Basis for Compositional Bullet Lead Comparisons
Forensic Physical Scientist
Materials Analysis Unit
Federal Bureau of Investigation
in Lead Composition Resulting from Manufacture.......Significance
of Bullet Lead Data.......References
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.
When a crime-scene bullet contains the same analytical elemental
concentrations (i.e., 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
(bullets and bullet cores) are produced with analytically indistinguishable
compositions. 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.
bullet lead analysis was developed in the early 1960s by researchers
at General Atomic (now General Activation Analysis, Incorporated,
Encinitas, California) under a federal grant to develop uses for
neutron activation analysis (NAA). Researchers developed procedures
for analyzing such materials as gunshot primer residues, glass,
paint, and bullet lead. The results of their research were published
in U.S. Atomic Energy Commission Reports (Lukens et al. 1970; Lukens
et al. 1970), the Journal of Radioanalytical Chemistry (Guinn
1982; Guinn et al. 1987), and the Journal of Forensic Sciences
(Lukens and Guinn 1971). In one research effort, the group acquired
and analyzed samples from bullet lead manufacturers. The results
of these analyses confirmed that a cast billet poured from a pot
of molten lead is relatively homogeneous, but that leads poured
from separate molten batches are distinguishable. As a result, comparative
bullet lead analysis has been adopted by laboratories and accepted
by courts internationally (Andrasko et al. 1993; Blacklock and Sadler
1978; Brandone and Piancone 1984; Capannesi and Sedda 1992; Cohen
et al. 1988; Desai and Parthasarathy 1983; Dufosse and Touron 1998;
Gillespie and Krishnan 1969; Guy and Pate 1973; Kishi 1987; Krishnan
1973; Krishnan and Jervis 1984; Sankar Das et al. 1978; Sreenivas
et al. 1978; Suzuki and Yoshiteru 1996).
The NAA technique
used at many laboratories has been replaced by inductively coupled
plasma-optical emission spectroscopy (ICP-OES), previously known
as inductively coupled plasma-atomic emission spectroscopy (ICP-AES)
(Peters and Koons 1988). OES was adopted because people confused
AES with auger electron spectrometry (Boss and Fredeen 1997). Since
the 1970s, ICP-OES has been widely accepted and is the method of
choice for most inorganic analyses (Koons 1993; Montaser and Golightly
1987). One advantage of ICP-OES is its ability to determine the
concentrations of as many as 70 elements simultaneously in some
samples. ICP-OES instrumentation is used in environmental, manufacturing,
research, and forensic laboratories throughout the world and has
been used by the FBI Laboratory in casework for the past 12 years.
The ICP-OES procedure currently used in the FBI Laboratory can determine
the concentrations of seven elements (antimony, arsenic, copper,
bismuth, silver, tin, and cadmium) in most bullet leads. The main
disadvantage of ICP-OES is that it is a destructive technique, requiring
acid digestion of approximately 60 milligrams of each replicate
sample of bullet lead.
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 75 to 100 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 processable. 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
(i.e., lead where antimony has not been added) is generally used
in jacketed bullets. The other elements are present in trace amounts
and can vary.
is generally delivered to the bullet manufacturers in several forms:
ingots which are 65 to 80 pounds (Figure 1); billets which are 100
to 300 pounds (often 125); and sows which are approximately 2,000
pounds (1 ton). If delivered in ingots or sows, the lead is remelted
in 7- to 10-ton pots (Figure 2) along with lead waste from the manufacturing
process that may include rejected bullets (coated or uncoated),
excess lead from bullet shaping, or any other scrap lead in the
factory. The molten lead is then poured into a billet mold (Figure
3) and allowed to cool and solidify (Figure 4). Wire is extruded
from the billets and cut into slugs (Figure 5). The slugs are formed
into bullets by swaging, then tumbled for smoothness (Figure 6),
and loaded along with gunpowder into primed cartridge cases (Figure
7). The cartridges are then loaded into boxes, which are stamped
with a packing code (also called lot number) (Figure 8).
in Lead Composition Resulting from Manufacture
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.
