Cessna,
Welcome, and a few recommendations for shooting the Bess.
First, they tend to have a long lock time, so you have to practice your hold. You want follow through, meaning you stand like a statue even after the ball has left.
Second, the trigger pull tends to be stiff. When I had a Bess my technique for dealing with this was getting a death grip on the wrist with my thumb and middle/ring/pinkie fingers. With a loose grip you'll have a tendency for the musket to "fall off" the trigger when the sear breaks, causing you to shoot low or to one side. Also slide your trigger finger to the very bottom of the trigger, right up against the trigger guard, even if it is hanging halfway off. That gives you the most leverage on the trigger.
Third, take your time getting a sight picture. All you have is the bayonet lug and the top plane of the barrel. Try sighting along it and bringing the muzzle up and down so that the lug/sight disappears, reappears, and then the top of the barrel appears. Drop it back down so the top of the barrel just barely disappears and your are sighting along that plane. Line that up on your bull and see where it shoots. You might have to take a 6 o'clock hold or maybe a 12 o'clock hold-over to get the ball in the middle (in which case you might want to up your powder charge a little). If you are shooting far too high you may want to line up the lug so it is just peeking over the breech.
Fourth, just before you go for your sight picture, pull your head back. The kick is hard and the drop in the stock is negligible. There will be a tendency for your right thumb to smack you on the tip of your nose, and that is disconcerting.
Brown Bess can be a fun shooting companion if you work on taming her faults.
