Below is a how to guide on breaking down, cleaning, and reassembling a Remington 870, complete with pictures of each step. The shotgun pictured is a Remington 870 XCS Marine Magnum, but the steps will be the same for any of the Remington 870 family. The steps for other shotguns are similar as well. Before beginning, ensure that the gun is unloaded, and that the hammer is not cocked. I can’t over stress the importance of ensuring the gun is unloaded.

Disassembly

Step 1: Assemble your cleaning suppliesShotgun cleaning supplies
Get your cleaning supplies together. Each person comes to prefer certain products; my favorites are shown. From left to right are a bore snake, a cleaning rag, a cleaning patch, and gun solvent/protectant. The bore snake will be used to clean the barrel, the cleaning rag and cleaning patch will be used to wipe down the interior and exterior surfaces of the gun, and the solvent/protectant will be used to clean the barrel, interior and exterior surfaces, and the trigger group. For a solvent/protectant, I use Break Free CLP, and highly recommend it.

Step 2:Unscrew the magazine capUnscrew the Remington 870 xcs marine magnum magazine cap
You should be able to unscrew this by hand, but if it is stuck then wrap it with a cloth (to avoid scratches) and then use pliers to gently unscrew it. Use caution, as the magazine cap is spring loaded and will pop off if you don’t hang on to it as you unscrew it.

Step 3: Remove the barrelRemove the shotgun barrel
once the magazine cap is off, the barrel will slide right off the gun.

Step 4: Remove the forend and bolt carrier assemblyremove the forend and bolt carrier assembly

Flip the shotgun upside down and use your fingers to push out the spring loaded metal strips. By “push out” I mean to press from the inside of the opening, attempting to flatten out the metal strips. This will release the forend, and you can pull it out just as the barrel was pulled out. You will likely need to press in the strips while pulling the forend, and then repeat the process a couple times. This step is the hardest to describe but is easy to actually do.

Step 5: Remove the trigger groupremove the trigger group
There are two pins that hold the trigger group in. To remove the trigger group, use a wooden dowel rod or tap about the size of the pin to tap out the pins. On my Remington 870, the larger of the pins comes out easily, while the smaller of the pins requires gentle tapping with a hammer. Note that you will want to use a wooden dowel rod or other tool that won’t scratch the gun. I’ve used screws, nails, and screwdrivers to tap out my pins, which is why there are superficial scratches. Once you remove the pins, the trigger group can be slid out from the bottom.

Cleaning

Step 1: Clean the barrelClean the barrel
Spray some of the solvent/protectant into the barrel, then thread the bore snake into the barrel from the breach end. Pull the bore snake through the barrel a few times, until the inside of the barrel looks clean and shiny. Don’t forget to clean the exterior of the barrel too.

Step 2: Clean the interior and exterior surfaces of the shotgunclean the gun's surfaces
Spray a some of the solvent/protectant onto the cleaning cloth and/or patches. Use them to wipe down the interior and exterior surfaces of the gun. A little of the solvent/protectant goes a long way – goal is to clean the gun, not to raise the stock price of the solvent/protectant manufacturer. I have used the same can of Breakfree CLP for over 30 cleanings of my Remington 870 XCS Marine Magnum.

Step 3: Clean the trigger Groupclean the trigger group
Ensure that the safety is on, since pulling the trigger now could cause the hammer to strike your finger, causing a good deal of pain and injury. You could also damage the trigger group. In fact, the hammer should not even be cocked (as mentioned above) but keep the safety on for good measure.
Remington states that the trigger group should be cleaned using RemOil, their particular. solvent/protectant brand. I happen to have a can of it, so I use it to clean my trigger group. That said, I seriously doubt that it is essential to use RemOil. To clean the trigger group, just spray it with the solvent, shake it off, and let it dry. If I’m in a hurry I’ll use compressed air and dab it with a cloth to remove excess solvent/protectant.

Reassembly

Step 1: Reinsert the trigger groupreinsert the trigger group
Slide the trigger group back in to place, and tap in the pins which hold it in place.

Step 2: Reinsert the forend and bolt carrierreinsert the forend and bolt carrier assembly
Slide the forend back in. You may need to press upon the same metal strips that you pressed upon when you removed the forend and bolt carrier.

Step 3: Reinsert the barrelreattach the shotgun barrel
The barrel will simply slide back on the shotgun

Step 4: Reinsert the magazine cap and related itemsreattach the magazine cap
Insert the (orange) shell follower into the magazine tube, followed by the spring. Then place the magazine cap over the spring and screw it in. It is spring loaded, but reinserting it gets very easy with practice. Hand tighten the magazine cap until you cannot turn it any further. Using pliers is generally not recommended because applying too much torque can damage the shotgun.

Your shotgun has now been disassembled, cleaned, and reassembled. Wasn’t that easy? :)

A video clip showing the disassembly process can also be seen here.