Before retiring as a BP builder & gunsmith I removed numerous loads from several original rifles, pistols & muskets without breech plug removal.
My recommendation;
FIRST MAKE SURE THE STUCK MINI OR BALL IS SEATED TO THE BREACH AREA !!
Always use a cleaning rod to assure the stuck Mini is seated at the breach area. If stuck use a brass or steel rod to seat the Mini down to the breach area before firing.
Failure to seat a projectile at the breach with a full load can create a bulge & ruin the barrel.
Now that the pyrodex has been contaminated with oil or solvents remove the nipple & immerse 6"
of the barrel's breach area in acetone or rubbing alcohol for a couple hours to remove the oil.
After soaking, allow the breech area to drain & air-dry dry for a couple more hours, then blow through the nipple's threads with compressed air to remove any remaining residual crud.
After breach area has dried, trickle 20-30 grains of 4F or 3F 'real' Black Powder through the empty nipple port, replace nipple
& fire the stuck Mini- Ball out.
*Not to fret, very few of us old-timers can claim we never dry-balled a load
Relic shooter