Semi
Under Stabilizing a bullet and it will tumble this cause instant inaccuracy it is much harder to over stabilize a bullet, it can be done but usually if you have the right twist for the caliber and type of bullet you won't.
If you blow the patch or sabot or strip the bullet it will shoot wild.
Most of the time what you are dealing with on high end loads with a muzzle loader is the whip or vibration of the barrel. A rifled barrel whips with a circular motion and the most accurate point is when it comes back to the point it started from because the barrel does not impart any side ways momentum.
They had a high speed picture that they showed in slow motion when I was an apprentice that showed the barrel whipping in a circular motion and they explained that that motion is added because of the stress of spinning the bullet.
I know a lot of you will find this had to believe but there are usually two or three loads that will shoot accurate and they are departing the barrel at the original position, the heaviest load will be the hardest to find because the barrel is whipping faster with a heavy load, heavy barrels slow this down and whip less which is why it is easier to get them to shoot accurate.