Identification of bullets fired from air guns using machine and deep learning methods

Muthu Rama Krishnan Mookiah; Roberto Puch-Solis; Niamh Nic Daeid

doi:10.1016/j.forsciint.2023.111734

Identification of bullets fired from air guns using machine and deep learning methods

Forensic Sci Int. 2023 Aug:349:111734. doi: 10.1016/j.forsciint.2023.111734. Epub 2023 May 19.

Authors

Muthu Rama Krishnan Mookiah¹, Roberto Puch-Solis², Niamh Nic Daeid²

Affiliations

¹ Leverhulme Research Centre for Forensic Science, School of Science and Engineering, University of Dundee, Nethergate Dundee DD1 4HN, Scotland, UK. Electronic address: mrkmookiah@dundee.ac.uk.
² Leverhulme Research Centre for Forensic Science, School of Science and Engineering, University of Dundee, Nethergate Dundee DD1 4HN, Scotland, UK.

PMID: 37267700
DOI: 10.1016/j.forsciint.2023.111734

Abstract

Ballistics (the linkage of bullets and cartridge cases to weapons) is a common type of evidence encountered in criminal cases around the world. The interest lies in determining whether two bullets were fired using the same firearm. This paper proposes an automated method to classify bullets from surface topography and Land Engraved Area (LEA) images of the fired pellets using machine and deep learning methods. The curvature of the surface topography was removed using loess fit and features were extracted using Empirical Mode Decomposition (EMD) followed by various entropy measures. The informative features were identified using minimum Redundancy Maximum Relevance (mRMR), finally the classification was performed using Support Vector Machines (SVM), Decision Tree (DT) and Random Forest (RF) classifiers. The results revealed a good predictive performance. In addition, the deep learning model DenseNet121 was used to classify the LEA images. DenseNet121 provided a higher predictive performance than SVM, DT and RF classifiers. Moreover, the Grad-CAM technique was used to visualise the discriminative regions in the LEA images. These results suggest that the proposed deep learning method can be used to expedite the linkage of projectiles to firearms and assist in ballistic examinations. In this work, the bullets that were compared were air pellets fired from both air rifles and a high velocity air pistol. Air guns were used to collect the data because they were more accessible than other firearms and could be used as a proxy, delivering comparable LEAs. The methods developed here can be used as a proof-of-concept and are easily expandable to bullet and cartridge case identification from any weapon.

Keywords: Air weapons; Automated feature extraction; Deep learning; Empirical mode decomposition; Firearm identification; Image analysis; Machine learning; Surface topography measurements.