the vertical curvature at the back of the receiver unless your bolt plate has some real rear wear.
Like OldStuffer says, you'll get insignificant "spots" in the forging and from cleanup. It is an Express; but it happens even on 870Ps. Unless you're getting significant or worsening wear marks on the bolt or shavings, I wouldn't worry about it. What's the year of manufacture of your Express (call Remington with S/N)? And I trust it's not a chromed WM bolt, right?
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