We could use some more specifics on the 20 gauge. Name, rank, serial number come to mind, a photo or two, especially of the flats of the barrels.

Could be you have something along the lines of a Nitro Special that was hunted hard, and simply looks old. It's happened here before. Oh, and a Nitro could use anything you could stuff in the pipes as well.

Knowledge is power, and all that. We just need some more information.

Best,
Ted