A Quantitative Understanding of Uniqueness and Reproducibility of Firearm Toolmark Surfaces

The forensic firearm and toolmark discipline is undergoing seminal changes as it moves towards objective methodologies based on 3D surface topography measurements. A largely unexplored area to stimulate advances in these state-of-the-art methods exists with respect to a comprehensive understanding of the tool and toolmark surfaces themselves. There is no quantitative information in the scientific literature regarding, 1) the range of spatial wavelengths of the individualizing features (uniqueness) found on firearm toolmark surfaces, nor 2) how consistently firearms generate toolmarks on fired cartridge components from test-fire to test-fire (reproducibility). A lack of this foundational knowledge impedes advancements in several ways.

Without a quantitative evaluation of the range of unique spatial wavelengths, practitioners and 3D instrument manufacturers have no authoritative guidance on the required measurement resolution for 3D surface topography toolmark measurements. Arbitrarily measuring at high resolutions involves expensive equipment, can lead to wasted data acquisition and analysis times, and may degrade similarity score and error rate evaluations. On the other hand, discovery of unique sub-micrometer scale toolmarks could enable significant advancements.

Arguably, the most important area in current objective 3D firearm identification research lies in determining error rates. A critical part of this effort requires an understanding of how various sources of uncertainty may influence a given error rate calculation. One source of uncertainty lies in the level of toolmark reproducibility of test-fires. A quantitative understanding of this reproducibility is essential to properly evaluate its contribution to firearm identification error rate uncertainties."