Manton Multi-Purpose Flint...and...

New guy here...gunbuilder of the old sort. I am in the process of building a Manton type double gun "system" if you will. An 18 gauge double shotgun, 32" barrels (damascus-already have), a SECOND set of barrels, also 32", but cape setup, an 18 gauge on the left (damascus), and a .50 caliber on the right, setup for roundball-via a chrome moly sleeve inserted into an 18 gauge damascus tube. The shotgun and combo set are on a flintlock platform.

Here is where I am need of input from those who have had field exposure with muzzleloaders:
If I were to at a future date get a side by side set of fluid steel barrels, in .50 caliber, what would I have to do to get performance close to a .500 nitro express, or equivalent cordite cartridge? There were a lot of early Mantons converted to percussion-the locks are still produced, and will mount directly into the mortises of the old flintlock with no stock modifications. This will likely be my first step-I will retain the flintlock mechanisms for the shotgun and combo barrels, but have the appearance of a percussion adapted set of rifle barrels. Second step...who here has experience with patented (chambered) breeches? I understand the principal, and will be employing them in the initial two barrel sets, but am in a quandry. Touch holes and percussion drums will have to allign, but the smaller the chamber, the higher velocity, maybe? I would like to regulate using traditional black powder, though I know subsititutes may give a boost...but availability varies.

Ultimately, I would like to have this weapon capable of taking anything-literally, from dove to elephant. Dangerous game bullets (steel core conicals) were once available for muzzleloader hunters, so I figure, why not?

Any info is appreciated!