Yesterday I received my preban 14.5" M4 upper back from Bushy. I had to return it to have the barrel re-indexed for excessive windage (13 clicks). Through the holes in the lower handguard, I could see a 3" long and .25" wide series of scratches on the barrel. I guess Bushy got a little rough with it when they stuck it in the vice. Anyway, they were not gouges in the steel...merely scratches in the finish. The silvery steel looked like hell shining through the handguards though. I was tempted to degrease the area and apply a little Birchwood Casey cold bluing but hesitated because I knew it would make the scratches BLUE instead of grey/green. Then I remembered my Birchwood Casey Aluminum Black...it's like cold blue for aluminum. I wondered if it turned aluminum black, what would it do to steel? So I used B-12 Chemtool to degrease the area and applied a little Aluminum Black in a small spot to test it. The scratch disappeared. It might as well have been manganese phosphate in a bottle. Even with the area completely degreased, the Aluminum Black blended the scratches in PERFECTLY with the surrounding finish. I honestly could not tell where the scratches had been. It certainly didn't look any worse when I applied a thin coat of CLP afterwards. Now there is absolutely no sign of a scratch anywhere. And I was so impressed, I wanted to share it with the board. I dunno if it will work on other barrels, but it worked like a charm on my Bushy.