When the physical markings of a fired bullet recovered from a crime scene are too mutilated for visual comparison or the firearm used in the crime is not recovered, the bullet can be compared with other bullets associated with a suspect by its elemental composition through the use of a criminological microscope. When a crime scene bullet contains the same analytical elemental concentrations or match in composition, as the bullets from known cartridges, a single source for these bullets cannot be excluded.
During the manufacturing processes, thousands of lead specimens like bullets and bullet cores are produced with analytically indistinguishable compositions that can only be seen using a specialized microscope like a criminological microscope. However, those lead specimens that share the same composition are generally packaged within the same box of cartridges, or in boxes of cartridges of the same caliber and type at the same manufacturing plant, on or about the same date. When the differences in element concentrations are small but analytically significant, a comparative examination can be used to differentiate among bullets made of different alloys or to exclude a single source for bullets of the same alloy.
Lead used in the bullet manufacturing process is generally obtained from secondary lead smelters where the raw material is made primarily of recycled automobile batteries. Under stringent environmental regulations, these smelters separate the batteries into plastic, acid, and lead components. This lead is then mixed with lead from other sources and melted in kettles with capacities of seventy five to a hundred tons. This scrap lead is reprocessed into ingots, also called pigs. Elements such as copper and tin may be present but are controlled within limits determined by the economics of the process and use of the product. For bullet manufacture, there are few physical requirements for the lead. Chiefly, the lead must be processed. Antimony may be added to harden the alloy, but its level will also vary with the requirements of the product and the economics of its use. Hardened lead is generally used in non-jacketed bullets, whereas soft lead like lead where antimony has not been added is generally used in jacketed bullets. The other elements are present in trace amounts and can vary.
The composition of lead reflects its inevitable heterogeneity at the secondary smelter, where the source material is usually a variable mixture of virgin and scrap lead. Differences in each batch may be attributed to environmental contamination, variations in mold erosion rates, and temperature variations. Typically, the extracted metal must be processed further before its final use. However, the ultimate goal is to produce an acceptable product at the lowest possible cost. One consequence of the economics is that variations in composition are tolerated as long as they do not adversely affect the physical properties of the products being manufactured. Maximum levels of certain deleterious impurities are defined and not exceeded; at the same time, alloying elements are kept between pre-established minimum and maximum levels.
When processing the lead to produce wire for bullets, the ammunition manufacturer may add rejected lead from previous runs, lead trimmings, rejected bullets, including copper-plated rounds, and virtually any other source of lead in the plant that may be recycled into the pot with the lead ingots. If it was not recycled, the scrap would become an environmental hazard. Thus, with the proportions of recycled materials undoubtedly varying from batch to batch, the composition of the lead mixture will inevitably vary.
This lead mixture occurring both at the smelter and the ammunition manufacturer provides meaningful information to forensic scientists. The homogeneity of each melt supplies an identity to a batch while it provides the ability to distinguish between batches. This enables bullets to be compared by the different mean concentrations of the elements in each. The variation of the concentrations within a source depends on both the homogeneity of the source and the analytical reproducibility of the instrument making the measurements. The number of distinguishable compositions that can occur in a given concentration range increases as the variability of the measurement is decreased. It is this ability to distinguish small differences, in fairly narrow composition ranges of the seven elements determined, that results in a high degree of discrimination between different melts.
The overall composition of the lead product is fixed after the bullet formation has cooled.
At most manufacturers, other scrap is frequently added to the lead in the melting pot throughout the dynamic process of bullet lead formation. As a result, bullets made from continuous pours may be analytically indistinguishable over only one to two tons. In one study, five bullets from each of two melts produced on consecutive days were sampled and were analyzed by using a criminological microscope. The measured percentages of antimony, copper, and arsenic determined in these samples show that, for each melt, the five bullets made from that melt are indistinguishable in their concentrations of all three elements. The bullets from different melts are readily distinguishable by the concentrations of antimony and copper, which are significantly higher in pour one than they are in pour two, and by the concentrations of arsenic, which are slightly lower in pour one than in pour two. In another study, a single bullet was extruded into a wire that was subsequently divided into the top, middle, and bottom portions of a bullet. The concentrations of each of the three elements exhibit no measurable variation among the samples, indicating that this bullet is homogeneous from top to bottom with respect to the measured element concentrations.
