First ask yourself: Do you you actually want to shoot it?
I answered this same question myself a while back, thinking of a 9.3x74R. I found that the Chapuis UGEX was a terrific gun for the money, but when I weighed one, I ordered an 8x57R instead, specifying regulation for the 196/200 grain bullet.
IMO, the 9.3 has too much recoil for this light double, if you actually want to shoot it a lot and still be able to take most anything in North America. The 8x57R (7x57R was not avialable at the time or that would have been my choice.)The 8x57R is a great cartridge: Brass is avaiable(S&B is actually pretty good), you use regular 8x57JS dies, bullets are plentiful(the 200 Partition is a fabulous bullet at 2350fps.) RL15 is THE powder.
Whatever you do, go with a RIMMED cartridge with heavy for caliber bullets at MODEST velocity, under 2500fps. Load developement is easy and accuracy will be much easier to obtain. The 30-40(220gr), 303 British(215gr), 8x57R,(196 or 215gr), and 7x57R(175 gr) all fill the bill and are traditional light double rifle cartridges. Rimless high pressure cartridges and light bullets may work but can be nightmare in a double. The 30-30 is cute but a bit anemic and will not satisfy.
