The gun will probably still shoot just fine, the rust will come out, the pitting won't. If you keep a pitted gun clean and well oiled it won't pit anymore and shouldn't be a problem.
I cleaned up the bad bore in the above picture and got 3" groups at 50 yards with it, I scrubbed the bore with a scotch bright pad and mild abrasive and got rid of more of the corrosion but couldn't hit squat with afterwards. I sent it off to Bobby Hoyt and it came back a tack driver.
I trusted Rem Oil to protect the bore on this Rice barrel, to my horror after being in the safe for a few months I found it had pitted. I used a scotch bright pad and soft scrub to clean it up and actually removed metal, I could see it on the pad. I thought I had ruined the barrel until I shot it, it shot better than it ever did before, on a good day I can get 1 1/2" groups at 100 yards with it. You can still see the pitting which is only down close to the breech.
I cleaned up the bad bore in the above picture and got 3" groups at 50 yards with it, I scrubbed the bore with a scotch bright pad and mild abrasive and got rid of more of the corrosion but couldn't hit squat with afterwards. I sent it off to Bobby Hoyt and it came back a tack driver.
I trusted Rem Oil to protect the bore on this Rice barrel, to my horror after being in the safe for a few months I found it had pitted. I used a scotch bright pad and soft scrub to clean it up and actually removed metal, I could see it on the pad. I thought I had ruined the barrel until I shot it, it shot better than it ever did before, on a good day I can get 1 1/2" groups at 100 yards with it. You can still see the pitting which is only down close to the breech.