It's not necessarily a "bad" idea. It just comes with caveats. Most automatics that were made without stainless barrels had steel exposed.
Using vinegar is a bad idea because, while it removes bluing, it does nothing to polish the steel. Polishing in itself will remove the bluing anyway- and the OP wants his bbl shiny. There is a right way to do things and far too many people spout out WRONG ways on the Internet.