This a common thing with the older 870 I would have to replace the complete ejector system including the riveits on the left side of the receiver you have 2 choices cold blue the new rivits or reblue the receiver. cost with out a reblue runs $85.00 or $150.00 with reblueing of the receiver.
just the facts!
RON KONTOWSKY
DEANS GUNSHOP II
GUNSMITHING DIV.
http://www.deansgunshop.com