Disclosed is a device and method for externally perforating a well-bore casing. The perforating apparatus is attached to the outside of the casing itself and is conveyed along with the casing when it is inserted into the well bore. The perforation is accomplished using two groups of charges which are contained in protective pressure chambers which are arranged radially around the outside of the wellbore casing. The pressure chambers form longitudinally extending ribs which conveniently serve to center the casing within the well bore. One group of charges is aimed inward in order to perforate the casing. A second group is aimed outward in order to perforate the formation. In an alternative embodiment, only one group of bi-directional charges is provided.