I'm thinking one could just have a BFR in 45-70 sleeved to handle the 9mm Parabellum and have a solid five shooter single action. Might even be able to skip the barrel and just used rifled sleeves.
Sometimes, the air you 'let in'matters less than the air you 'let out'.
Might even be able to skip the barrel and just used rifled sleeves. 
