Depends on the degree- rusty or rusted?
For light rust, bronze bore brush, solvent, clean. Repeat a couple of times. Use some J-B Boreshine to do final cleanup.
Rusted- as in large flakes of rust, looks like a sewer pipe? Clean, have evaluated by a gunsmith. Will depend on degree of damage, pitting, etc. May NOT be salvagable in extreme cases.
What we have here is... failure- to communicate.