3
Firearms Identification and the Use of Ballistics Evidence

This chapter provides a key context for the next part of the report by examining the nature of ballistics evidence itself and the ways it has been analyzed and used over the past century. Since the primary focus in this study is the ability of computer-based systems to discern unique characteristics in ballistics evidence on the basis of images of different sorts, it is useful to first consider how uniqueness and reproducibility of characteristic marks have been discerned through the particular media of the comparison microscope and the human eye.

We briefly describe the general nature of toolmarks as evidence (Section 3–A) and then outline the basic history and theory of firearms identification (3–B). Section 3–C summarizes the literature on the uniqueness, reproducibility, and permanence of marks as detected by traditional methodology. The circumstances of firing a weapon, the manufacture of firearms and ammunition, and measures taken (or not taken) by firearms users can all pose various complications to the identification and linking of ballistics evidence; we review some of these issues in Section 3–D. Section 3–E offers key commentary on the scope of the committee’s work in the context of the material in this chapter. The final section (3–F) briefly describes the general role of imaging and photography in firearms identification, as a prelude to the next section of the report.

3–A
TOOLMARKS AS EVIDENCE

Various branches of forensic science—dealing with the analysis of evidence as diverse as handwriting samples and soil or mineral samples—are



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3 Firearms Identification and the   Use of Ballistics Evidence This chapter provides a key context for the next part of the report by  examining the nature of ballistics evidence itself and the ways it has been  analyzed  and  used  over  the  past  century.  Since  the  primary  focus  in  this  study  is  the  ability  of  computer-based  systems  to  discern  unique  charac- teristics in ballistics evidence on the basis of images of different sorts, it is  useful to first consider how uniqueness and reproducibility of characteristic  marks have been discerned through the particular media of the comparison  microscope and the human eye. We briefly describe the general nature of toolmarks as evidence (Sec- tion 3–A)  and  then  outline  the  basic  history  and  theory  of  firearms  iden- tification (3–B). Section 3–C summarizes the literature on the uniqueness,  reproducibility, and permanence of marks as detected by traditional meth- odology. The circumstances of firing a weapon, the manufacture of firearms  and ammunition, and measures taken (or not taken) by firearms users can  all pose various complications to the identification and linking of ballistics  evidence; we review some of these issues in Section 3–D. Section 3–E offers  key commentary on the scope of the committee’s work in the context of the  material in this chapter. The final section (3–F) briefly describes the general  role of imaging and photography in firearms identification, as a prelude to  the next section of the report. 3–A TOOLMARkS AS EvIDENCE Various branches of forensic science—dealing with the analysis of evi- dence as diverse as handwriting samples and soil or mineral samples—are  

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 BALLISTIC IMAGING increasingly  compared  with  one  of  their  sister  branches:  the  analysis  of  deoxyribonucleic  acid  (DNA)  markers  and  identification  based  on  those  markers. As we note in Section 1–C, the term “ballistic fingerprinting” has  also  come  into  common  (albeit  somewhat  inaccurate)  usage  to  describe  some  features  of  firearms  identification,  suggesting  a  comparison  with  fingerprint evidence.  Firearms toolmark evidence differs from DNA and fingerprint evidence  due to their basic point of reference: the former links to a particular firearm  while the latter two link to a particular person. Links between pieces of bal- listics evidence can point to a common gun from which exhibits were fired,  but not necessarily to the same person pulling the trigger. A potential match  suggested by a national reference ballistic image database could suggest a  link  from  a  piece  of  crime  scene  evidence  to  an  original  firearm  point  of  sale, but that link is at least doubly indirect: the link is only to the location  of the transaction and not immediately to the firearm’s purchaser, and sub- sequent identification of the purchaser does not necessarily mean that the  purchaser still possesses the gun (or fired the shot in a crime). However, it  is important to consider that—alone, absent any other evidence or knowl- edge of circumstances—even person-specific fingerprint and DNA evidence  is  necessarily  one  step  removed  and  indirect.  It  is  possible  for  fingerprint  or DNA evidence to be present and retrievable at crime scenes without its  source  person  having  been  the  crime’s  perpetrator.  Forensic  evidence  can  be  used—in  combination  with  other  investigative  findings—to  develop  a  theory of what transpired at the scene and who may have committed the  crime, but the link to any specific person from the ballistics evidence alone  is necessarily indirect. Toolmark evidence and DNA evidence are markedly different in another  crucial respect, which is the subjectivity inherent in the analysis. Firearms  identification  ultimately  comes  down  to  a  subjective  assessment—specifi- cally, a subjective probability statement (although practitioners often render  these as absolute statements). Firearms examiners observe concrete, objec- tive  phenomena,  but—as  Thornton  and  Peterson  (2002:24–25)  observe,  “there  is  an  incredible  amount  of  difficulty  attached  to  the  development  of a statistical basis for evidence evaluation” in forensic science fields like  firearms examination: Behind every opinion rendered by a forensic scientist there is a statistical  basis. We may not know what that basis is, and we may have no feasible  means of developing an understanding of that basis, but it is futile to deny  that  one  exists.  .  .  .  The  most  common  and  coherent  theory  of  forensic  identification  is  that  where  there  is  a  high  degree  of  variation  among  a   ttributes (of toolmark striations, writing, friction ridges on skin, and so  on), then where a “match” is observed the probability that the match is 

