A few thoughts, some of which you probably already know:
Minie' bullets can be finicky things and if the clearance between the bore and the bullets outside diameter is much larger than .003, they often shoot poorly.
Minie' bullets must be cast from pure lead.
Relying on the expansion of the skirt to seal the bore and grip the shallow rifling, harder alloys work poorly even if the fit of the bullet to the bore is correct.
Heavy powder charges are thought to "blow the skirt" out as the bullet leaves the barrel.
This is especially true of Minie's with thin skirts.
If the skirt "blows" or enlarges unevenly, accuracy goes to hell in a handbag.
Speaking of blown skirts and limited powder charges, I never bothered to notice this before but, the data in the "Lyman Black Powder Handbook & Loading Manual" (2nd ed) shows that Pyrodex RS in the .58 caliber, 1:48 twist barrel consistently produced less breech pressure than 2Fg GOEX.
This happened for all of the Minie's tested between 460 and 566 grain.
I can only guess how this effects pressures "blowing the skirt" on a Minie' but it may be that when loads of 70 grains of 2Fg black powder is nearing the limit, possibly 90 grains of Pyrodex RS could be used? The breech pressures are quite similar.
The velocities of a given powder load of 2Fg GOEX and Pyrodex RS were similar despite the difference in breech pressures.
As for shooting Pyrodex, I've found it to be more likely to misfire than real black powder.
On the positive side, the fouling produced is much less than real black powder.
This allowed continuous shooting with Pyrodex without the need to swab the bore. Something that I can't say when real black powder is being used.
Often, the trigger pull on these Rifled Muskets is poor to say the least and the heavy pull makes precision target shooting very difficult.
The guns were designed to be used against man sized targets under battle conditions so the shooter shouldn't get his hopes for great accuracy when shooting small targets.
Feel free to pass on these tidbits of thought if you think it will help the new shooter.