Was lookin' at the Davide-Pedersoli.com website and I noticed a few details about these double-rifles that got me thinkin' ???
First, the rifling twist rates for the .50, .58 & .72 are all 1-in-48",
while the .54 is about 1-in-65".
In .50 caliber, a 48" twist is a compromise for both round balls or conicals
In .58 & .72, the same 48" twist is getting close to conicals only
and at 65", the .54 caliber seems to be made specifically for round balls only
Why ???
2nd thought: I tend to like round balls for many reasons. One of them is that, once seated on top of the powder charge, tightly patched RB's tend to stay put. I've had Minnie and Maxi(type) balls slide up the barrel, just from carrying the rifle around the woods for awhile. (with black powder, this empty space, could result in a bulged barrel, at best!) If you load conical style bullets in a double-rifle, will the bullet in barrel #2 stay seated, after all the recoil of firing barrel #1 ???