Chambers and bores are susptable to rust as they need to have little or no oil in them when the rifle is in use. A little steel wool wrapped around a worn out bore brush, screwed into a section of old rod and chucked in an electric drill will, when spun in the chamber, polish or rather burnish the netal.. This will make the chamber as clean as is possible. Oil the chamber to stop re-rusting but run a patch thru it before firing. Oil is a liquid and if a rifle is fired with oil (or any liquid) in the bore there will damage to bore. It takes quite a bit of rust in the chamber to cause problems. Usually this will make itself known as a difficulty in extraction of fired rounds.