It does not matter what was or what was not fired in the barrel. If it was nice and shiny and is now dark, then it is dark. To improve the condition of your bore you might want to consider using a good copper solvent to remove any fouling in the barrel. Hoppes is not a particularly effective copper solvent. Shooter's Choice and Sweets 7.62 are effective when used according to directions. You will find that use of a good quality phosphor-bronze bore brush will help remove the fouling. Use a muzzle guide and a solid steel rod. Do not use a jointed rod as it has the potential to cause damage to the crown of your barrel. One thing I have done with very fouled barrels is to plug the barrel from the chamber end with a piece of rubber and then fill the bore with solvent. On a M-1 you will need to also plug the gas port. After a reasonable period of time (follow label directions) pour out the solvent and then brush and patch the bore. You might also consider using a Outer's Foul Out. It is very effective in removing fouling from neglected bores. A final suggestion... but this is only based on my own experience. If you are not able see any improvement in the condition of the bore, then gather all your cleaning materials, patches, brushes, solvents, etc., load up about 100 rds. of good ball ammo and head for the range. Clean the barrel and wipe it dry. Fire off a clip or two of ammo. Get the barrel hot and then brush and clean it. You may just find that this method works. I have found that sometimes after doing the best job I can do, a bore just simply will not clean up to my satisfaction. I have on such occasions simply taken the rifle and fired plain ball ammo at a steady cadence in 5 to 10 rd. strings. After say 100 rds., see how the bore cleans up. For 30 years I've enjoyed shooting a variety of military rifles, some of which had badly neglected bores. This method has helped me restore some of them to remarkably good condition. HTH. Sincerely. Brucev.
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