A feller gave me some cast soft lead .41 Mag. semi-wadcutters a couple of years ago to use in my .40 flintlock. He had swaged them in a cut off section of a GM barrel (like mine). In doing this, he pre-engraved the rifling into the bullets. That made a mechanical fit similiar to the Whitworth or Brunswick rifles. I loaded without a patch, although I suppose it wouldn't hurt to use a WonderWad or something like that to help seal the gases. They weigh about 172 grains, nearly double that of a RB.
He asked me what charge I normally use and I said 50 grs. He said try 60 with these and you should be able to keep the same sighting that I normally use. I did and it worked. At least out to about 60-70 yds. Where I hunt, I rarely get a longer shot. Since I had so few, every time I came in without seeing any deer, I pulled the bullets and saved them, reloading them the next time I went out. The ball screw of course turned the bullet into a hollow point. When I got my deer that year, that little .40 tore a right sizable hole in him and he only took 2 steps before dropping.