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 FIREARMS IDENTIFICATION coincidental rather than reflecting a shared source will be very small. . . .  Forensic individualization sciences that lack actual data, which is most of  them,  have  no  choice  but  to  either  intuitively  estimate  those  underlying  probabilities and calculate the coincidental match probability from those  subjective probabilities, or simply to assume the conclusion of a miniscule  probability of a coincidental match (and in fact they do the latter). In the specific context of firearms and toolmark examination, derivation  of an objective, statistical basis for rendering decisions is hampered by the  fundamentally random nature of parts of the firing process. The exact same  conditions—of  ammunition,  of  wear  and  cleanliness  of  firearms  parts,  of  burning of propellant particles and the resulting gas pressure, and so forth— do not necessarily apply for every shot from the same gun. Ultimately, as  firearms identification is currently practiced, an examiner’s assessment of the  quality and quantity of resulting toolmarks and the decision of what does or  does not constitute a match comes down to a subjective determination based  on  intuition  and  experience.  By  comparison,  DNA  analysis  is  practically  unique among forensic science specialties as having a strong objective basis  for determination and as being amenable to formal probability statements.  Thornton and Peterson (2002:Fig. 1) rank various forensic science sub- fields on a continuum of relative subjectivity. On the low end of that scale  is DNA analysis, along with serology (blood type determination) and drug  and narcotic identification. They identify firearms and toolmark identifica- tion as having relatively high subjectivity, on par with fiber identification.  They  identify  blood  spatter  interpretation,  voiceprint  analysis,  and  bite- marks as a group of forensic science specialties just slightly more subjective  than toolmark identification, and handwriting and hair identification as a  cluster slightly more subjective yet. 3–b TRADITIONAL FIREARMS IDENTIFICATION Smith  (2004:130)  succinctly  summarized  the  basic  task  of  a  firearms  examiner in making an identification between pieces of evidence: Before a microscopic comparison begins, a foundation is built by measur- ing and comparing available class characteristics, such as General Rifling  Characteristics  (GRCs).  These  objective  criteria  are  used  to  narrow  the  pool of candidates for determining a common source. Once an available  foundation  has  been  established,  a  common  source  often  can  be  deter- mined by evaluating individual microscopic marks of value using pattern  recognition. In  traditional  firearms  identification—part  science  and  part  art  form,  still carried out today using the same basic tools that gave rise to the field 

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 BALLISTIC IMAGING in the 1930s—the firearms examiner faces the formidable cognitive task of  forming a mental pattern of identifying marks and features on bullet and  cartridge case evidence. That pattern must then be matched to those from  other exhibits.1  3–b.1 Individualization and Identification: Class, Subclass, and Individual Characteristics The  label  “firearms  identification”  is  another  instance  in  which  lan- guage can be a bit elusive. Although “the terms ‘identification’ and ‘iden- tity’ are used constantly by practitioners” of criminalistics, or the forensic  analysis of evidence, Kirk (1963:236) argued that the usage is an “unfor- tunate failure of nomenclature.” Rather than identification (as that term is  commonly  understood),  Kirk  argues  that  “criminalistics  is  the  science  of  individualization:” The criminalist does not attempt identification except as a prelude to his  real function—that of individualizing. The real aim of all forensic science is  to establish individuality, or to approach it as closely as the present state of  the science allows. . . . What was actually done was not the identification  of the fingerprint, but rather the individualization of a person as the one  who left the fingerprint. . . . [Likewise,] if the firearms examiner said that  the bullet was a Colt 45 A.C.P. but could not individualize the gun that  fired it, his value would be relatively slight. Thornton  and  Peterson  (2002:8,  9)  further  differentiate  between  the  two terms: “Individualization is the process of placing an object in a unit  category which consists of a single unit. Individualization implies unique- ness; identification, strictly speaking, does not require it.” They also note  the  frequent  use  in  forensic  science  of  “identification”  when  “individual- ization” is meant, but recognized that “it is a constraint imposed by our  language”; relying on the term “individualize” would likely lead to public  confusion. “It should be appreciated, however, that the process of identi- fication means one thing to the forensic scientist, and another thing to the  botanist or the zoologist.” (See also Champod [2000:1077] on “individu- alization” versus “identification.”)  The phrasing of “individualization” as the act of associating an object  1  urther  information  on  current  practice  in  firearms  identification  and  images  connected  F with that work are available through resources at the Association of Firearm and Tool Mark  Examiners  (http://www.afte.org)  and  the  Scientific  Working  Group  for  Firearms  and  Tool- marks (http://www.swggun.org). Additional information and images—and tools for simulat- ing the use of the comparison microscope for comparing bullet and cartridge evidence—are  accessible at http://www.firearmsid.com.

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 FIREARMS IDENTIFICATION with  a  category  that  contains  only  one  unit  is  a  useful  one  in  addressing  the basic approach of firearms examination. Firearms examiners typically  give testimony to the effect that this particular gun, and not any other gun  in the world, was used to fire particular shots. Yet even in the profession’s  earliest days, Hatcher (1935:266) anticipated a common reply to this type  of argument: The  firearms  witness may expect to  be asked how he can be  sure of his  findings, in view of the fact that he can have examined only a few of the  countless  thousands  of  guns  that  exist.  But  how  does  [an  eyewitness  to  the crime] that identified the prisoner know that some other person may  not look exactly like him? He will say that he has seen enough people in  his life to know that the rule is that all people look different, and that the  chances are overwhelmingly against finding two people who look so much  alike that they cannot be told apart.   In precisely the same way, the firearms expert, who admittedly cannot  have actually examined more than the tiniest fraction of a percent of all the  guns in the world, can still have had enough experience to be well aware  of what differences do occur, and to know that the chance of finding two  bullets  from  different  guns  that  are  exactly  alike  in  every  detail  of  their  surface markings is infinitesimally small. The same thing applies with equal  force to the finger-print method of identification.  Subsequently,  he  posited  this  idea  more  succinctly:  “It  may  be  quite  common for two or more prominent individual marks on bullets from two  entirely different guns to match exactly, but the chance that there will be a  correspondence of a great many of the individual characteristic marks on  two bullets that came from different guns is so remote as to amount to a  practical impossibility” (Hatcher, 1935:288). The basic approach to identification in forensic science developed into  the concept of class characteristics and indiidual characteristics. Thornton  and Peterson (2002:5–6) define these concepts: Class  characteristics  are  general  characteristics  that  separate  a  group  of  objects from a universe of diverse objects. In a comparison process, class  characteristics serve the very useful purpose of screening a large number of  items by eliminating from consideration those items that do not share the  characteristics common to all the members of that group. Class characteris- tics do not, and cannot, establish uniqueness. Individual characteristics, on  the other hand, are those exceptional characteristics that may establish the  uniqueness of an object. It should be recognized that an individual charac- teristic, taken in isolation, might not in itself be unique. The uniqueness of  an object may be established by an ensemble of individual characteristics.  A scratch on the surface of a bullet, for example, is not a unique event; it is  the arrangement of the scratches on the bullet that mark it as unique.

