The effect of composition and morphological features on the striation of .22LR ammunition

In this paper, the effect of the chemical composition of .22LR bullets was studied and correlated to the extent and quality of the markings left after shooting them with the same gun. Scanning electron microscopy and optical comparator microscopy equipped with 3D and profilometry modules were used as non destructive techniques with two main purposes. The first is to assist the firearm examiner in the choice of the ammunition most suitable for preparing the test bullets. The second is to propose an approach, crossing data from optical microscopy, profilometry and space-resolved chemical analysis, for explaining the morphology of striae, whether continuous or interrupted. Among the most notable results is the finding that commercial .22LR ammunition show very wide interbrand, interlot and intralot variability. A selection of test ammunition solely based on the same brand and model used on the crime scene is therefore not suitable, urging the need for a more accurate choice, based on a preliminary chemical analysis.