How did Mike Beliveau become the be all end all of traditional muzzleloading?
Best I can tell all his "data" comes from range testing, a static and controlled environment. Nothing real world.
Now, I admit, I don't watch a lot of his videos, I don't like YouTube and I don't see him hauling himself and his gear up the mountain or through rough woods to actually hunt,,,, and certainly not fight.
Yes, one won't be shooting off the bench in the field. But load development and sight regulation done from the bench,,,, correctly,,,, tells one what the gun/load/sights are capable of. If the group is good and shooting to point of aim off the bench,,, you know it works. If it isn't when one leaves the bench we know it is a shooter issue. If the group doesn't grow much but consistently shoots to the same spot,,,, different from point of aim/different from the bench,,, then maybe move the sights to correct for it, but otherwise correct the shooter issue. In this case that correction may be a rear sight or something placed to the rear of the gun as a reference.
There are many homemade rear sight suggestions on this forum. The one on my Fusil des Chase is just a small brass rear sight off an old junk gun that's glued in place. It would be really easy to shape one like it from a small piece of brass.