Stomachache post pics and information about special spacer for Remington 870 shotguns one piece magazine tube on Remington 870 Forum:

Remington 870 Spacer for Tactical Models with One-piece 6 Round Magazine Tube

Remington 870 Spacer for Tactical Models with One-piece 6 Round Magazine Tube

Remington 870 Spacer for Tactical Models with One-piece 6 Round Magazine Tube

Remington 870 Spacer for Tactical Models with One-piece 6 Round Magazine Tube

From Stomachache:

“Remington sells a spacer, part number F402819. Sells for $22.39. Just call their customer service line and select parts. Takes 7-10 days.”