AFTE Store - Comparison and Statistical Analysis of Land Impressions from Consecutively Rifled Barrels

The validity and reliability of firearm and toolmark analysis has been debated, often revolving around the subjectivity of the methods examiners use. This study attempts to confirm an examiner’s conclusions through objective computer analysis. Bullets, knowns and unknowns, fired through ten consecutively rifled barrels were used for the study. Unknown bullets were identified to the barrels from which they had been fired using traditional comparison techniques. Each land impression (LI) was photographed, and the distances of the prominent striae to one shoulder of the (LI) were measured using computer software. Two methods of selecting measurable striae were used. The data from these measurements was then converted into a barcode representative of the LI from which it was taken. Barcodes were subjected to Principle Component Analysis (PCA), and a Support Vector Machine (SVM) was employed to evaluate the computer’s ability to correctly identify which LI was represented by the barcode, based on SVM analysis error rate (ideal error rate =5%). Optimal error rate varied based on selection technique, with 19.444% and 1.149% being the optimal values obtained by each method. The second result, generated by the majority of bullets analyzed, indicated the computer was able to adequately group barcodes according to their common origins, supporting the examiner’s identifications. This research and described methodology may provide support for the reliability of firearm and toolmark analysis.