I wouldn’t stress about a bit of rust. A shot of Ballistol and a few swabs down the barrel should clean that up nicely. If it is accurate just shoot and enjoy.R
I have read the thread and I feel your pain with the sight of brown rust on such a beauty. One thing that should be mentioned with any type of salt blued finishes, such as the AA finishes are famous for, is that if the bluing salts are not fully removed or neutralized they will continue to quietly and slowly corrode further. This action does not produce a lustrous blued finish but the oxidized brown rust color you are seeing. Your bore looks very easily neatralized and saved. It has not produced anything in the realm of pits so you are good to go. Take the time to clean it very well with solvents and then dry patch that bore until it pulls through white. I personally would shoot it a bit afterwards and let it sit a few days then clean it again until you can see that no brown is coming out after each cleaning. A watchful eye over an extended period of time will ensure you have removed any of the old bluing salts for sure. I collect and encounter older rifles that appear to never have been removed from their stocks and still show signs of blueing salt oxidation after 50 years. They still clean up nicely and look great. What you are seeing is sometimes simply the price we sometimes pay for old school salt blueing on steel.My.02