tamara
Flinching and target panic may be related. Then again, maybe not.
Anyway, the flinching many people do is related to the sub-conscious mind and it usually is the result of pain inflicted in the past.
It may not be the kind of pain that one gets when they whack their thumb with a hammer but the sub-conscious remembers it.
Later in life, when the same set of circumstances arise, like releasing the bow string or pulling the trigger, without a person even realizing it the sub-conscious says, "Get AWAY, NOW!" and the body reacts to it.
I don't know if you have ever been slapped with a bow string on the forearm but that's the kind of thing that can condition the sub-conscious into these kinds of responses.
In the case of shooting a modern hard kicking rifle or pistol can have the same effect on the sub-conscious.
That said, the first thing to realize is that a muzzleloading rifle, shooting a roundball with a moderately hot powder load is not the same as shooting a modern rifle. The "kick" is quite different.
Where a modern high power rifle seems to instently slap the shooters shoulder, the black powder rifle gives a hard shove. It has to do with the speed of the powders burning.
I can't explain it any better but once you have fired your muzzleloader a few times you will see what I'm talking about.
Your conscious mind can over-ride your sub-conscious mind if you want it to.
Knowing this you will need to convince your sub-conscious that no pain or other unpleasant things are going to happen to you. Then direct all of your thoughts to holding the gun on target and to keeping it in that exact position after you hear the shot.
Although the gun will move when the shot is fired if you are concentrating on keeping it held in exactly the same position that it was before the shot went off your flinching will become a thing of the past.
It does take practice and repeatedly shooting your gun but it can be done.
Once your flinching problem is under control, then, and only then can you start experimenting with the more powerful powder loads.
As for these moderate powder loads and hunting, IMO, most hunters tend to overload their guns. I think it is a carryover from modern Magnum thinking.
In a .50 caliber muzzleloader a powder load of 75 grains is more then enough to take a deer.