Relax, your problem is very common. First drain all the oil out of the barrel. Get a good solvent like Hoppes #9 or Sweets Bore cleaner and run a couple wet patches down the bore. Let it sit and work for about 5 mins. Use a wire bore brush about 20 times up and down. Follow up with dry patches until they come out clean.
By now all the rust should be out of the barrel and you can look for pitting in the bore. If the pitting isnt to bad J-B Bore Paste works very well. use a clean patch and about 100 up and down strokes ( add paste as needed) then use your solvent again.
Good luck and let me know how it works out.