When you’re loading with black powder, case volume is often the limiting factor on how fast you can drive the bullet, and if your rifle needs more velocity to get both barrels to hit at the same spot, then duplexing is one option, as you’re substituting a small amount of more powerful smokeless for the black powder. That smokeless charge goes in first and has the benefit of better ignition of the BP, plus helping that black burn more completely and leave less fouling in the barrel.
I’ve not tried it yet but that’s the gist of it.
