Trying to disassemble the shotgun and I can't get the damn barrel off. I'm assuming this is due to the duracoat that is applied to the barrel and receiver. Any ideas on how to get the barrel to come off?

Start with pulling the slide to the rear about 3/4 way and unscrewing the mag tube cap.

That extended magazine is really cranked on. I had
To get a pair of pliers out to loosen the tube and nut.  Don't mar up the nut like I did. Oh we'll it's a tool ;-)

Magazine extension and the cap is completely off. For some reason two fairly strong young men can't pull the barrel off.

Is this a shotgun on which someone would have had to remove the mag tube dimples? If so, and they used the socket method to swage out the dimples, they may have expanded the end of the mag tube right where it fits in the barrel ring.

More than likely the Duracoat added thickness to the barrel extension where it fits into the receiver and /or on the mag tube where it fits into the barrel rings causing the tight fit.

Hold the barrel in a padded vice and pull harder. Don't twist the receiver side to side while pulling, you could break off the tip of the ejector.

soak with kroil then pull harder.

one shell ought to do it

Don't feel alone I balled up the one in my wife's 1100 so bad I had to order a new collar and tube trying to get it out. Any way a lot of Rems stuff has been shipping out almost impossible to take down. It's probably just the finish or a combination of that and a rubber mallet which also works well getting old 870 barrels freed up.