As a follow up to the McNaughton Post below here is a project on “a 4-bore falling block rifle under construction from a unique vintage MacNaughton 4-bore shotgun. Where there is a will, there is a way!”
As one commenter said, “The torque from the hunk of lead engaging the rifling wants to twist the rifle out of your hands!”
So why do you ask? Boys and toys, I guess. Extremes and obsessions are hard to ignore.
I had a muzzleloader, American I believe, from Maryland. Will look for pics.