This discussion is going over my head, too. However, I think the same amount of force would be applied on a disk-shaped projectile and on a spherical one, because the forces acting on opposite sides of the hemisphere would cancel out, except for the vectors--and the vector force would be the same as that of a disk. That's just a theory, though...
EDIT: I was also sure that the wadding (I forgot the correct term) has a flat surface, but I may be wrong...