Its takes a year of more of frequent CORRECT practice for most of us to be able to shoot a flintlock as well as we were shooting a Percussion lock before the Enlightenment. :rotf:
Shoot of the bench, or a standing rest. Teach yourself to focus on only the front sight, looking past the frizzen, and the flash. You should be seeing the muzzle flash around your front sight, and even the ball or shot leaving the barrel before recoil and smoke hide the rest. You have to learn to count to 5 in your follow through. When the gun comes down out of recoil It must return to your last sight picture at the instant the shot went off. If it doesn't, it means your are not mounting the gun to your shoulder and cheek consistently, nor at the correct angle to the target.
Use light loads. You don't need to learn to deal with BOTH recoil and the flash in your face.
The reason you seem to be able to shoot a flintlock without flinching when shooting at birds, or clay targets, is that these activities require you to focus on the target, and NOT ON THE FRONT SIGHT. Shooting a single projectile requires you to focus on the FRONT SIGHT ONLY, and not the target, the rear sight, or the frizzen. It take lots of dry fire practice, at home, daily, and a lot of practice on the range. Begin shooting only off the bench. When you find that you are not Noticing the flash, then, and ONLY THEN, move on to take a few shot off-hand. Only when you see that your follow through skills have been transferred from bench shooting to off-hand shooting should you consider shooting more Off-hand, than bench shots.
Read my article on Off-hand and trick shooting. It can be found on the index page, here, under Member Resources. Scroll down to Articles, click on that word, and then click on articles again. The third article is the one on Off-hand shooting. That should give you the fundamentals you have to master to do very good off-hand shooting, and believe me when I tell you that shooting off-hand at 50 yards is very difficult. You never master it, until you finally move targets out to 75 and 100 yards, where your struggles there make the problems shooting small groups at 50 yards Nothing at all! But you have to spend a lot of time at both 25 and 50 yards before you are ready to shoot at longer targets and have a clue why your balls are going somewhere other than your POA. Along the way, you will perfect your loading and cleaning techniques, which are both extremely important to long distance accuracy. A lot of men who can shoot one-hole groups at 25 yards all day long, can't hardly hit the black at 50 yards. Most get frustrated and go back to shooting modern guns, unwilling to learn what they are doing wrong with loading and cleaning their guns.