Overkill, 43Shooter pretty much nailed it. The residue from the shells I think is a lube they use in manufacturing so shells go easily through the machinery. I've only seen it in trapshooting situations, hot barrel and lots of rounds. More likely the factory preservative in the chamber turned gummy.
I've seen shotguns stuck so hard a strong man couldn't open the action. The trick is to run a range rod down the barrel and pop the shell while trying to open the action. Opens like magic. See, the extractor is forcing the hull against the side of the chamber for more friction. A pop on the base of the shell straightens it out.
Now polishing may be the thing where the chamber has a little rust. Usually 000 steel wool is enough.
Modern shotguns like the 870 do best with a minimum of lubrication. Modern steel requires almost none, the excess just picks up carbon and dirt. Makes a nice sludge which will bugger things up. Keep it clean and get thyself a chamber brush.
