Volume 9, Number 1, March 2017 Experimental Studies of Terminal Performance of Lead-free Pistol Bullets in Ballistic Gelatin using High Speed Video Elijah Courtney1, Amy Courtney, PhD2, Lubov Andrusiv, PhD3, and Michael Courtney, PhD4 Abstract Quantifying terminal performance may provide clues when reconstructing some shooting events, and it also informs potential ammunition selection for duty use. Due to lead hazards, lead-free ammunition is coming into wider use. This paper uses the established technique of studying bullet impacts in ballistic gelatin to characterize the terminal performance of eight commercial lead-free handgun bullets for comparison with earlier analysis of jacketed lead bullets. Peak crushing force of the bullet and energy loss in calibrated ballistic gelatin are quantified using high speed video. The temporary stretch cavities and permanent wound cavities are also characterized. Keywords: lead-free; pistol bullets; incapacitation probability; wound channel; crushing force; ballistic gelatin Introduction Changes in pistol ammunition have important implications for police investigators and medical examiners, because the wound channel is often important in reconstructing shooting events (McCormick et al., 1996; Haag and Haag, 2011; Coupland et al., 2011). Increasing use of advanced imaging techniques as a part of modern autopsies and in diagnosis of surviving gunshot victims allows documenting of wound channels throughout the internal bullet path and not simply entrance and exit wounds (Leth, 2007; Persson et al., 2011; Levy et al., 2006). Comparing documented wounding from medical imaging and forensic examinations with wounding potential documented by shooting known bullets into tissue simulants allows investigators to make some inferences about which bullets may have been more or less likely to cause certain wounds (Buck et al., 2013; Harcke et al., 2007). 1 BTG Research, 9574 Simon Lebleu Road, Lake Charles, LA, 70607 [email protected] 2 Exponent, Inc., 3440 Market Street, Philadelphia, PA, 19104 [email protected] 3 United States Air Force Academy, 2354 Fairchild Drive, USAF Academy, CO 80840 [email protected] 4 BTG Research, 9574 Simon Lebleu Road, Lake Charles, LA, 70607 [email protected] www.InvestigativeSciencesJournal.org Vol.9, No.1, March 2017 Most pistol and rifle ammunition contains lead in the bullet itself and in the primer (in the form of lead styphnate). The development of current ammunition was influenced not only by cost and ease of manufacture, but also by the good and reliable performance of lead-based components. Over the past two decades, interest has grown in removing lead from duty and hunting ammunition due to environmental and health concerns. For example, the US Army issued the new M855A1 load with lead-free bullets to troops in Afghanistan. An Air Force study showed that transitioning to training ammunition with lead-free bullets and primers can reduce instructor exposure to lead by 70% in indoor ranges and 41% in outdoor ranges (Cameron, 2006). However, the priority for selection of lead-free bullets for duty use must be that they perform equal to or better than the lead-based bullets they replace. The shift toward lead-free bullets has several implications for law enforcement. Solid copper designs tend to penetrate more, thus being more likely to exit the target and require a search at the scene of a shooting than to be recovered by the attending medical personnel or medical examiner. Many times, bullets that exit the target go unrecovered, and reconstructing shooting events requires making inferences from the wound channel. If the make and model of the ammunition can be narrowed from recovered cartridge cases or other recovered bullets, inferences about which bullet caused which wound may be possible by comparing observed wound channels in the human target with analysis from ballistic gelatin like the kind reported here, especially if investigators are distinguishing between bullets with significantly different wound channels. A second significant difference between lead-free and lead-based bullets in the same cartridge is that lead-free bullets tend to have significantly lower muzzle energies for two reasons. Lead-free bullets are less dense than jacketed lead bullets, so they tend to occupy more volume as the base of the bullet sits deeper in the cartridge case, resulting in a smaller effective powder volume and less cartridge potential at the same standardized peak chamber pressure. Further, solid copper bullets have greater barrel friction than jacketed lead bullets, so additional energy is used to push the bullet through the barrel that would otherwise contribute to muzzle energy. As discussed below, this leads to lower potential for wounding and incapacitation. Two physical mechanisms shown to contribute to the wounding and incapacitation potential of penetrating projectiles are the permanent cavity and the temporary cavity (Courtney and Courtney, 2012). The role of a ballistic pressure wave radiating outward from the projectile has been widely debated (Maiden, 