I hadn't removed the tube in two years. No, I hadn't used the shotgun in two years, either!
Well I'm guessing that's 66% of the problem, the other 34% was likley the prosess you used to clean it after you shot it last.
Not to knock all the new wiz-bang features of the "Traditional" gun,,but them new wiz-bang cleaners that usually get sold with'm could have had some adverse affect,,,
Most anti-sieze products do work well, but anything can be neglected, even chrome can rust! I know, I have a chrome lined barrel from Cabelas, shot good for years,,til I skipped a few days before I cleaned it, that's all it took, it was steady downhill from there.
Good luck in your choice,