It breaks like a Model 12 and on top of the mag trube there are screws. Remove them and the cap comes flying off along with spring and plug. Do not loos the parts, hard to find. Replace the same way.
To remove the plug on a Remington 870 series all you need to do is unscrew the cap on the end of the magazine tube. Carefully because it holds in the magazine spring. the plug is the rod that is inserted in the spring so it cannot compress fully to limit capacity so just take it out and screw the cap back on (the cap compresses the spring) and voila. takes about twenty seconds.
Remove the magazine tube cap. The plug is inserted inside the spring, and cap replaced. You may want to have a gunsmith do this. It will take less time for him to do it than it will for you to find the springs that got loose.
Unscrew cap from end of plug chamber .Take your barrel off there will be a little tin cap at the end of your plug barrel take it out to get to your plug
You can request an owner's manual for free at the Remington website.
take bolt off
You can request an owner's manual for free at the Remington website.
Remove the magazine cap and forearm, then remove the barrel and mag tube. At the end of the tube will be a metal cap that you can carefully pry out with a screwdriver, some press in and turn to remove them. Use caution, it is under spring pressure.
You can get a manual from Remington by going to their website
The "plug" is just a stick slipped inside the magazine to limit the amount of space, but there's a big long spring in magazine too, so be VERY careful when you take the cap off the muzzle end of the magazine tube or stuff's going to go flying. (and save the stick, you may want to put it back some day)
eBay has reproduction manuals on occasion. Keyword Remington Model 12 and it should take you where you need to look.
Since there is no one Remington 20 g model, there is no one answer. If the owner's manual (available free thru the Remington website) does not help, take it to a gunsmith.
Take it to a gunsmith