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 BALLISTIC IMAGING In  the  context  of  examining  bullet  and  cartridge  case  evidence,  such  parameters as caliber and the number of lands and grooves are class char- acteristics that can be used to screen or filter out possible comparisons that  could  not  possibly  have  been  fired  from  the  same  gun.  Other  class  char- acteristics include firing pin shape (for instance, the distinctive rectangular  firing pin impressions left by Glock firearms2) or pronounced “drag marks”  that  can  be  caused  by  firing  pins  as  the  cartridge  case  goes  through  the  ejection process. Individual characteristics—the fine striations on a bullet’s  surface  or  peculiar  microscopic  textures  in  the  firing  pin  impression,  for  instance—allow for the set of possible guns from which an exhibit could  have been fired to be narrowed. However,  a  binary  split  between  class  and  individual  characteristics  proved  too  limited  to  describe  the  full  range  of  phenomena  observed  by  experienced examiners. In 1985, the Association of Firearm and Tool Mark  Examiners (AFTE) convened a committee to develop a consensus document  on a theory of toolmark identification and a range of basic conclusions that  could be reached from comparison of toolmark evidence. The result, pro- duced in 1989 and unanimously approved by the organization’s member- ship, is reproduced in Box 3-1. Importantly, the 1989 standard also added  a new term to the firearms identification lexicon: subclass characteristics,  defined as follows (Moran, 2002:228): Discernable surface features of an object, which are more restrictive than  “class characteristics” in that they are •  Produced incidental to manufacture •  Significant in that they relate to a small group source (a subset of the  class to which they belong) •  Can arise from a source which changes over time Caution should be exercised in distinguishing subclass characteristics from  individual characteristics. As a middle ground between class and individual characteristics, subclass  characteristics covered marks or features that—arising from specific manu- facturing  techniques,  or  flaws  in  said  techniques—could  induce  similar  marks  on  pieces  of  evidence  even  though  they  originated  from  different  2  ectangular firing pin marks were a telltale sign of Glock pistols “prior to the introduction  R of Smith & Wesson’s Sigma Series, Model SW40F, semi-automatic pistol,” the design of which  is  very  similar  to  Glock  standards.  Nichols  (1995:133,  134),  in  documenting  the  similarity  b   etween this firearm’s marks and the known Glock characteristics, notes that the two highly  s   imilar gun types can be distinguished by ejector and extractor marks, as well as a character- istic “dimple on the [casing] head above the firing pin drag” on Glock rounds.

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9 FIREARMS IDENTIFICATION BOX 3-1 Association of Firearm and Tool Mark Examiners (AFTE) Theory of Identification and Range of Conclusions Theory of Identification as It Relates to Toolmarks A. The theory of identification as it pertains to the comparison of toolmarks enables opinions of common origin to be made when the unique surface con- tours of two toolmarks are in “sufficient agreement.” B. This “sufficient agreement” is related to the significant duplication of random toolmarks as evidenced by the correspondence of a pattern or combination of patterns of surface contours. Significance is determined by the compara- tive examination of two or more sets of surface contour patterns comprised of individual peaks, ridges, and furrows. Specifically, the relative height or depth, width, curvature, and spatial relationship of the individual peaks, ridges, and furrows within one set of surface contours are defined as compared to the corresponding features in the second set of surface contours. Agreement is significant when it exceeds the best agreement demonstrated between tool- marks known to have been produced by different tools and is consistent with the agreement demonstrated by toolmarks known to have been produced by the same tool. The statement that “sufficient agreement” exists between two toolmarks means that the agreement is of a quantity and quality that the likeli- hood another tool could have made the mark is so remote as to be considered a practical impossibility. C. Currently, the interpretation of individualization/identification is subjective in nature, founded on scientific principles and based on the examiner’s training and experience. Range of Conclusions Possible When Comparing Toolmarks The examiner is encouraged to report the objective observations that support the findings of toolmark examinations. The examiner should be conservative when reporting the significance of these observations. The following represents a spectrum of statements: A. IDENTIFICATION: Agreement of a combination of individual characteristics and all discernible class characteristics where the extent of agreement ex- ceeds that which can occur in the comparison of toolmarks made by different tools and is consistent with the agreement demonstrated by toolmarks known to have been produced by the same tool. B. INCONCLUSIVE: a. Some agreement of individual characteristics and all discernible class characteristics, but insufficient for an identification. continued

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0 BALLISTIC IMAGING BOX 3-1 Continued b. Agreement of all discernible class characteristics without agreement or disagreement of individual characteristics due to an absence, insufficiency, or lack of reproducibility. c. Agreement of all discernible class characteristics and disagreement of individual characteristics, but insufficient for an elimination. C. ELIMINATION: Significant disagreement of discernible class characteristics and/or individual characteristics. D. UNSUITABLE: Unsuitable for comparison. SOURCES: Excerpted from AFTE Criteria for Identification Committee (1992) and Grzybowski et al. (2003). sources. Two examples of carryover of subclass characteristics are described  in Box 3-2. The AFTE theory of identification is rooted in the recognition that “the  interpretation  of  individualization/identification  is  subjective  in  nature.”  However, it melds that recognition with more objective, quasi-quantitative  benchmarks—“sufficient  agreement,”  “significance,”  “likelihood  .  .  .  so  remote,”  and  agreement  in  both  “quantity  and  quality”—but  no  specific  empirical definition is given for these terms. Importantly, the AFTE theory  does set up two cognitive thresholds that must be crossed in order to arrive  at the conclusion that two exhibits share the same source, to the exclusion  of  all  others.  In  describing  the  comparison  of  fingerprints,  Stoney  (1991;  quoted in Moran, 2002:233) wrote of the basic cognitive process: When more and more corresponding features are found between two pat- terns, scientists and lay person alike become subjectively certain that the  patterns could not possibly be duplicated by chance. What has happened  here is somewhat analogous to a leap of faith.3 It is a jump, an “extrapo- lation,”  based  on  the  observation  of  highly  variable  traits  among  a  few  characteristics,  and  then  considering  the  case  of  many  characteristics.  Duplication  is  inconceivable  to  the  rational  mind  and  we  conclude  that  there is absolute identity.  3  he  “leap  of  faith”  involved  in  extrapolating  to  all  possible  sources  in  the  world  was  T echoed,  in  specific  reference  to  firearms  identification  by  Rowe  (1991).  Understandably,  some  firearms  examiners  have  objected  to  some  overtones  of  the  “leap  of  faith”  phrasing;  for instance, Miller and McLean (1998:17–18) “respond [to the phrasing] by stating that the  published  data  which  does  exist,  certainly  presents  enough  statistical  data  to  indicate  more  substance to the identification theory than a ‘leap of faith.’’’

