What Jon & Bill said. I've posted this image of a 16g Smith victim of chamber lengthening (and likely short magnums) with a bulge at the start of the forcing cone



The barrels otherwise have only minor pitting and good wall thickness. I think 28g adapters that reach past the bulge would be an option. Scroll down to the bottom here
http://www.gaugemate.com/info/sub-gauge-adapters

Briley will sleeve the chamber for $375, but I have heard they will not work on Damascus barrels (but would very much like to be proven wrong)
http://www.brileygunsmithing.com/shotgun_gunsmithing_pricing.html

https://sites.google.com/a/damascusknowledge.com/www/home