You are going to have to remove the dimples in the mag tube that holds the spring and mag cap retainer - abortion that it is. Some bigwig's son must have come up with that. The old metal retainer and detent retained cap was 200% better.
What could have happened... did.
ive tricked my 870 express with tactical pistol grip stock.....sent barrell off to polychoke .barrell is now 20''with 9 different choke settings on it..#4 buck patterns 18''circle at 35yds.....perfect for defense.....when set between mod /full