Consider that when a cylindrical object intersects a sphere the resistance increases as the area of penetration grows. The result is an elongated elipse in the skull even though the weapon is cylindrical. The shape of the broken part of the skull will be small and not related to the length of the weapon but rather to the diameter. The smaller the diameter of the cylinder the longer the elipse. So the long 7x4 inch bruised area shows it was not a localized puncture like you might get from a hammerlike object, but rather a failure caused by a long cylindrical object.