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. For example,
if the antimony level in a melt of lead were known (with 95 percent
confidence) to be 0.12 % +/- 0.1, it would not be possible to distinguish
(with 95 percent confidence) an alloy that contained 0.20 percent
antimony from this melt on the basis of the antimony level. On the
other hand, if the variability was only +/-0.001, 28 distinguishable
antimony levels could exist. With modern ICP-OES instrumentation,
the high precision achieved (3-5 percent relative standard deviation)
in determining most elements in lead results in millions of potentially
distinguishable lead compositions. It is this ability to distinguish
small differences, in fairly narrow composition ranges (e.g., 0.01-0.05
percent) of the seven elements determined, that results in a high
degree of discrimination between different melts.
overall composition of the lead product is fixed after the billet
formation has cooled (Figure 4). 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 billets from each of two
melts produced on consecutive days were sampled at Winchester Western
Company in 1974 and were analyzed by NAA. The measured percentages
of antimony, copper, and arsenic determined in these samples are
presented in Table 1. These results
show that, for each melt, the five billets made from that melt are
indistinguishable in their concentrations of all three elements.
The billets 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 billet was extruded into a wire that
was subsequently divided into the top, middle, and bottom portions
of a billet. These samples were collected and analyzed by ICP-OES,
the results of which are presented in Table
2. The concentrations of each of the three elements exhibit
no measurable variation among the samples, indicating that this
billet is homogeneous from top to bottom with respect to the measured
variability of the final product can be affected by the final step
in cartridge manufacturing, the packaging of cartridges into boxes.
As a result of casework, in which lead from many boxes of cartridges
from major ammunition manufacturers was analyzed, it has been widely
demonstrated that most boxes of ammunition contain bullets from
more than one melt. As previously discussed, bullets produced from
a single wire (i.e., from the same billet) are analytically indistinguishable.
However, during the processes of cutting, swaging, finishing, and
jacketing bullets; assembling cartridges; and packaging boxes; bullets
from various melts are intermingled. This was demonstrated in a
published study involving 200 bullets from each of four manufacturers
(Peele et al. 1991). A small part of the results of this study is
shown in Tables 3 through 6. For
one box of cartridges from each manufacturer, the average concentrations
of five elements in each of the distinctive compositional groups
are shown. The results are typical of those found in the larger
study. One conclusion from this study is that for the ammunition
studied (.38 special caliber cartridges loaded with lead round-nose
bullets) within each box of 50 cartridges, Federal has one or two
compositional groups, Remington and CCI (Cascade Cartridge Incorporated)
have approximately five compositional groups, and Winchester has
as many as 15 compositional groups. Although each alloy is specified
by the manufacturer to contain certain antimony content, its concentration
varies from 0.58 to 0.81 percent in the box of Remington ammunition
and from 0.24 to 0.66 percent in the box of Winchester ammunition
shown in the tables. These levels of variability in antimony and
the other trace elements account for a large number of distinct
compositions of bullet lead.
of Bullet Lead Data
bullet lead comparisons are possible because each melt of lead has
its own characteristic composition. There are enough identifying
elements with concentrations that are measurable with good precision
in the lead alloy to distinguish among most melts. Years of analysis
in the FBI Laboratory have demonstrated that the distinctiveness
of a melt is defined not only by the number of elements measured
but also by the relative scarcity of other alloys in that melt.
Not all measured elements are equally effective at discriminating
among lead sources, however. In general, for most lead products,
the relative source discrimination power of the measured elements
decreases in the following order: copper, arsenic, antimony, bismuth,
and silver (Peele et al. 1991). Tin is not included in this list
because in many lead sources it is not present at detectable levels.
However, when tin is present, it provides excellent discrimination
among melts of lead. Antimony, specified by the ammunition manufacturers,
is alloyed with lead in order to harden the bullets. The other elements
are present in trace amounts and can vary from one product to another.
