When rust bluing I always remove the sights. It's hard to get a perfect seal at the bottom of the slot and the rusting agent will creep in there and rust and rust and rust. Also any oil that might have been in there will creep out and give you yellow spots. I, however, do not plug the bore. As soon as the barrel comes out of the boiling water I empty it out, and wipe with a dry patch. That dries the bore instantly. On a slow rust blue that might take a week to do depending on my time demands and the temperature any moisture that got in there on the first boil will have a week to do some serious damage. I've done lots of barrels without plugging them, shotguns, rifles, longrifles and never had any bore damage. Any slight discoloring is removed with a wipe or two of a tight patch with some JB bore cleaner.
Browning is a different matter. The parts aren't being put in boiling water but may be being rinsed with cold water which is a pain to dry out of the bore. I typically plug those and use the plug for a handle to coat and card the piece.
Tom
