Forensic Analysis: Weighing Bullet Lead Evidence (2004)

As explained in Chapter 4, there is a need for the meaning of technical evidence to be elucidated for use by attorneys, judges, and jury members. This appendix is intended to serve as a rough guideline for such information to be included in a “boiler plate” document. Such a document would be attached to or incorporated in laboratory reports dealing with compositional analysis of bullet lead (CABL). It is not necessarily intended to be used as is.

CABL can provide useful and probative information regarding a possible association between known bullets and questioned bullets or bullet fragments in a number of case situations. However, CABL has its limits, and the strength of an indicated association will vary from case to case. Care should be taken that these limitations and caveats are appreciated and understood; this may require expert interpretation.

CABL uses a chemical technique called inductively coupled plasma-optical emission spectroscopy (ICP-OES). It is capable of detecting and measuring the concentrations of several elements that occur as trace impurities in or minor alloying elements of bullet lead. The concentrations of those elements that are most useful in discriminating among bullet leads can be measured with good sensitivity, accuracy, and precision. Close correspondence of the quantitative measurements between two samples (the samples are “analytically indistinguish-

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

extruded wire will remove much of the wire that has noticeably different lead characteristics. After this point, the lead will maintain the same composition indefinitely.

The extruded wire is cut into segments to form slugs that will become bullets and bullet cores; these may be stockpiled in bins, possibly with slugs from different wires with different compositions, before they are assembled with other components to form cartridges. Bullets from multiple bins (also with different compositions) may be assembled into cartridges at the same time. That results in the possibility that different compositions of bullet lead are present in a single box of ammunition.

Details about the manufacturing and distribution of lead bullets and finished ammunition are largely unavailable. Therefore, distribution patterns and their effect on random matches cannot be estimated. Calculations can be used only to offer general guidance in assessing the significance of a finding that certain bullets are analytically indistinguishable.

The previously mentioned uncertainties arising from factors related to manufacturing make it difficult to define the size of a “source,” hereafter referred to as a compositionally indistinguishable volume of lead (CIVL). The analytically indistinguishable regions of wire could be considered a CIVL, but other wires extruded from billets from the same melt (assuming there was no additional material added to the melt while the lead was being poured) could also have regions that are analytically indistinguishable from this first wire (although this has not been confirmed by a quantitative, scientific study). A CIVL may range from approximately 70 lbs in a billet to 200,000 lbs in a melt. That is equivalent to 12,000 to 35 million .22 caliber bullets in a CIVL out of a total of 9 billion bullets produced each year.

Although it would be extraordinarily difficult—or impossible—for a large-scale industrial operation (smelter or bullet manufacturer) to purposefully duplicate a given CIVL, the possibility of recurrence of a composition over time as an occasional random event cannot be dismissed. Theoretically, the number of these that might repeat would depend in part on the number of elements measured, the

permitted concentration range for each element, and the discrimination of the analytical technique for each. Considering the thousands of “batches” of lead produced over a number of years, there is a reasonably high probability that some will repeat. However, the probability that any given composition would repeat within the next several years could be expected to be quite low. Furthermore, the likelihood that such a coincidental match would occur from such a source and appear in a given case would be smaller still.

In summary, a CIVL would be large in comparison with the amount of lead in the ammunition in the possession of a typical single purchaser. Thus, multiple people would be expected to have ammunition with the same lead composition. It is not known how many of these would be in the same geographic area. As time passes and some of the ammunition is used, the likelihood of a false association because of the distribution of ammunition with lead from the same CIVL would decrease.

If several evidence bullets in a case have similar but distinguishable compositions, and each of these compositions has a counterpart in a known source, such as a box of ammunition, the association would be stronger.