The major difference in power would come from how tightly the ball fits the barrel. Most muskets were subsonic, that is the ball exited the barrel at slower than sound speed, but sometimes they would become supersonic due to fouling. Unburned powder would build up on the walls of the barrel after several shots, which would create a tighter fit and therefore more pressure would build up. The ball would be accelerated faster and sometimes break the sound barrier. With more energy it could travel farther and inflict more damage. Some muskets were naturally more inclined to this phenomenon than others due to having less leeway in the barrel.
The end results would still be the same though, musket ball hit = death. Even a non vital area would most likely than not be a death sentence, due to infection and blood loss.