I never lend out my guns but a friend of mine convinced me to let him use my muzzleloader. I took him out shooting and showed him how to use the gun. After a few hours shooting we went back to my place and cleaned the gun. Told him how important it was to clean right after using it. Well, guess he didn't think I knew what I was talking about and didn't clean the gun. So now the bore is rusty. I have scrubbed it with all the cleaning stuff I have and can't get it clean. What should I do now?