Use some Hoppe's 9 or some light gun oil, soak the area with a drop or two, take a utility single edge razor blade and shave the rust off, a little at a time. Rust is softer than steel, and you will feel the blade as it scrapes through the rust and contacts the case hardened receiver metal. You will not mar the steel or case finish if you keep the angle shallow and feel the rust. Just like the skin on your cheek. If the rust is more than a couple of thousandths thick, you will need to re-oil it and shave it again until gone. You will have fine dark pits in the remaining steel, but not as bad as the surface rust. Steel wool of any grade will leave a burnished appearance on the receiver which is a dead give-away. Shaving will not.
