Fireball2,
Here is what you need to read on the subject of making a DR on a shotgun frame. It is known as "the book" on the subject and was written by a gunsmith/gunmaker in Colorado. It is also an interesting and informative read.

Building Double Rifles on Shotgun Actions
by W. Ellis Brown

Buy it from Bunduki Publishing --- www.BundukiPublishing.com

Enjoy

CRS, NRA Benefactor Life Member, Whittington Center, TSRA, DWWC, DRSS
Android Reloading Ballistics App at http://www.xplat.net/