Back when I used to muzzleload, I had a breech plug that allowed a 25 acp brass and a small primer could be rifle or pistol. They idea was that the 209's were hot, but provided more initial pressure and at times unseated the bullet before ignition was complete, therefore decreasing accuracy. The smaller primers were plenty hot, just not as much ignition pressure. They in turn would give tighter groups.
The acp case allowed you to experiment with 4 different levels of ignition, SR, SRM, SP, SPM.
My muzzleloader really shot well with it.
I THINK THIS HAS SOME RELATION TO THE OP'S QUESTION. MAYBE???