The old gunsmith treatment is to plug the chamber and fill the bore w/10 % nitric acid for 5 minutes. You can also use Birchwood Casey Rust and Blue Remover. You must take precautions to protect blued metal because it will be damaged by the acid if any gets on it. I usually wax the metal or spray it with lacquer to protect it. Then remove it when done. You can also pour about a gallon of boiling water through the bore, then brush it out with bronze cleaning brushes and oil. The boiling water will neutralize and soften the rust.
I will plug the bore and fill it with Evapo-Rust. Let it sit for at least 24 hours, then drain and brush. Like the previous suggestion, blued metal must be protected as the Evapo-Rust will remove the bluing.
Blue Wonder Bore Cleaner - ain't much better than anything else as a bore cleaner, but it does a great job on removing light rust from firearms without doing any further damage to the metal. It's great for removing light surface rust from blued finished without harming the bluing.
If the barrel is more than just a light surface rust, Blue Wonder may not be worth the expense. Kroil would be my first choice for any heavier firearms rust. Follow up with some Hoppes, or better yet, GI bore cleaner. If there's no deep pitting or undue bore damage, fire a dozen or so rounds out at the range, then immediately clean whiule it's hot.
I'd be hesitant to use anything caustic, or acidic in a firearms bore.
You can polish the chamber using a spent case. Drill out the primer and put in a small diameter machine screw with nut and lockwasher, use the shaft to chuck in a drill and use valve grinding compound for deeper pits, finer polishing compound for light rust and final polishing.
I dipped my SVT-40 in a bathtub of Evapo-Rust without ill effect on the bluing.. It was highly diluted with water though. Don't take my word for it.
Scrubbed the bore, ran out side and blasted the whole thing with brake cleaner then 3 in 1 to prevent flash rusting and a wipe down with a rag dampened with pennzoil as final protection.
The bore is as good as it will ever be. Strong, dark but not rusted anymore.
As for chamber polishing.. get a big bore or chamber mop and run it in the chamber on a drill bit with a little bit of polishing compound available at automotive stores. Hand Glaze for final polishing, anything heavier for smoothing out pits.
Don't use the blue and rust remover! It uses phosphoric acid as the main ingredient and though it will remove the rust, it will also activate the metal due to hydrogen popping and it will frost over. B&R remover is only if you immediately intend to re-blue or paint over a clean surface.
From a similar discussion a while back at britishmilitariaforums comes a somewhat oddball remedy that apparently works well if you have the patience: mix one part molasses with 4-5 parts water. Coat the rusted parts and let sit two to three weeks. Rust will be gone... pitting will remain.