For anyone contemplating converting a shotgun to a double rifle, the book mentioned above by Ellis Brown does indeed contain a lot of useful information, and is a very good place to start.

However now might be the right time to again mention that there are some errors concerning Breech Thrust calculations in the first edition of this book.

These errors are on page 19 of the first edition.
I don't know if these errors have been corrected or not in later editions.

The errors relate to calculations using Proof pressure, cartridge case dimensions, and chamber pressure, to obtain breech thrust figures.

The math is not done correctly, resulting in a GROSS over estimation of the breech thrust that a particular action supposedly endured and passed during proofing.

Breech thrust is a very important factor with DR's, and getting it wrong is a very bad idea.