At one time CVA sold a double barreled muzzle loader that had the barrels aligned in a jig with lasers. It was not notably successful, they switched to the adjustable muzzle concept.

The Kodiak barrels are also aligned in a jig with some what better results than the CVA but still not perfect.

BTW: One of the reasons for so many proprietary cartridges in the old British trade was so the maker had control over the loading and would not be dependent on the whims of the ammo manufacturer. (I'm not saying this was the only reason, a captive market may have been an object as well.) For example, some doubles were chambered for the 30-30. Of late Winchester has altered the loading. So where are you with factory loads in one of them?