1) Have you tried
adjusting your charge? That can move the point of impact.
2) Filing the end of the muzzle - If the late Paul Valdilligham (sp?), who used to be VERY active on here, was still alive, he would tell you to CAREFULLY shave off the muzzle, more off the top than the bottom, whereas she is hitting 'high'. That puts more gas to the 'top' of the ball to lower's it trajectory.
To raise the impact, you'd file some off the bottom of the muzzle from 4 to 8 o'clock, with the intent to have gas below the charge exit the muzzle first, thereby adding more lift to the charge.
To lower the impact, you'd file some off the top of the muzzle from 4 to 8 o'clock, with the intent to have gas above the charge exit the muzzle first, thereby pushing the charge down.
Now with that all said ... even though I've been on here for 20+ years ...
BEWARE that I don't ever recall someome doing it with solid roundball load, but it does work for shot patterns, as
@Britsmoothy does it. In this post here, someone claims that they successfully moved their point of impact to be 'dead on' by barely filing the end of the barrels.
Here's that post!
https://www.muzzleloadingforum.com/threads/raising-pattern.105997
3) Project to Test! - Hmmmmmm, I have a shootable, but junkie 69-cal smoothbore barrel, in a stock with a lock, just lying around. I'll PM Message
@B P Maniac Shooter to see if he'd test this 'filing the muzzle' hypothesis with roundball loads, if I send him the parts gun. I'll have to fashion a trigger, but that's no biggie.