Felix,
I took, in gunsmithing school, a class on double shotgun to double rifle conversion, from W. Ellis Brown, prior to his writing book on same subject, now in its second edition; that is essentially where I learned the process, including techniques of regulation. Thus, I highly recommend his book for anyone doing or attempting to do such conversions, or anyone considering doing some double rifle regulation work. However, his book, and discussion with him, still does not resolve the issue of a suitable action for conversion to .500, due to problems I previously mentioned above.

500 Nirro,
I have no experience using the Greener boxlock action, for double rifle conversions, but I am familiar with these actions, and am confident that they, along with many other British/German and other actions found around the world, are most suitable. But, American actions are NOT suitable, though they tend to be robust enough, they lack good proper bolting. For nitro for black loads, including the .500 N. for black, the Greener is more than suitable, as are a good number of other actions, since heavier overall rifle weight is not needed with N. for black, as is needed for full nitro rifles.