Colorado,

I didn't build them. The barrels were custom made in Argentina. I don't know the manufacturer's name, but I have seen other barrels he made, mostly for Mausers, and they are very well made and very accurate. Then, it was just a matter of fitting them to the existing block by turning them on a lathe.
Brown's book is a must for anyone intending to make a project like this. There are a few differences from his method and my project, but he gives very good guidance. I also miss a few more details on some topics, like what outside diameter to have on a barrel, or how to fit a scope mount on a double, and maybe some alternate constructions, like an adjustable regulation system, which may be beneficial in some calibres.
I think that his book is very good in how to evaluate a shotgun action for the purposes of DR building. During the past months, this book and Graeme Wright's "Shooting the British Double", have been my constant references and helped me on critical decisions. Hope it will also help you.

Fred