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 FIREARMS IDENTIFICATION BOX 3-2 Examples of Subclass Carryover • Lomoro (1974:18) described test firings from F.I.E. Titanic revolvers, in a particular serial number range (but later shown to include some of a different make from the others). These firings produced seemingly similar striation patterns on fired bullets that could be naively matched to each other even though they had been fired from different weapons. “This pronounced stria [pattern] was only pres- ent on the lands of the bullets and very little if any matching stria was found on the grooves of the bullets. Examination of the bore revealed that either a worn or a very poor rifling tool was used during manufacturing,” causing the similarity be imparted within a batch of separate barrels. • Matty and Johnson (1984) observed that the particular tooling used on some Smith & Wesson firing pins results in a pattern of concentric rings that is re- peatable in consecutively tooled pins. This pattern appears in the firing pin impres- sion, meaning that examiners need to downweight the ring patterns and focus on other individual features to make correct matches on the firing pin impression. He continues that this “leap” occurs—in fingerprinting, as in other branches  of forensic science, “without any statistical foundation.”  In a sense, the AFTE theory of identification requires two extrapola- tions:  first,  that  marks  are  sufficiently  consistent  with  true  matches  (pro- duced by the same tool) to have come from the same source, and, second,  that  the  quality  and  similarity  of  corresponding  features  exceed  the  best  known apparent agreement between true nonmatches (produced by differ- ent tools). It follows that both of these extrapolations—but particularly the  latter—require considerable experience in comparing exhibits and training  in recognizing significant features.4 3–b.2 Historical Evolution Some  of  the  major  advances  in  the  field  of  traditional  firearms  iden- tification are described in Box 3-3. Calvin H. Goddard, a founder of the  Bureau of Forensic Ballistics in the New York City Police Department, is  typically credited as the “father” of forensic ballistics in the United States;  4  iasotti  and  Murdock  (2002:219–220)  advocated  the  addition  of  the  known  nonmatch  B criterion,  beginning  with  their  conclusion  in  1984  that  “existing  research  was  insufficient  to validate the quantitative objective criteria necessary to conclude that a working surface is  unique.” The known nonmatch criterion “added a quantitative dimension” to the mix.

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 BALLISTIC IMAGING BOX 3-3 Highlights in the History of Traditional Firearms Identification • 1900: The Buffalo Medical Journal published “The Missile and the Weapon” by Albert L. Hall, commenting on his observations that firearms of different mark impart different striated marks and impressions on fired bullets. • 1907: Cartridge case evidence was examined as part of the investigation of the 1906 Brownsville, Texas, riots in which black soldiers allegedly fired upon a white crowd in retaliation against racial slurs. Thirty-nine cartridges were recov- ered; all but six were matched to four rifles, based on examination of enlarged photographs of firing pin impressions. [Examiners concluded that no identifications could be made on the basis of recovered bullet evidence.] • 1915: Charlie Stielow, an illiterate farmer in New York state, was convicted and sentenced to death for killing his employer and the employer’s housekeeper, the latter of whom was found dead on the doorstep of Stielow’s nearby home. Stielow’s conviction was due in part to testimony by an examiner who claimed to find nine abnormal scratches on the recovered bullets that corresponded to particular defects in the alleged murder weapon. However, a more careful subse- quent analysis of the evidence by Charles Waite concluded that Stielow’s revolver could not have been the murder weapon; Stielow was pardoned and released from prison. • 1925: Philip Gravelle was credited with the development of the comparison microscope for side-by-side comparison of ballistics evidence in Calvin Goddard’s article “Forensic Ballistics” in Army Ordinance. Goddard used the technique in the article, and the comparison microscope technique grew in popularity after the Saturday Evening Post’s two-part “Finger-printing Bullets” article in 1926 profiled New York City’s Bureau of Forensic Ballistics, which Goddard, Gravelle, and Waite helped found. • 1929: On February 14, six members of George “Bugs” Moran’s North Side gang (and one acquaintance) were brutally killed in a Chicago warehouse by six men impersonating police officers, believed to be operatives of rival Al Capone’s South Side gang, in what quickly became known as the “St. Valentine’s Day Mas- sacre.” As part of the investigation, Goddard prepared a detailed examination of recovered bullet and cartridge case evidence, connecting them to two Thompson submachine guns and a 12-gauge shotgun. Goddard—considered the father of modern firearms examination—subsequently left New York City to establish the Scientific Crime Detection Laboratory of Chicago, affiliated with Northwestern University. • 1934–1935: First editions of two seminal texts on firearms examination were published: Gerald Burrard’s The Identification of Firearms and Forensic Bal- listics and Julian Hatcher’s Textbook of Firearms Investigation, Identification and Evidence. • 1958: John Davis’ An Introduction to Tool Marks, Firearms and the Stria- graph argued for the use of the striagraph, an early tool for direct measurement of the surface contours of ballistics evidence. Though the striagraph never advanced beyond the research stage, it was a precursor to the use of imaging and profilom- etry techniques for firearms identification.

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 FIREARMS IDENTIFICATION BOX 3-3 Continued • 1962: J. Howard Matthews, a retired chemistry professor from the Univer- sity of Wisconsin who became interested in forensic science after being called in to assist on a bombing case, published the two-volume Firearms Identification. • 1969: At a 35-member conference hosted by the Crime Laboratory of the Chicago Police Department, firearms identification practitioners form the Asso- ciation of Firearm and Tool Mark Examiners (AFTE). The new association’s news- letter developed into the AFTE Journal in 1973 and continues as a quarterly, peer-reviewed publication; the international association continues to hold an an- nual Training Seminar. SOURCES: Information from Hamby and Thorpe (1999), Hatcher (1935), and Goddard (1999). the  text  by  Hatcher  (1935)  is  generally  considered  the  text  on  which  the  modern  field  of  firearms  and  toolmark  examination  is  based  (Nichols,  2003). The modern field of firearms identification owes much to the develop- ment of the comparison bridge, a system of mirrors or prisms that permits  the views from two separate microscopic lenses to appear side by side in  the same field of view. The device equipped with this optical bridge, called  the comparison microscope, was not designed for analysis of bullet or car- tridge case evidence, nor was it first used in such forensic applications (see,  e.g.,  Thornton,  1978;  Goddard,  1980).  Even  early  advocates  of  forensic  firearms analysis cautioned against making too much of the technology at    hand, noting that the comparison microscope, on its own, has limitations.  Foreshadowing some contemporary claims about the capability of ballistic  imaging systems, Burrard (1962:131–132) lamented:  The  most  fantastic  claims  have  been  put  forward  for,  and  the  most  r   idiculous descriptions of, [the comparison microscope] which are enough  to  suggest  that  it  has  magical  properties,  and  that  it  automatically,  and  wholly of its own accord, rings a bell or utters some similar warning, when  the  two  cartridge  cases  under  examination  exhibit  the  same  [individual  markings]. Unhappily there is no foundation for this comforting belief. In  the hands of a trained microscopist the comparison microscope can be of  great  value  in  determining  the  identity  of  fired  bullets;  but  for  cartridge  cases I have come to the conclusion that a high-class single instrument is  preferable. . . . And even for fired bullets the comparison microscope offers 

