When working up an accurate load with both muzzle loaders or my modern center-fire rifles, I use exactly the same method.
I use a heavy, adjustable, molded Lyman rifle rest with a sandbag under the rifle's forearm. I use a couple of sandbags under the "toe" of the stock and hold the rifle tight against my shoulder, pulling the rifle back against my shoulder using my right hand on the front of the trigger guard. This is a very solid shooting position for me.
I adjust the sandbags so that THEY "hold" the rifle's sights exactly on the center of the bullseye so that my only effect on the rifle is pulling it back tightly against my shoulder. I use this same method when bore-sighting a rifle after putting a new scope on it.
No... I do
NOT put scopes on black powder, muzzle-loading rifles. Such a set-up is an abomination!
Am I a "purist"? No... but some things just go too far or don't SEEM "right"... and a scope on a traditional muzzle-loader is simply one of those "things".
Make
GOOD smoke...
Strength & Honor...
Ron T.