I believe I would contact Arrietta to ask these questions. Don't you recon Arrietta would have better knowledge of the subject than the members of this board?
Aside from that, let me say that most Continental double rifles chambered up to 9.3x74R are built on frames the same size as a 28ga. shotty. Notice I say "the same size" because they use frames originally intended for rifles and not shottys. The practice of converting a shotty to a rifle is questionable at best.
Here are a couple photos of my Francotte SLE chambered in 9x57R. It's built on a 28ga. size frame.
Curl