Years back I did a series with photos on the website Nitro Express.com on how to build a double rifle using early 1960's Brno ZP 49 shotgun action. This series is still on that website and it was titled: "Building, Regulating and engraving a DR with Shoe Lump Barrels" You will need to be a member of Nitro Express.com to access it, I think.
Others are fond off using the Brno ZP 49 action as well, such as Aaron Little. The 1960's ones are very high quality and therefore preferred and not only that as they have not been discovered in America they are inexpensive. Well under $1,000. If you use this action buy the 1950's or 1960's series which are dated on the action flats. The guns were not imported to USA in that time frame, but a great deal were shipped into Canada and you can find them in USA states that border Canada.
http://forums.nitroexpress.com/showflat.php?Cat=0&Number=297834&an=0&page=1#Post297834 I cannot recommend the mentioned book as a guide to this task because the author uses a screw in barrel method that weakens the barrels and which he later stated (after the book printing) was not a good idea. The book will give you some ideas, however even though the machining work pictured is crude.
The Brno ZP 49 of the early 1960's was made with excellent strong alloy through hardened electric furnace steel (similar to 4140) and is a good action to use. It has firing pins discs and you will need to make new discs with smaller opening for the reduced diameter firing pins that are necessary for safe rifle shooting. A significant task is to mount the barrels onto the action where the firing pins are centered to the rifle barrels after they are mounted to the action---this must be well thought out.
Although I did not build a .45-70 double rife, that is the caliber I recommend for such a task now days. I built a .38-55 wildcat that I named the 38-55 McPherson Nitro Express with a service pressure of about 34,000 psi. I use a virtual ballistics software and you may want to consider that as well. I also built a double rifle using the 20 gauge Remington sabot slugs that duplicate standard .45-70 ballistics.
Regulating the barrels is a long task, but using the .45-70 shortens that task a great deal.
If you are serious about building a DR on a shotgun action, I will give you as much information on how to do as you can digest. You will learn a great deal by reading and viewing the series on Nitro Express.com. Feel free to send me a PM.
Kind Regards;
Stephen Howell
