The only difference between a standard 870 and the Marine Magnum is the extended magazine.
To disassemble the magazine, unscrew and remove the screw from the barrel clamp and slide the clamp forward and off the barrel and magazine tube.
VERY CAREFULLY unscrew the magazine tube, leaving the grooved collar in place.
Be very careful, since the long, STRONG magazine spring will attempt to force it's way out as soon as the tube is completely unscrewed.
Wear glasses and maintain control or the magazine tube.
A good technique to maintain control is to unscrew the extension and allow it to move up JUST enough to get your fingers between the extension and the magazine tube.
Grab the spring and hold it firmly in place as you remove the extension.
Then, carefully allow the spring to slide out of the tube until tension is relaxed.
Remove the magazine spring from the gun.
Open the action about half way, then unscrew the grooved collar on the front of the gun's magazine, and pull the barrel forward and off.
The magazine follower can then be pushed out from the rear of the tube.
The rest of disassembly is described in the owner's manual, and is the same for all 870's.
To reassemble, replace the magazine follower.
With the action about half way open, carefully insert the barrel into the receiver, fitting the barrel's support ring over the magazine tube until it seats in place.
Screw the grooved collar in place on the magazine.
Screw the collar down snug, then turn one more "click".
The collar needs to be tight, but NOT too tight.
Insert the magazine spring into the magazine.
Put the extension tube over the magazine spring and feed the spring into the magazine as you force the extension into place.
Screw the extension in until it just stops turning.
This will insure the extension is properly butted against the gun's magazine, but not so tight as to cause misalignment.
Slide the barrel clamp over the extension and barrel until the hole lines up with the groove in the extension, and replace the screw.
