Interesting read. The rifle I had, would not shoot slugs within a foot of each other, at 50 meters & they crossed badly. I did not try more powder. Slugs in muzzleloaders can work their way down the barrels and can therefore be dangerous.
With 100gr. 2F, it put both barrels into the same 1" group at 50meters from a rest, holding the barrels in my left had, back of the hand resting on the bags. It also shot a 2" group at 100 meters (our range distances).
It shot identically (together and crossing) at 50 and 100 meters with that load, 100g.r 2F.
With 110gr. 2F, it shot parallel.
With 120gr., it crossed by 2" at 50 meters as well as shooting 4" difference in elevation. Frustrating.