Stick scotch tape on the gun around the spot of rust. Leave the rust spot exposed. Then use the fine steel wool (I think it can go as fine as 0000 grade?) and rub in a clock wise circle motion directly on the rust spot, and then rub again in a reverse clock motion. Use wood dowels, shaved down, to the same diameter size as the rust spot, itself, and wrap the end of the dowel with a small ball of the steel wool about the same size of the rust spot. You'll need to keep replaceing the ball of steel wool as it loses it's strength to rub of the rust. The idea of the tape is to offer a protective layer over the blue parts of the gun so the wool will not take off the blue finnish around the rust spot. Replace the tape if the steel wool starts to go through it, but you should just be rubbing the rust spot, only, and in circle motions.
Sometimes, if the rust is built up, you'll need to start with a stronger grade of steel wool. Keep an eye on the tape, and replace it if it starts to wear through.
You'll need a lot of patents, and time, work on it a little bit a day, not all at once.