I have a .243 Win M70 that has been leaving a scratch on the brass for a long time. I finally want to tackle this problem but am unsure as to how to proceed. I'm not a gunsmith but am a real hands on handy, work with my hands and tools kind of guy. I've thought through the problem and my solution is to take a full length sized case attach a mandrel of sorts in place of where the primer would go and "gob" the whole thing in polishing paste and hand polish the chamber.
What would the professionals do?
What would be wrong with my method of repair?
Thanks in advance for your responses and opinions.

I`d try a bore mop before a case If I was to polish the chamber. You don`t want to remove any more metal then needed to stop the scratching.
I`m no smith though and don`t really know the best way to handle your problem. I do know you should take a good look for the cause of the scratch before polishing anything. I had a Rem M700 I thought was doing the same and after some playing it turned out a burr on the feed rail was the cause, not the chamber.

Pull the barrelled action from the stock and place it in a padded vise. Take a piece of 1/4" rod about 12" in length and cut a slit in one end. Insert and wrap a piece of "Scotch Brite" pad around the end. Place the other end in a drill motor and carefully run it in and out of the chamber a few times. Use oil on the pad if you want, but keep the pad moving. Don't let it sit in one spot too long. Unless there is serious damage, this should clean up the chamber pretty well without getting you in trouble with headspace issues. Be sure not to let end of the rod poke through the pad and damage the chamber.

Snellstrom:

Question, is the scratch lengthwise or circular around the case?

If it is lengthwise your chamber may not have been chamfered after reaming? If this is the case the barrel will have to be removed and chamfered.

If the scratch is circular it may be removed by taking a piece of 320 or higher grit wet dry sand paper and wrap it around a wire chamber cleaning brush and twisting it in the chamber. If the chamber has a scar in it from a chip in the reamer it most likely can not be polished out. This will require a set back.

Is the barrel a factory barrel or a custom barrel? In either case if the fist method does not work send it back to the manufacture or gunsmith who rebarreled it.

It could be as simple as trach in the chamber or as complex as a rhip ring in the chamber.
Good luck
Longshot

Thanks guys I'll give the scotchbrite a try. The scratch is 1/4" long or so along the length of the case with a slight "L" to it I believe from the extraction. The barrel is a factory .243 on a Win M70 Lightweight Carbine. Bought the gun new in 1984? or so for the ex-wife, she recently (2 years ago) gave it back for our 12 yr. old boy to use. I don't remember it scratching cases when it was new but it has been doing it for quite a few years. The gun is tremendously accurate and I want to avoid taking the barrel off because I would be sick if it never shot the same, if I had to I can get by with it scratching cases its really not a big thing just something I wanted to clean up.

Thanks everyone.

I have seen a couple of cases of scratches being caused by the feed rails rather then the chamber. You might want to check this as well as it will alleviate a lot of other options quickly.

Wink and Ol' Joe the feed rails can't be it because it scratches whether it is fed from the magazine or inserted into the chamber by hand and bolt closed on it ( it is a push feed ).
Thanks for the heads up though I do appreciate everyones attention to detail on this forum when someone is troubleshooting.