Wow. I have no 1st hand knowledge about this...
But if this was my project, and if I wanted to get rid of the rust as much as possible... then, well, I would use a wood dowel with a glued piece of sandpaper around it (high grit: 800 or more) impregnated with oil, and then try to sand some of the rust out.
However you say the gun is shooting GOOD. Is it good power? Good accuracy? Both? If you are satisfied with both, then you probably should leave the gun as is.
BTW. I love restoration projects. If you can bring up a beaten gun to life, and also make it look good, it is very satisfying. Keep up with your project!!