Robert, I would first do asgoatbrother suggests with the Kroil or Rusty Duck and allow it to soak and work for at least 24 hours before wiping it off and degreasing. I would then very carefully tape the sight and barrel areas right up next to the rust and evenly on both sides of the sight and at the muzzle end of same. I would try to keep the tape squared to the exisiting lines of the barrel. (plastic tape or even a carefully cut piece of thin aluminum taped in place might be better).

Then use jeweler's files or perhaps a Dremel to slowly work away the remaining rust. Then clean, degrease, re-tape, and cold blue.