OK, great shooting, but I am interested in your technique (not equipment) that produces this result at 100 yards. Among the guns I'm working with is a P-H musketoon; I believe it will hit where it's aimed if the shooter can hold it on the same spot every time.
Alright, get some coffee and relax. The secret is there is no secret. You can't have technique without the gun and get what you're after and the reverse is true.
You mentioned a musketoon and black powder is my wheelhouse. There are two types of Parker Hale 'toons. One was made in Birmingham, we call those first gen. The others were assembled by Mario and Luigi from parts when EuroArms bought Parker Hale. Nearly every first gen I've seen can be made to shoot. Not so much with the spaghetti ones.
Now to load. Different guns shoot different minies to varying degrees of accuracy. There are some basic rules to follow. Minies MUST be cast from pure lead and sized to .001 under the MEASURED bore size. Powder must be real deal black, best you can get which currently is 3f Swiss accept no substitutes. Caps must be best you can get, currently RWS followed closely by Schuetzen. Lube must be from natural products that play nice chemically with black powder fouling and not some commercial concoction designed for round balls, 50/50 beeswax/lard works good with minies. Finally, be careful who you get advice from. Round ball guns are completely different critters and I have seen very few round ball guys who know their stuff on minies. There are those who will dispute what I've said but hey, go look again at the picture.
Now technique. The ignition process in a black powder gun is best defined as glacial speed. Any error in technique will be greatly magnified from a modern gun. In nearly every gun I've worked up, the best method for bench firing is straightforward. Sit as erect as possible imitating a standing posture, don't "hunker" down on the gun. Rest the back of your forward hand on the bag and grip the forearm in that hand. Rear hand at the same place every time on the wrist of the stock. Do not attempt to crush the stock in your hand. Trigger pull- one smooth quick motion to the rear. This is one place lots of noobs screw up and they will move the stock with the rear hand. KEEP your HEAD DOWN on the stock as the gun fires. You will be able to see your shot later. Finally, do not go to the spotting scope after each shot. You will subconsciously start to "chase" the group and it will open up. We'll cover offhand shooting later.
In the world of Muzzleloading, there are tons of variables in shooting. And just as in modern shooting, accuracy is all about recognizing and controlling the variables.
But hey, what do I know.....