The National Institute of Standards and Technology (NIST) has developed the NIST Ballistics Identification System (NBIS) based on three-dimensional (3D) topography measurements on correlation cells . The Congruent Matching Cells (CMC) method is used for ballistics identifications using three identification parameters of the paired correlation cells including the cross correlation function maximum CCFmax, registration angle θ and x-y registration position. The number of congruent matching cell pairs C is determined by the three identification parameters associated with their thresholds . The numerical identification criterion is suggested as C ≥ 6, by which the correlated topography pair is considered as a Match [1, 2]. In order to test the CMC method and to verify the proposed numerical identification criterion, 40 cartridge cases fired from handguns with 10 consecutively manufactured pistol slides are measured by a 3D confocal microscope and correlated by the CMC method. The breech face topographies are divided into cell arrays (7 × 7 or 6 × 6) for correlation. There are a total of 780 correlations including 63 known-matchings (KM) and 717 known-non-matchings (KNM). Initial tests support both the proposed CMC method and the numerical identification criterion C ≥ 6 for ballistics identifications. Test results show a significant separation between the KM and KNM distributions without any false positive or false negative identification. That represents the highest identification accuracy for the same set of cartridge cases that have been tested at NIST thus far. The identification accuracy can be further improved by optimization of the cell numbers and the thresholds of the identification parameters.
Citation: Journal of the Association of Firearms and Toolmarks Examiners
Pub Type: Journals
Ballistics identification, cartridge case, correlation cell, congruent matching cells (CMC)