I'd suggest getting right with it on Rocky Mountain brass cases as the business is for sale and limited production. Their doors are still open but who knows how much longer. I have their Brass for a Lyon & Lyon 12 bore full double rifle, they are perfect, and will last forever.
RMC will need a chamber cast and expected ball/bullet size.
I would go with .839"- .840" projectile to fit the groove dimension so long as you are getting custom cases anyway.
- Mike