I've not heard of such a rust condition as you describe here. (What make you believe it's rust as opposed to some other kind of fouling?)
That said, I recommend you get some Evapo-Rust, push a patch soaked in it down into the shoulder section of the chamber and leave it there for an hour or two (keep it soaked with a 1/4 teaspoon of solvent over that time.)
http://www.evapo-rust.com/
This stuff is incredible at removing rust compeletely without touching anything else. Having discovered it a few years ago, I use it for everything from godawful motorcycle parts to fine jewelry connectors. Like any rust remover, however, keep it away from that rust we call "blueing".