I work at a Rem service center and here are a couple of issues with 870's we see frequently.

1. Poor heading cut. We see many 870's that have chambering and extraction issues not related to chamber finish. Most of these can be traced to an incomplete heading cut to the chamber mouth. The fix is to re-cut with a piloted reamer available thru Brownells. This will clean up the rim recess at the chamber mouth. You may also need to use a Dremel with a ganged cutoff wheel disc or thick diamond wheel to relieve interference at the 12 o'clock position on the barrel extension. Only do these remedies if you see bright spots or metal movement at the 9 o'clock position at the chamber mouth or a bright ring at the 12 o'clock position on the barrel extension.

2. Action binds, locks up, or locking bolt assy comes out of the receiver without shell latch depression when barrel is removed. This is usually caused by a bad shell latch (left) and/or bent battered action bar leg. Often the left leg is bent in and the end of the leg overrides the action bar lock in the trigger group. Remove the fore-end tube assy and make sure legs are straight. Straighten as necessary or replace with new. If the assy can be removed from the receiver without depressing the shell latches check the interface between the shell latch notch and the slide. This needs to be 90 degrees or the the slide will push it over and the assy will carry forward. Replace and stake a new shell latch if necessary. You'll need to use a notched staking tool made from a punch to allow clearance of the receiver.