My research considers the interaction between post-conviction relief mechanisms in the USA, the application of forensic science in the criminal justice system, and the doctrine of finality. My research is inspired by work as a Fellow at the Arizona Justice Project. The Project considers claims of “manifest injustice” and “actual innocence” by Arizona inmates, and is one of over sixty such projects. To date, there have been over 300 post-conviction DNA evidence exonerations in America, and half of these can be attributed to invalid and/or improper forensic evidence. In recent years, the Project has investigated numerous cases where an inmate has been convicted on the basis of what is now considered to be unreliable forensic science. Any resulting legal claims, however, are rarely successful. My research considers why this is the case.

This blog post shares an extract of my forthcoming paper “Judicial Responses to Challenges to Firearms Identification Evidence: A Need for New Perspectives on Finality.”


Forensic science is a popular feature of the American criminal justice system, with firearms identification evidence being admitted into American courtrooms since the early 1990s. Firearms identification is premised on the notion that a weapon leaves unique tool-marks on the ammunition it fires, and those marks are reproduced each time the weapon is discharged. As such, many firearms examiners believe they can conclude that a particular gun fired a particular bullet to the “exclusion of all other[s] . . . .”  Such claims are termed “individualization.” However, in its 2009 landmark report – Strengthening Forensic Science in the United States: A Path Forward – the National Academy of Sciences raised significant concerns about this discipline’s scientific underpinning. The report coincided with, and in some instances propelled, numerous American courts to discourage individualization testimony and curtail the language used by experts to connect weapons to suspect ammunition. These courts began instructing experts to replace “matches” and “degrees of certainty” with “descriptions of observations” and phrases such as “more likely than not.”

In light of this conservative shift, and increased national initiatives towards improving the use and reliability of forensic science post-2009, it is unsurprising that defendants have continued to challenge firearms identification admitted against them at trial. Appellate courts routinely reject challenges on these grounds, however, with some still allowing experts to use unfettered testimony. The obvious theoretical reason for this pattern is finality. Judges and scholars routinely argue that restricting post-conviction review, and increasing the finality of judgments, benefits society. As Professor Andrew Chongseh Kim describes, the term “finality” is considered to be “shorthand for a collection of interests scholars assume are furthered by any restrictions on review.” These interests include ensuring respect for criminal judgments, conserving state resources, furthering the efficiency and deterrent and educational functions of the criminal law, satisfying the human need for closure, incentivising defence counsel to “get it right first time” and preventing a flood of non-controversial claims from masking the fewer, credible ones.

These latter two “interests” are prominent in post-2009 cases in which courts have rejected challenges to the admissibility of firearms identification evidence. In relation to preventing non-controversial claims flooding the system, courts often conclude the admission of such evidence was “non-prejudicial” in light of other evidence against the defendant. In other words, courts are terming the (legally sound or unsound) admission of firearms identification evidence as non-controversial. However, this rationale arguably overlooks the impact scientific evidence has on jurors, with numerous studies showing ““jurors place special trust in expert[s]” and scientific evidence.” Moreover, studies have shown jurors to rate firearms examiners as among the most honest, competent and influential experts. It also arguably overlooks the complications jurors have in relation to understanding such evidence. In relation to incentivizing defence counsel, the courts emphasise the importance of the adversarial system i.e., defence counsel’s ability to weed out frailties in forensic evidence via cross-examination. However, again, this view arguably overlooks the difficulties lawyers have in resourcing, making and understanding challenges to forensic evidence. As, Professor David L. Faigman states, as consumers of science, lawyers (including judges and extended legal personnel), “have very little understanding of the product they are buying.” It also overlooks the limitations of the adversarial process itself.

By focusing on firearms identification evidence, this article sets out this controversy between finality and the impact of forensic identification evidence. Part I outlines the theory of finality. Part II considers the process of firearms identification, the recent criticism aimed at it, and the initial shift in court approaches to firearms identification evidence. Part III reviews recent judicial responses to challenges to the admission of firearms identification evidence, highlighting the influence and implications of finality on judicial decision-making. It will also expand upon how jurors and lawyers engage with forensic evidence in order to demonstrate how the courts, by favouring finality, are arguably overlooking the difficulties both these groups have in handling such evidence. Part IV concludes that the courts should consider taking new perspectives on these finality interests in such cases, and give more meaningful consideration to the issues that arise when law consumes science in this way.


Bonnie Lanigan, Firearms Identification: The Need for a Critical Approach to, and Possible Guidelines for, The Admissibility Of “Ballistics” Evidence, 17 Suffolk J. Trial & App. Advoc. 54 (2012).

Donald E. Shelton, Twenty-First Century Forensic Science Challenges for Trial Judges in Criminal Cases: Where the “Polybutadiene” Meets the “Bitumen,” 18 Widener L.J. 309 (2009) at 336.

Adina Schwartz, Challenging Firearms and Toolmark Identification — Part One, Champion,  Oct. 2008, at 14.

Committee on Identifying the Needs of the Forensic Science Community, National Research Council of the National Academies (NRC), Strengthening the Forensic Sciences in the US: A Path Forward (The National Academies Press, 2009) at 155.

United States v. Green, 405 F.Supp.2d 104; United States v. Monteiro, 407 F.Supp.2d 351 (2006); United States v. Diaz, 2007 WL 485967 (N.D.Cal.); and United States v. Glynn, 578 F.Supp.2d 567 (2008).

U.S. v Casey 928 F.Supp.2d 397 D. Puerto Rico, 2013.

Andrew Chongseh Kim, Beyond Finality: How Making Criminal Judgements Less Final Can Further the “Interests of Finality”, Utah L. Rev. 561 (2013) at 563.

Paul M. Bator, Finality in Criminal Law and Federal Habeas Corpus for State Prisoners, 76 Harv. L. Rev. 441 (1963); Henry J. Friendly, Is Innocence Irrelevant? Collateral Attack on Criminal Judgments, 38 U. Chi. L. Rev. 142 (1970); Carrie Sperling, When Finality and Innocence Collide, Controversies in Innocence Cases in America, Chapter 8, (Ed. Sarah Lucy Cooper), Ashgate Publishing Ltd, 2014.

Brandon Garrett & Peter Neufeld, Invalid Forensic Science Testimony and Wrongful Convictions, 95 Va. L. Rev. 1 (2009).

Michael Saks & Roselle Wissler, Legal and Psychological Bases of Expert Testimony, Surveys of the Law and of Jurors, Behav. Sci. & L. 361, 445 (1984).

David L. Faigman, Legal Alchemy: The Use and Misuse of Science in the Law 53 (1999).

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Sarah Cooper

Sarah Cooper

Sarah is a Senior Lecturer in Law and Director of Birmingham City University's Mooting Society. She has been a Fellow at the Arizona Justice Project in the United States since 2010.