able”) may suggest that the two samples were derived from a common “source.” However, several poorly characterized processes in the production of bullet lead and ammunition, as well as in ammunition distribution, complicate the interpretation and render a definition of “source” difficult. For that reason, unlike the situation with some forms of evidence (such as the DNA typing of bloodstains), it is not possible to obtain accurate and easily understood probability estimates that are directly applicable. It is necessary for the finder of fact to have a general understanding of the possible complicating factors.
Virtually all the lead used in the manufacture of lead bullets and lead bullet cores in the United States is purchased from secondary lead smelters that use recycled automotive batteries as their primary source of lead. It is not economically feasible to attempt to remove particular elements below some point. To meet user specifications during the refining process, smelters must keep the concentrations of specified elements in the lead within a range or below a maximum set by the bullet manufacturers. The variation in several elements from the ore, from use as battery lead, and required by the bullet manufacturers (arsenic, As; antimony, Sb; tin, Sn; copper, Cu; bismuth, Bi; silver, Ag; and cadmium, Cd) provides the basis of discrimination used in CABL. The smelter casts the refined molten lead into molds, where it cools and solidifies to form castings for shipment to customers, including bullet manufacturers. A variety of mold sizes can be used to produce castings known as pigs, sows, ingots, and billets.
Bullet manufacturers produce bullets from continuous cylindrical wires of lead. The wires are produced by extrusion, when the billet is forced through a circular orifice of a specified size to produce the lead wire. The diameter of the wire produced depends on the caliber and design of the bullets to be made. Some bullet-manufacturing plants obtain billets for wire extrusion directly from the smelter. Others produce their own billets from large melts made from larger castings obtained from the lead smelter.
Additional steps in the bullet-manufacturing process can introduce changes in the lead’s elemental composition. When ingots are melted in the bullet-manufacturing plant, multiple ingots of different composition may be melted together in a large vessel. In addition, the composition of the melt may change because of oxidation of some elements by exposure to air, the addition of lead recycled from other parts of the operation, and drawing off of molten lead for casting while lead is being added to the vessel. Thus, small but important changes in the composition of the lead can take place during many steps in the smelting and bullet-production steps.
Furthermore, as a billet cools, any radial segregation that occurs tends to be homogenized during extrusion of the wire. Top-to-bottom variations still exist, but it is probable that the industry practice of removing the first several feet of