Around 17 or 18 years ago I had a CVA muzzle loading rifle with adjustable muzzles. Biggest problem with it was that at the time I was woefully ignorant of how a double regulates and suspect other owners were afflicted with the same problem, accounting for its lack of success.

With a very mild load it shot wall eyed beyond the range of adjustment. At the time I though the heavier the load, the greater the recoil, and the more a rifle would shoot wall eyed. For this reason I gave up on it. Not until later did I learn of the importance of barrel dwell time to the regulation and that a hotter load might have brought the groups together.