Bullet LEA and GEA measurements: Evaluating the suitability of replacing traditional manual measurements with automated measurements.

Land engraved area (LEA) and groove engraved area (GEA) widths on bullets are class characteristics entered in a General Rifling Characteristic (GRC) database search that returns a list of the make, model, and type of a firearm that may have discharged the questioned bullet. LEAs and GEAs have historically been measured using a comparison microscope and stage micrometer (manual measurements). LEA and GEA width measurements are also captured during the acquisition of bullets onto the Integrated Ballistics Identification System (IBIS) (automated measurements). This study aimed to determine if automated IBIS measurements can be used as an alternative to the manual stage micrometer measurement for GRC searches, the main benefit being that LEAs and GEAs can be measured more quickly with IBIS. The study had two phases: the first involved the manual and automated LEA and GEA width measurements of one hundred 9 mm caliber class bullets; the second involved the manual and automated LEA and GEA width measurements of 103 bullets of various handgun and rifle calibers. Statistically significant differences were observed between the manual and automated LEA and GEA measurements in both phases of the study, leading to the application of a binary approach to determine practical difference. To determine if automated and manual measurements would return the same GRC list, a recommended tolerance range of +0.003" was applied to the automated IBIS measurements. If the manual measurement falls within the tolerance range, the same GRC list should be returned. After application of this binary function, only 58% of the Phase 1 samples and 30% of the Phase 2 samples (44% of all samples), fell within the applied tolerance range. Due to the large proportion of instances where the applied tolerance was exceeded, it is not recommended that the automated method be used as an alternative to manual measurements for GRC searches.