Easiest way to polish the bores is to take a cylinder of hardwood, brass, etc, mounted on a steel rod, with a split to accept a slip of abrasive sheet. Wrap the abrasive until the cylinder just fits the bore. Drive it with an electric drill. This will rapidly remove rust, and reveal just how bad - or good - the bores are.
I have never had much luck trying to unscrew the plugs from double guns. Singles, usually no problem.
You want to be satisfied that the nipples are well threaded in.
Bought a ml 14 bore gun last Saturday, gave the barrels a bit of a polish and cleaning. Checked the nipples. Shot it yesterday, then cleaned it thoroughly. Bores don't look too bad at all. Going to do some more polishing, now that I have a better idea about the bores' condition.
It is hard to tell exactly how deep pitting is. Have a nice single; was 12 bore. Pulled the breech, cleaned the bore; it was bad. Decided it would be a good candidate for a sleeve. Reamed it to 3/4". Pitting was still evident, obviously over 20 thou. deep. As it was, it was a wallhanger; had the sleeving not worked out, it would still have been a hanger.
Nice little gun now.
Next barrel I reamed cleaned up beautifully, pitting gone. Still going to sleeve it.