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 BALLISTIC IMAGING fired  at  all  is  likely,”  missing  possible  hits.  Corrosion  is  also  problematic  because some means of dealing with it—including scrubbing with metal or  other brushes—may also damage the weapon for the purposes of generat- ing matchable marks. Corrosion is a concern not only for the internal parts of firearms, but  also for recovered ballistics evidence. Though it is certainly ideal to process  and collect evidence from crime scenes rapidly, such is not always possible.  Bullet and cartridge case evidence may be exposed to the elements for days,  weeks, or months until they are found and recovered, and bullet evidence  may be irretrievable from shooting victims because rapid extraction could  be harmful to their health. See Larrison (2006) for a study of the rate of  degradation  of  bullets  and  cartridges—monitored  every  6  months  over  2  years—in  four  different  and  demanding  environmental  conditions:  soil,  water, open air, and an animal carcass.  3–D.2 Countermeasures Smith’s (1971) listing of “jokers,” which introduced this section, includes  some deliberate countermeasures that might be taken by criminals—perhaps  after firing a lethal shot—to try to mislead investigators. Aside from Smith’s  rough ordering of situations by frequency and the rough impressions formed  by examiners over their years of experience, there are no firm and system- atic data on the frequency with which criminals deliberately alter firearms  parts (or attempt to obliterate serial numbers) in order to avoid detection. Some countermeasures, like crude filing of the barrel, are relatively easy  to perform (if not always effective).14 As Smith (1971) articulates in fleshing  out his list of problematic situations, some countermeasures are particularly  rare because they are inherently more difficult to perform. The barrel of a  revolver, for instance, is a more integrated part of the firearm assembly than  the more modular barrel of a semiautomatic, so swapping out a revolver  barrel requires special tools and considerable knowledge. With  expert  knowledge,  the  range  of  countermeasures  that  might  be  taken is immense. For instance, Schecter et al. (1996:97) concluded that it  was possible to replace the entire breech block of a SIG Sauer P226 9mm  Luger  pistol,  so  that  “only  the  ejector  will  remain  to  give  possible  indi- vidual marks on the cartridge case base area from the ‘original’ weapon.”  However, the registration of “good ejector and chamber marks on weapons  14  f course, it is possible for quick measures to be very effective at changing toolmarks. At  O the committee’s December 2004 meeting, firearms examiner Lucien Haag shared an evocative  presentation entitled “‘New’ Bores and Breechfaces in 60 Seconds,” which suggests two quick  ways in which a knowledgeable user could rapidly alter the striations left by the barrel or the  breech face markings left on cartridge casings.

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9 FIREARMS IDENTIFICATION of this quality can be poor to non-existent,” and this technique would not  be immediately obvious on inspection of the weapon. But the issue is how  often people who use guns in crime would go to such lengths, akin to the  proverbial question of why more criminals do not wear gloves while com- mitting crimes (given the ease of leaving fingerprints).  Smith (1971:18) argued that replacing pistol barrels “may happen not  infrequently;” he personally reported having seen two such cases in criminal  cases.15 He, however, “never personally encountered [a case of replacing a  firing pin in an automatic pistol], and it is obvious that such replacement  of  other  parts,  (extractor  and  ejector),  refiling  of  breech  face  exists,  still  the possibility of the replacement of this part alone must be kept in mind”  (Smith, 1971:19). Refiling the breech face is problematic, but “much refil- ing would leave the breech surface bright & fresh in appearance”; for full  concealment,  “it  would  be  much  simpler  to  discard  the  arm  entirely  and  replace it with another, rather than go to the effort” to make it appear that  an arm with a refiled breech face had not been altered (Smith, 1971:19).  It should be noted that while some countermeasures may successfully  alter the “signature” markings left on ballistics evidence, they also serve to  create new sets of markings—and, in some instances, prominent signs of the  alteration—that can be used to match to evidence from later cases. Konior  (2006) describes a case in which three cartridge casings and a pistol were  recovered in investigating a homicide. The evidence casings did not appear  to match test firings from the firearm; the latter all showed much greater  levels  of  flowback  around  the  firing  pin  impression.  Subsequent  analysis  determined that an attempt had been made to alter the firearm by drilling  the entire length of the pistol’s barrel; however, the person attempting the  modification  apparently  inserted  the  drill  bit  into  the  firing  pin  aperture,  widening the hole but also damaging the tip of the firing pin. The flowback  and a new distinctive mark in the firing pin impression could be attributed  to this attempted modification. 3–D.3 “Settle-In” Effect From the standpoint of establishing an RBID, a phenomenon of extreme  interest  is  what  might  be  called  a  “settle-in”  or  “breaking-in”  effect:  the  notion  that  it  takes  several  firings  for  a  firearm’s  unique,  characteristic  markings  to  stabilize.  When  the  settle-in  effect  has  been  documented,  it  15  n this context, Smith (1971:18) notes that the interchangeability of barrels was made vivid  I “in the famous Sacco-Vanzetti case.” Specifically, “the barrel of the fatal arm was, following  a demonstration before the court by an expert for the defense, found present in one of the  two other arms of the same type which had been introduced as exhibits, all three having been  disassembled and reassembled in the course of the demonstration.”

