all is not lost. For the exterior, fine steel wool will remove losts of the surface rust. Glass beading will remove the tougher stuff. You can reblue, parkerize, teflon, baked on teflon/laquer, etc. Would be tough to tell anything bad happened to it.
For the bore however, you will need to lap most of the rust out of it. Start with a nylon (stiff) brush and gun solvent. See how much will simply come off. Then handlap with JB lapping compound followed by non embedding stuff. The bore hopefully, will be back and shiny.
If there are still very obvious pitts above the surface, wrap fine steel wool around a smaller brush and lap. you will feel that area smooth up.
Any below surface pits will not affect your accuracy too much. Work up your loads again and see what happens.
Too bad...
Jerry