Go to the Remington Law Enforcement web site and read the FAQ's.
http://www.remingtonle.com
They say that they spray the Police guns, inside and out, with hot cosmoline as a preservative for shipping and storage.
This needs to be removed before using the gun.
Use the owners manual to strip the gun down, including the magazine tube.
The 870 is one of the easiest shotguns to disassemble.
Follow the directions, but here's how to disassemble the magazine extension:
Open the action about 1/2 way.
Remove the screw on the barrel clamp, and slide the clamp off the gun.
Unscrew the extension tube....leave the collar in place for now.
NOTE: The spring is LONG, and STRONG.
Be VERY careful.
The spring will try to "blow" the extension tube off as soon as it's unscrewed, and the spring will try to force it's way out with great force.
I unscrew the tube, allow it to move off about 1/2 inch, then stick my fingers inside the gun's magazine tube and restrain the spring, allowing it to come out slowly.
With the extension and spring out, unscrew the collar, then slide the barrel forward and off.
If the collar is really tight, use some well-padded pliers to grip and unscrew the collar.
Follow the manual for the rest of the gun.
Spray the gun, inside and out, dripping wet with Rem-Oil or CLP Breakfree.
Let soak 20 minute to allow it to dissolve the cosmoline and flush it off.
The new lube will also "soak" into the rough parkerized finish and seal it.
Wipe the parts off with a clean rag and reassemble.
To treat the trigger group, spray it thoroughly, and let drain. Then shake off and wipe off the excess.
Before shooting, wipe the bore out.
Every time you use the gun or handle it, use a clean patch to replace the Rem-Oil or CLP.
I used CLP Breakfree, and this works VERY well. The CLP builds up a layer of Teflon on the finish and prevents rust.
