The only "serious" and not real common problems with 870's are a broken magazine tube, or a broken ejector. The ejector replacement requires drilling out the old rivets and refinishing the receiver. Magazine tube is soldered in and will also need the receiver to be refinished.
A couple years ago a friend had a brand new Express Super Mag that would not eject, it would just dribble the case out the side. He brought it to me to look at, the ejector was installed backwards by Remington.