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0 BALLISTIC IMAGING has been almost exclusively in the context of the striations left on bullets  and not on markings left on cartridge cases. This—in addition to the time- intensive  process  of  firing  into  a  water  tank  or  other  nondestructive  trap  and retrieving individual bullets—is a major reason that the restriction of  RBIDs to cartridge case entries is sensible. Murdock  (1981:90)  analyzed  10  test-fired  bullets  from  each  of  three  new and consecutively rifled barrels. He found that, “in general, the com- parison between the first, second, and third firing from any one barrel failed  to result in an identification. Some good agreement was present, however.  The third, fourth, and fifth test firings from any one barrel could, however,  generally be identified as having been fired through the same barrel.” From  this,  “it  became  obvious  that  each  barrel  needed  to  have  a  few  bullets  fired  through  it  before  it  began  marking  in  a  reproducible,  identifiable  m   anner.”  Similar  work  by  Hall  (1983:43)  using  a  set  of  consecutively  reamed polygonal barrels noted “rapid change during the first few shots,”  so that firings 1 and 2 from the same barrel looked much different com- pared to each other than did firings 19 and 20. Hall found a “lack of iden- tifications where bullets from any of the first five shots were compared to  any others.” Matty (1985:65) noted similar behavior but was not as precise  in  suggesting  the  number  of  firings  after  which  individual  characteristics  were discernible; instead, he observed that “it was not possible to make a  conclusive match between test bullets #1 and #2 [from the same barrel, but]  bullets #9 and #10 . . . were very similar in appearance.” From these and other works, Bonfanti and De Kinder (1999a:5) con- cluded that “about five to nine shots” are needed “so that [barrels] transfer  a signature to a bullet.” They note that the studies of consecutively manu- factured  barrels  that  they  summarize  in  their  review  meet  this  standard.  However, at least one of those studies—work by Lutz (1970:25) using two  new and consecutively manufactured .38 Special Smith & Wesson barrels— began  with  the  barrels  in  a  completely  unfired  state  and  downplays  any  settle-in effect. Indeed, Lutz described the completely unfired nature of the  barrels as the key advantage of the study, as it “offer[s] the ultimate as far  as similarities between two successive barrels. It is certain that the similar  markings visible now would become less prominent after being subjected  to normal firing, cleaning, and wear.” Even though all the test firings Lutz  examined were among the first 12 uses of the barrel, “sufficient matching  individual striae were noted on the bullets to enable the examiners to eas- ily identify the barrel of origin for each of the bullets. Similarities in class  characteristics were noted; however, microscopic comparison of the bullets  revealed that each barrel had caused different markings to such an extent  that  each  land  and  groove  impression  on  each  of  the  bullets  had  a  great  number of individual identifying striae.” Though a prominent settle-in effect for the breech face or firing pin—

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 FIREARMS IDENTIFICATION the  prominent  markings  on  cartridge  cases—has  not  been  documented,  there may be structural or design features that may cause a similar effect.  The Thompson (1996) investigation of highly similar marks resulting from  different Lorcin L9 9mm pistols (see Section 2–D.1) included some firings  from guns right off the manufacturing line, with no previous proof firing.  It was noted that the “breech face impression . . . can change considerably  shot-to-shot [in early firings] due to the paint wearing/chipping off.”  3–E COMMENTARy As  detailed  in  Chapter  1,  it  is  not  the  function  of  this  committee  to  assess  the  general  validity  of  firearms  identification  and  toolmark  exami- nation nor their admissibility in court proceedings. The discussion in this  chapter on the nature of toolmark evidence and the context in which it is  applied, as well as an overview of existing research among firearms exam- iners on the uniqueness and reproducibility of toolmarks,  is  presented to  frame the discussion in the rest of this report. For instance, understanding  situations that may pose particular challenges for associating two images of  evidence requires some knowledge of situations that are generally known  to  be  complex  in  the  field;  likewise,  recommendations  for  the  setup  and  maintenance  of  any  ballistic  imaging  system  that  would  do  harm  to  the  maintenance of clear chain of custody—so important in the legal context  of toolmark evidence—would be ill-advised. However, as we also note in  Chapter 1, we understand that some readers may try to infer a position—a  leaning, one way or the other—based on the preceding analysis.  Accordingly, we believe it important to make the committee’s finding  clear and unambiguous:  Finding: The validity of the fundamental assumptions of uniqueness and reproducibility of firearms-related toolmarks has not yet been fully demonstrated. There is one baseline level of credibility, however, that must be demon- strated lest any discussion of ballistic imaging be rendered moot—namely,  that there is at least some “signal” that may be detected. In other words,  the  creation  of  toolmarks  must  not  be  so  random  and  volatile  that  there  is no reason to believe that any similar and matchable marks exist on two  e   xhibits fired from the same gun. The existing research, and the field’s gen- eral acceptance in legal proceedings for several decades, is more than ade- quate testimony to that baseline level. Beyond that level, we neither endorse  nor  oppose  the  fundamental  assumptions.  Our  review  in  this  chapter  is  not—and  is  not  meant  to  be—a  full  weighing  of  evidence  for  or  against 

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 BALLISTIC IMAGING the assumptions, but it is ample enough to suggest that they are not fully  settled, mechanically or empirically. Another  point  follows  directly:  Additional general research on the uniqueness and reproducibility of firearms-related toolmarks would hae to be done if the basic premises of firearms identification are to be put on a more solid scientific footing. A designed program of experiments covering  a full range of sources of variability is important to test the fundamental  assumptions,  as  well  as  to  better  document  phenomena  like  “settle-in”  effects. In such a program, it could be useful to study the level of agreement  of marks generated by the whole system of parts that make up a firearm,  rather  than  treating  each  mark  type  in  isolation.  For  example,  in  a  large  number of test firings, how comparable is the quality of breech face mark- ing with firing pin impressions, and how do those compare with the clarity  of striations etched on bullets? Fully assessing the assumptions underlying  firearms identification would require careful attention to statistical experi- mental design issues, as well as intensive work on the underlying physics,  engineering,  and  metallurgy  of  firearms,  but  is  essential  to  the  long-term  viability of this type of forensic evidence. A  third  point  is  important  in  reading  this  report—stopping  short  of  commenting on whether firearms toolmark evidence should be admissible:  Conclusions drawn in firearms identification should not be made to imply the presence of a firm statistical basis when none has been demonstrated.  Specifically,  as  described  in  Section  3–B.4,  examiners  tend  to  cast  their  assessments  in  bold  absolutes,  commonly  asserting  that  a  match  can  be  made “to the exclusion of all other firearms in the world.” Such comments  cloak an inherently subjective assessment of a match with an extreme prob- ability statement that has no firm grounding and unrealistically implies an  error rate of zero. Thornton and Peterson (2002:24–25) note the basic flaw  in this reasoning:  Since the basis of all forensic identification is probability theory, examiners  can never really assert a conclusion of an “identification to the exclusion  of all others in the world,” but at best can only assert a very small (objec- tive or subjective) probability of a coincidental match. . . . It is ironic that  those areas of forensic science that have real underlying data offer more  modest statements of individualization, while those limited to subjective or  impressionistic data make the strongest statements, sometimes of absolute  certainty. As described in Box 3-4, recent court decisions in which admissibility  of firearms toolmark evidence was in question have generally accepted that  the field has validity but have refused to accept “exclusion of all other fire- arms”  arguments.  The  committee  agrees  with  the  basic  point:  statements 

