Angle of impact determination from bullet holes in a metal surface Eugene Liscio a, * , 1 , Rabbia Imran b, 1 a i2-3D Forensics, Vaughan, Ontario, L4L 8A4, Canada b Department of Anthropology, University of Toronto, Mississauga, Ontario, L5L 1C6, Canada A R T I C L E I N F O Article history: Received 11 April 2020 Received in revised form 30 August 2020 Accepted 7 September 2020 Available online 13 September 2020 Keywords: Terminal ballistics Bullet path Impact angle Ellipse method Balthazard CloudCompare Ellipser App A B S T R A C T Using the best-fit ellipse method, single bullet impacts in thin sheet metal were assessed to investigate the accuracy of impact angle estimation. When a bullet passes through a metal panel, the yielding nature of metal causes changes to the metal surface and the resultant hole. This deformation of the metal complicates the assessment of single impacts using the ellipse method. Determining the correct impact angle may not be obvious and results in considerable errors between the known and calculated angle. To determine if the calculated angle varies in any particular way to the known angle, impacts were created on metal panels using six different types of 9 mm ammunition and seven angles from 90  to 14  . Impact angles, determined using the ellipse method, were compared with known firing angles and the error pattern assessed. The results show an error pattern with a significant quadratic relationship for three ammunition types, with the error pattern for the remaining three ammunitions not explained by a quadratic formula and requiring further study. Results suggest that single bullet impacts for a given type of ammunition with a quadratic error pattern, can be assessed with accuracy due to a more consistent behavior. This characteristic pattern of error requires further study but is a promising step for determining an accurate impact angle and bullet path from a single impact point in a metal surface. © 2020 Elsevier B.V. All rights reserved. When automobiles are involved in shooting incidents, often the body of the automobile, manufactured from thin rolled steel, is struck and perforated by bullets. Shooter placement and assess- ment of bullet pre-impact path using impact damage is often a valuable piece of information. The estimation of the bullet path can be determined from the shape and dimensions of the bullet hole and involves determining the angle of impact which can be resolved into two angles: the vertical angle (elevation angle, side view angle) and the horizontal angle (azimuth angle, top-down view angle) [1,2]. Previous research has shown that the bullet impact angle can be successfully approximated using the trigonometric relationship between the major and minor axis (the length and width) of the bullet hole, a relationship first noticed by Victor Balthazard to determine the angle of impact of a blood droplet [3–5]. Unlike the predictable fashion in which a blood drop impacts a solid and non-porous surface, production of a bullet hole or impression is less predictable as tearing and destruction of the surface results upon impact [1,3,6]. The variability of the impact shape increases if the target surface is thin sheet metal, as the malleable surface bends upon contact with a projectile, causing deformation of the impact shape [7–9]. There is less information about the bullet path when a single bullet impact point is present at a shooting scene. The purpose of this study is to analyze the error present in the calculation of the impact angle from a single bullet impact in thin sheet metal and observe any discernable pattern or trends using different types of ammunition. If a pattern in error can be discerned, implementation of a correction factor may be used to increase the accuracy of determining the bullet impact angle and pre-impact path from a bullet hole. Characterizing the error present and increasing accuracy has implications on the effective- ness of reconstructing shooting incidents with single bullet impacts in sheet metal. 1. Background 1.1. Bullet impact in thin sheet metal Perforation of any material by a bullet can occur by failure mechanisms such as front or rear petal formation, ductile hole enlargement, plug formation, scabbing and the fragmentation of the metal [7]. In cases where the projectile does not perforate or prior to complete perforation, two additional failure mechanisms, dishing and bulging, can occur [7,8]. For thin sheet metal, a ductile material that undergoes high levels of deformation before reaching failing conditions, it is important to understand specifically petal * Corresponding author. E-mail addresses: eliscio@ai2-3d.com (E. Liscio), rabbia.imran@mail.utoronto.ca (R. Imran). 1 These authors contributed equally. http://dx.doi.org/10.1016/j.forsciint.2020.110504 0379-0738/© 2020 Elsevier B.V. All rights reserved. Forensic Science International 317 (2020) 110504 Contents lists available at ScienceDirect Forensic Science International journal homepage: www.elsevier.com/locat e/f orsciint