Using chemical rust removers leaves you with bare metal and no bluing.
Soak the gun down with WD 40. Let it sit for a day or two, and scrub all the WD 40 off with a rag. Much of the rust will come off on the rag. When dry, if you still have rust spots, use 0000 steel wool (or bronze wool) to gently scrub the rust away. This will leave the bluing, but remove the raised rust.
When finished, flush the gun with WD 40. Dry, and oil as necessary.
Stubborn rust spots can be scraped off with the edge of a copper penny. WD 40 and a rag will remove the copper streaks-again leaving the existing bluing intact.