Bullet leads analyzed from CCI, Federal, Remington, and Winchester
have contained up to 0.42 percent arsenic, 6.8 percent antimony,
2.5 percent tin, 0.2 percent bismuth, 0.22 percent copper, 0.031
percent silver, and 0.011 percent cadmium. The wide ranges in concentrations
of all of these elements within sources provide for thousands of
distinguishable melts of bullet lead at any one time.
of a molten pot of battery lead can change because of volatilization
of selected elements, segregation during solidification, as well
as other factors (Schmitt et al. 1989). However, in experimental
studies of bullet lead ingots, no compositional variations have
been observed. That is, once a composition is created, it does not
change appreciably merely by being held at the pouring temperature.
Even if there are several compositions within a melt due to the
factors cited, the probability of a random match between unrelated
melts of material would still be low because of the huge number
of compositions that could potentially occur.
The more practical
reason for a compositional match is that the material more likely
is derived from consecutively poured billets than from a random
match among the millions of possibilities among unrelated melts
of material. Accordingly, the assumption of homogeneity of the melt
is a conservative approach because it results in an overestimate
of the number of analytically indistinguishable bullets produced.
In order to
assess the significance of a compositional match, it may be helpful
to know the number of bullets that can be manufactured from a homogeneous
melt. A simple calculation can determine the number of bullets that
can be produced from one ton of lead, though the number per ton
will vary according to the weight of the bullet. For example, a
.38 caliber lead round-nose bullet typically weighs 10.23 grams,
which is equivalent to 158 grains. There are 454 grams in a pound,
and therefore 44 bullets of this caliber are produced per pound.
Because as much as 20 percent of the lead can be lost as waste during
production, only 35 bullets are actually manufactured per pound,
or about 71,000 bullets per ton. A .22 caliber long rifle lead round-nose
bullet weighs 2.6 grams, which is equivalent to 40 grains. Therefore,
with allowance for waste, approximately 140 bullets per pound or
280,000 bullets per ton can be produced. To appreciate the significance,
compare this with the fact that there are approximately 9 billion
cartridges produced annually by ammunition manufacturers in the
to the number of bullets manufactured within one melt, other factors
must be considered, such as the distinctiveness of the melt, the
distribution of ammunition, the relative concentration of the elements,
and the date of manufacture. A manufacturer's distribution of cartridges
throughout the United States is generally on a case-by-case basis.
Since a single melt can be represented across many boxes of ammunition,
it is expected that one source of lead can be distributed to more
than one geographical area. Exceptions to this distribution might
be bullets produced for law enforcement and the U.S. military, who
order large amounts of ammunition at one time. If a packing code
(lot number) is found on a box of ammunition, then the assembly
date may be obtained from the manufacturer. Another factor that
must be considered is a case where multiple shots of various calibers,
manufacturers, and compositions are fired at a crime scene. If multiple
compositions present in the crime-scene lead are analytically indistinguishable
from lead groups in partial boxes of ammunition, it is much more
likely that the crime-scene bullets came from those boxes than it
is when only one compositional group is present.
All the aforementioned
factors are considered when interpreting the compositional analysis
data to determine if there is an association between specimens.
If such an association exists, an example of the conclusion reached
by the FBI Laboratory may read as follows, "The bullet removed
from the victim and 10 of the 15 analyzed cartridges from the suspect
residence are analytically indistinguishable from one another. Therefore,
they likely originated from the same manufacturer's source (melt)
of lead." This conclusion does not associate a bullet to a
box but rather to a melt of lead that has bullet specimens within
that box and perhaps other boxes.
expanding on the early work by General Atomic and others, the FBI
Laboratory has successfully defended challenges to the scientific
validity of compositional bullet lead comparisons and its application
to individual cases in federal, state, and local court systems since
the 1970s. Recently, as a result of a Daubert hearing, the
United States District Court in Columbia, South Carolina, admitted
the technique in United States v. Jenkins 1997; and as a
result of a Frye hearing, New York admitted the technique in People
v. McIntosh 1998. The admissibility of this examination in court
has also been affirmed on an appeal by the New Jersey Supreme Court
in State v. Noel 1997.
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