When a firearm is fired it leaves marks on the bullet and cartridge case. Examples are striated marks on the bullet imparted by the barrel rifling and impressed marks on the soft primer surface of the cartridge case imparted by the firearm breech face or firing pin. Forensic examiners compare these firearm toolmarks to evaluate whether two bullets or two cartridge cases were fired from the same firearm. Results of this comparison may link a ballistic sample found at a crime scene to a test fire from a suspect’s firearm or to a sample found at another crime scene.
Firearm identification is based on two premises: uniqueness and reproducibility. Uniqueness means that marks left by a firearm have individualizing features that distinguish them from marks left by another firearm. These individualizing features typically originate from random microscopic defects in the working surfaces of the firearm acquired during manufacturing and use. Reproducibility means that the individualizing features are similar for different firings from the same firearm. The degree of uniqueness and reproducibility affects the accuracy of assessments as to whether ballistic samples originated from the same firearm, both for examiner and algorithmic evaluations.
This first-of-its-kind foundational study is an objective, quantitative, evaluation of the degree of uniqueness and reproducibility of impressed marks left by the breech face of a firearm on the primer of fired cartridge cases. For different lateral scales, we assessed the uniqueness of features in the topography of firearm breech face surfaces, measured using casts, and the uniqueness and reproducibility of the features on fired cartridge cases. The three-dimensional (3D) topography of firearm and cartridge case surfaces were measured at extremely high lateral resolutions. The topography images were compared algorithmically, yielding objective similarity scores for known matching and known non-matching samples at different wavelength bands. Representative 9 mm pistols of three manufacturing models were studied, having different parallel or granular features on the breech face. For each brand, we evaluated firings from ten consecutively manufactured slides, which poses a higher challenge for uniqueness. Two ammunition parameters were evaluated: primer material and ammunition pressure.
Results of the study enable advancements in comparison methods and provide immediate benefits to the discipline:
- Guidance to practitioners and instrument manufacturers on requirements for lateral measurement resolution. Results show that, for breech face impressions, investments in higher lateral resolution of the images or more accurate form metrology are unlikely to provide benefits.
- Selection and demonstration of data analysis parameters (band-pass filter cutoffs) for accurate and efficient algorithmic toolmark comparisons.
- Support for error rate estimates with respect to firearm manufacturing method and ammunition characteristics. The observed reproducibility was best for brass-primer, regular-pressure, ammunition.
- Demonstration of a method for examining inoperable or dismantled firearms based on casting of firearm surfaces.
- Data supporting the scientific validity of the discipline.
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