For reasons unknown, recent 870's with the extended magazine have been coming out of the factory with the coupling screwed on WAY to tight.
This is not right and can cause problems with the gun.
So far, no one has gotten an explanation from Remington why this is happening, or why they haven't corrected it.
Here's how to disassemble a Remington factory extension:
Unscrew and remove the screw from the barrel clamp, then slide the clamp off the gun.
Unscrew the magazine extension tube. (Leave the coupling ON).
WARNING: When the tube is unscrewed the magazine spring will try to force it's way out of the gun with GREAT force.
Wear safety glasses and be careful.
What I recommend is unscrewing th extension tube, and allow it to move off JUST enough so that you can get your fingers between the gun and the extension.
Grip the spring and allow it to escape a little at a time.
With the extension tube off and the spring out, unscrew the coupling.
If it's been installed too tight at the factory, pad a pair of pliers with leather and use that to unscrew the coupling.
If you slip and scar the coupling, call Remington and complain and make them send you a new coupling.
With the coupling off, the barrel is slid forward and off, and the rest of the gun comes apart per the owner's manual.
To reassemble, replace the barrel, then screw the coupling on FINGER TIGHT, then give it ONE more "click".
You should be able to remove the coupling with just your fingers.
Install the spring, then screw the magazine extension tube on until it JUST contacts the gun's magazine tube.
DO NOT over tighten the extension, you want it to JUST contact the magazine tube.
Too loose, and there's be a gap between the two tubes that will cause jams.
Too tight, and the tube can mis-align, again causing jams.
Reinstall the clamp and tighten the screw down snug.
The clamp prevents the extension tube from unscrewing.