2009). Since the force between bullet and tissue is the only direct interaction, these mechanisms originate with the crushing force between bullet and tissue (Peters, 1990). The permanent cavity is often described as being caused by crushing of tissue by the bullet as it penetrates. The temporary cavity is caused by the crushing force of the bullet accelerating tissue forward and outward from the bullet path until elasticity causes the tissue to spring back into place. The temporary cavity can contribute to incapacitation and wounding when stretching beyond tissue's elastic limit damages tissue and enlarges the permanent cavity. In addition, stretching can also cause nerve damage, and impact of the temporary stretch cavity to the spine can cause injury and hasten incapacitation. 2 www.InvestigativeSciencesJournal.org Vol.9, No.1, March 2017 Calibrated 10% ballistic gelatin has become widely accepted as a homogeneous tissue simulant that adequately reproduces average bullet forces and penetration depths of projectiles in tissue. Consistency has been reported between performance in calibrated 10% ballistic gelatin and autopsy findings (Wolberg, 1991). It is common to use ballistic gelatin to quantify penetration depth, permanent cavity, and temporary cavity volumes. It has recently been demonstrated that high speed video can be used to quantify the crushing force curve between bullet and tissue (Gaylord, 2013). Bo Janzon and colleagues seem to have originated the work of quantifying the crushing force and emphasizing its importance in wound ballistics (Janzon, 1983; Bellamy and Zajtchuk, 1990). The energy lost by the projectile as it crushes tissue can be related to the force on the projectile by the Work-Energy Theorem of physics. Using Newton's third law, the crushing force of the bullet on the target, which is equal to the retarding force of the target on the bullet, can be obtained. The crushing force can also be estimated from Newton's second law, F = ma. High speed video has an increasingly useful role quantifying bullet interactions with tissue simulant (Dean and LaFontaine, 2008; Zhang et al., 2005; Zhang et al., 2007). Previous work has shown that a high-speed camera can record the location of the bullet as a function of time, which is required to quantify the bullet-tissue interaction using the Work-Energy Theorem (Gaylord et al., 2013; Keys et al., 2015). With an appropriate scale in the plane of the gelatin surface, pixel identification can be converted to position, and then the change in position between subsequent frames of the video can be computed. This computation yields velocity of the projectile which decreases with time, from which acceleration can be calculated. The change in kinetic energy can also be computed. In the video, temporary and permanent cavities are also visible. In this study, the wounding potential of eight commercial off-the-shelf lead-free handgun bullets was analyzed using the high-speed video method to determine crushing force vs. penetration depth. The results were compared with earlier work (Gaylord et al., 2013) in four 9mm NATO loads. It is notable that the video method above also allows the bullet energy lost as a function of penetration depth to be computed, when velocity is expressed as a function of distance rather than time. This approach can be used to estimate the volume/diameter of wounded tissue as a function of penetration depth (Janzon, 1983). It can also be used to estimate the probability of incapacitation given a hit, P(I/H), on an enemy combatant (Neades and Prather; 1991, Dean and LaFontaine, 2008). The details of the mathematical model the Ballistics Research Laboratory (BRL) and later the Army Research Laboratory (ARL) have used to estimate conditional incapacitation probabilities, P(I/H), have changed slightly over the years but are consistently based on knowing the energy deposit as a function of penetration depth. The approach in this study employs the energy lost by the bullet in the first 15 cm of penetration, E15, and the incapacitation probability model of Bruchey and Sturdivan (1968). Estimating probability of incapacitation is more likely useful for selecting duty ammunition than 3 www.InvestigativeSciencesJournal.org Vol.9, No.1, March 2017 reconstructing shooting events, but combined with the documented wound channel information, may occasionally be useful if the gunshot victim's response to the hit is well documented. Method The experimental and analysis methods as described in Gaylord et al. (2013) and Keys et al. (2015) were applied to a number of different lead-free bullet loads. The IDT Motion Pro X4 high speed camera has a frame rate of 20,000 frames per second. The authors have verified the frame rate by comparing free flight projectile velocities determined with the video camera to those from an optical chronograph. Uncertainties are dominated by the pixel resolution (512 horizontal pixels in a 34-cm span) rather than by the frame timing. Rounds in 9mm NATO, .357 SIG, and .40 S&W were fired from an appropriately chambered SIG P229 pistol.