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 FIREARMS IDENTIFICATION BOX 3-4 Recent Court Decisions: “To the Exclusion of All Other Guns” As part of a larger trial on racketeering, assault, and gun charges, Judge Nancy Gertner of the U.S. District Court for Massachusetts rejected a motion to exclude ballistics evidence but strictly limited the scope of the testimony (United States v. Green, 405 F.Supp. 2d 104; 2005 U.S. Dist. LEXIS 34273). Fourteen .380 caliber shell casings were recovered from two sites in September 2000, six of them at the site of a shooting; about a year later, a loaded .38 caliber pistol was found in the front yard of a residence. As part of the trial, prosecutors wanted to introduce testimony from a Boston Police Department sergeant that all of the casings came from the same gun, namely, the recovered pistol. At a preliminary hearing, the sergeant indicated that he could make this determination “to the exclusion of every other firearm in the world.” Gertner’s opinion noted that the “exclusion of every other firearm in the world” claim was “needless to say, extraordinary, particularly given [the sergeant’s] data and methods”: the examiner “took no notes, recorded no measurements, made no photographs, and drew no diagrams.” After reviewing toolmark issues in some detail, the court concluded: [The examiner in question] is a seasoned observer of firearms and tool- marks; he may be able to identify marks that a lay observer would not. But while I will allow [him] to testify as to his observations, I will not allow him to conclude that the match he found by dint of the specific methodology he used permits “the exclusion of all other guns” as the source of the shell casings. Separate rulings in United States v. Monteiro—also from the U.S. District Court for the District of Massachusetts, and coming before and after the Green ruling—adopted similar stances on testimony “to the exclusion of all other fire- arms.” Defendants in Monteiro challenged the specific examiner (a Massachusetts State Police sergeant) as not being qualified in firearms identification; challenged firearms identification as unreliable under Daubert ; and asserted that—even if fire- arms identification were reliable—that the examiner did not apply the techniques properly. The defense also challenged the validity of any identification because the examiner replaced—“among other parts”—“the firing pin, recoil spring, barrel, and trigger lever (but, significantly, not the breech face)” of the gun in order to get it back into firing condition. Prior to Green, Judge Patti Saris ruled on a motion to exclude ballistics evi- dence (2005 U.S. Dist. LEXIS 39062), allowing testimony in part but giving the government “two weeks to ensure that its proffered firearms testimony comported with the established standards in the field.” As in Green, no notes or photographs were made on the identification: Saris ruled that “until the basis for the identifica- tion is described in such a way that the procedure performed by [the sergeant] is reproducible and verifiable, it is inadmissible under Rule 702.” Saris further continued

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 BALLISTIC IMAGING BOX 3-4 Continued directed that the identification be subjected to independent review and verification in order to be admissible. Judge Saris’ subsequent ruling on the general challenge to firearms identifica- tion (407 F.Supp.2d 351, 2006) concluded that “the underlying scientific principle behind firearm identification—that firearms transfer unique toolmarks to spent cartridge cases—is valid under Daubert. However, the process of deciding that a cartridge case was fired by a particular gun is based primarily on a visual inspec- tion of patterns of toolmarks, and is largely a subjective determination based on experience and expertise.” The court ruled that the government must demonstrate basic standards for the qualification of the examiner. However, like Judge Gertner, Saris precluded—in any event—any testimony of a match to the exclusion of all other guns in the world. Saris noted that “examiners testified to the effect that they could be 100 percent sure of a match,” a statement that could not be sustained. An examiner may “testify to a reasonable degree of ballistic certainty,” but “an expert may not assert any degree of statistical certainty, 100 percent or otherwise, as to a match.” Most recently, in United States v. Diaz (2007 U.S. Dist LEXIS 13152), Judge William Alsup of the U.S. District Court for the Northern District of California accepted that: the theory of firearm identification . . . is reliable under Daubert. While there is some subjectivity involved, it is the subjective judgment of trained professionals with a keen practiced eye for discerning the extent of match- ing patterns. . . . The record, however, does not support the conclusion that identifications can be made to the exclusion of all other firearms in the world. Thus, the examiners who testify in this case may only testify that a match has been made to a ‘reasonable degree of certainty in the ballistics field. In defense of firearms identification, Alsup remarked: It is important to note that—at least according to this record—there has never been a single documented decision in the United States where an incorrect firearms identification was used to convict a defendant. This is not to say that examiners do not make mistakes. The record demonstrates that examiners make mistakes even on proficiency tests. But, in view of the thousands of criminal defendants who have had an incentive to challenge firearms examiners’ conclusions, it is significant that defendants cite no false- positive identification used against a criminal defendant in any American jurisdiction.

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 FIREARMS IDENTIFICATION on  toolmark  matches  (including  legal  testimony)  should  be  supported  by  the work that was done in the laboratory, by the notes and documentation  made  by  examiners,  and  by  proficiency  testing  or  established  error  rates  for individual examiners in the field and in that particular laboratory, but  should not overreach to make extreme probability statements. 3–F ROLE OF IMAgINg IN FIREARMS IDENTIFICATION In  the  next  several  chapters,  we  explore  the  current  state  of  ballistic  imaging technology. As context for this discussion, we note that imaging  and photography have a long and somewhat controversial history in tradi- tional firearms identification.  Moran (2003) summarizes some of the historical debate over the use of  comparison photographs, culling relevant quotations from source materials.  Some  of  the  pioneers  of  the  field  of  firearms  identification—Goddard,  B   urrard, and Hatcher—considered photography to be valuable, if not essen- tial, Burrard going so far as to comment that “any evidence unsupported by  photographs cannot be regarded as being anything more than an expression  of opinion. Photographs are, accordingly, essential; and such as are deemed  necessary  must  be  taken  through  the  microscope”  (Moran,  2003:175).  However, Hatcher sounded a note of concern: “There is a difference in the  ability  of  the  various  experts  to  use  the  microscope  and  camera,  so  that  in the hands of a very skilled operator they may show the correspondence  or  lack  of  correspondence  very  clearly,  while  in  the  hands  of  a  poor  or  mediocre  operator,  they  may  show  the  same  thing  faintly,  or  may  even  fail to show them at all” (Moran, 2003:176). However, with the passage  of time, the practice of using photographs to document identifications fell  out of favor, so much so that a 1957 revision of Hatcher’s 1935 text now  stipulated that “photo micrographs are now rarely used,” for a variety of  reasons (Moran, 2003:177): •  Courts  tended  to  accept  examiners’  testimony  on  identifications  without the “visual proof,” obviating the need to prepare the photographs. •  Preparation of the photographs took time, time that laboratories  were unwilling to commit due to increased caseloads. •  Static  views  of  an  evidence  match  were  deemed  unsatisfactory,  relative  to  the  full  range  of  panning  and  rotation  possible  during  direct  manipulation of the evidence. •  “These pictures were not understood by juries,” and “a good deal  of knowledge and experience are necessary to evaluate them.” •  “Some men after years of working in Firearms Identification refuse  to make a positive identification from pictures alone.”

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 BALLISTIC IMAGING •  Photographs  depict  striations  and  features  that  do  not  match  as  well as those that do, which could create doubt among jurors. In subsequent decades, the use of photographs as part of documenta- tion remained a matter of personal and agency tastes, with some favoring  it and others opposing it: for example, Heard (1997:113) stated that “the  use  of  comparison  photomicrographs  in  a  court  of  law  to  illustrate  stria  comparisons should be discouraged” (although he suggested that a video  might  be  more  informative).  Ultimately,  in  2005,  the  AFTE  membership  approved  a  “standardization  of  comparison  documentation”  stipulating  that “photography is the preferred method of documentation” but noting  that  other  forms  of  documentation  (including  “narrative  descriptions”)  “may  serve  to  satisfy  the  requirements  of  this  standard.”  This  standard,  revised as of June 13, 2005, was promulgated in a 2006 issue of the AFTE Journal (Vol. 38, No. 1). Prior to the use of automated ballistic imaging, law enforcement agen- cies  such  as  the  Los  Angeles  Police  Department  (LAPD)  relied  on  cruder  means—and a certain amount of luck—to make connections with evidence  in their open case files. Quite literally, making those connections depended  on reference to Polaroid photomicrographs, posted on a bulletin board or  shared with colleagues, in order to jog memories and generate possibilities.  As of 1994, the process used by the LAPD was to post Polaroid images on  two walls in “a hallway that we pass through constantly. On one side are the  ‘unknowns,’ like cartridge cases collected as evidence at a homicide scene.  On the other side are ‘knowns.’ When we recovered a gun from a suspect or  a crime scene, we test-fire it in the lab—that makes it a known. . . . When  we get a new known, we compare it to all the unknowns. When we get a  new unknown, we compare it to all the knowns and all the unknowns,”  and the pictures are added to the hallway for future reference (Maruoka,  1994b:214).16  The  next  chapter  begins  to  describe  efforts  to  go  beyond  Polaroids in the hallway: to use computer imaging to make it possible to  more readily draw connections with exhibits in other open cases.17 16  he Maruoka article is, in fact, a reprint in the AFTE T Journal of a profile written for the  Polaroid Corporation’s Instant Eidence newsletter for law enforcement officials, circulated  in  1993.  As  such,  it  touts  the  specific  Polaroid  equipment  and  film  used  for  the  standard  imaging. 17  s an example of the way a large-scale search of exhibits is conducted using conventional  A methods,  without  imaging  assistance,  Grooß  (1995:29,  30)  describes  a  series  of  three  mur- ders in West Germany in 1984–1985. Cartridge cases from the second and third crimes were  easily matched by examiners but the cases from the first crime bore only limited similarity in  markings to the others. Still, the headstamp information on the casings from all three crimes,  and the bullet evidence, strongly suggested that a member of the police might be the killer. So  strong was this suspicion to investigators that all 7,862 Walther P5 duty pistols then in use by  the police forces throughout the state of Baden-Württemberg were test fired and the cartridges 

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 FIREARMS IDENTIFICATION compared with the crime scene exhibits. “As the comparison work in this specific case [was  done] using conventional microscopes,” it “was very complicated and time-consuming”; up to  3 firearms examiners at a time worked continuously on the comparisons. Ultimately, test-fired  casings from the 3,704th pistol in sequence matched the casings from the second and third  murders;  this  led  investigators  to  an  officer  in  Stuttgart  who  was  ultimately  found  dead  in  southern Italy, having also murdered his wife and sons. Grooß concluded that the experience  was strong testament to “the individuality of marks on fired bullets and cartridge cases”: that  the German examiners were able to observe “marks . . . which left no doubt that they were  identical  to  those  observed  on  the  evidence  ammunition,”  against  a  backdrop  of  “approxi- mately 4000 pistols of the same manufacturer, same model, approximately the same age and  same degree of wear.” Apparently, the comparisons with test-fired exhibits were halted with  the positive result on the 3,704th pistol, even though the linkage to the casings from the first  murder was unclear.  Even  when  the  officer’s  pistol  was  recovered  and  test  fired  with  a  variety  of  ammunition  brands, no casing could be generated that matched the evidence from the first murder. It was  originally suspected that the pistol “got a new blue finish” (a refurbishing process that may  affect the marks left by the gun) at a time between the first and second murder, which might  explain  the  differences.  However,  “a  more  careful  examination  showed  that  the  pistol  had  been  blued  in  the  period  between  the  second  and  third  murder”  (why  this  did  not  impede  the  ability  to  match  the  second  and  third  crimes  is  left  unspecified).  It  was  also  speculated  that the officer might have deliberately planted a different casing at the first crime scene to  mislead detectives.

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