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FIRED CARTRIDGE CASE EJECTION PATTERNS FROM
Movement of the firearm during firing created a very different pattern of spent cartridge casings. Again restricting the study of the distribution pattern to the cartridges ejected from the Glock 17, we found the distribution as shown in Figure 10. This figure represents test eight and illustrates spent cartridge casings ejected from a firearm held with a two-handed grip, pointed downwards 22 degrees, and the shooter going through a 45 degree body rotation with his/her weapon. Spent cartridge casings landed in all but one 30-degree section. Nearly 30 % landed to the left and rear of the shooter. This was very different from test one with the same semi- automatic weapon. From this test, weapon motion is evident to have contributed to the location of spent cartridge casings as the reader can see from comparing Figure 8 with Figure 10.
The spent cartridge casing locations shown in Figure 10 are found in a 360-degree circle about the shooter illustrating the impact of firearm motion on the exact location of spent cartridge casings. Variability within quadrants is also impacted by firearm motion as the reader can see in the distances traveled by the spent cartridge casings; 7.21 inches (18.31 cm) to 143.13 inches (363.55 cm) away from the zero point. This gave an average distance of 44.43 inches (112.85 cm) from the shooter or a difference of over 8 feet (2.44 m) from the maximum distance. As noted elsewhere the pilot studies where the shooters were able to hold the firearm in the test position and shoot at a target while they were moving rapidly produced a much greater scatter of spent cartridge casing to the left of the shooter than was obtained here.
Ammunition type is yet another variable said to affect spent cartridge casing patterns (Hueske, 2006; Haag, 2006). In this experiment both the Glock 23 and Sig Sauer 229 used 40- caliber ammunition from the same Federal lot. In these tests when the ammunition is held constant but the firearm is altered and later when the manipulation of the firearm is altered, very significant differences were found among the spent cartridge casings. Therefore, using two different weapons (Glock 23 and Sig Sauer 229) both containing the same ammunition from the same Federal lot, the reader can see variability still exists. Even when firearm type, firearm position, firearm movement, grip, and ammunition type are accounted for, significant variability still existed in where spent cartridge casings landed during the study. This is illustrated in Figures 12 - 13. These figures show that most often the spent cartridge casings land to the right and rear of the shooter with both firearms but there is still a very significant dispersal within the right rear quadrant. In Figures 14 - 15 where a specific movement pattern was added, we still found significant variability in the placement of the spent cartridge casing. The scatter plots which follow show the difference on the variability in spent cartridge casing locations even within the same quadrant when ammunition is held constant.
The scatter plots below (Figures 16-19) of the same semi-automatic firearms and test positions as shown in Figures 12 – 15 above, show in detail the dispersion of the spent cartridge casings. The Glock 23 scatter plots show a vertical tendency of spent cartridge casing locations, while the Sig 229 scatter plot indicates a more horizontal trending of spent cartridge casings. In both cases, however, significant variabilty and uncertainty existed about the location of where a spent cartridge casing would come to rest. This again emphasized the imprecision of identifying shooter location based solely on the location of a spent cartridge casing.
Not all of the variability in this test can be attributed solely to the ammunition variable. It is thought that even with all of the human factors held constant as conducted in this study, each person will still hold or fire the firearm in his/her own idiosyncratic fashion. Further, there may be variables in the firearm itself, such as the placement and sequence of the bullet in the magazine, that contribute to differences. The only way to really determine the effect of ammunition on spent cartridge casing location is to control for all of the firearm variables and most importantly eliminate all of the human variables by not having humans fire the firearm. As previously noted, ammunition manufacturers inform us there is still a 3% variability when all of these ammunition factors are accounted for. The effects of this variability upon cartridge case ejection is not known but may also contribute to the uncertainty involved in attempting to make determinations from the ejection pattern of a particular handgun and ammunition.
As this study has shown, factors previously listed including firearm design, firearm condition, ammunition type, position firearm is held when fired, movement of the firearm and person during firing, and grip factors such as how, where and how tightly the firearm is held during firing can affect the locations of spent cartridge casings (Hueske, 2006). This study illustrated that even when accounting for the above factors, significant variability occurred in the landing locations of spent cartridge casings. This variability must be considered before efforts are made to establish the location of a shooter based solely on the location of even an undisturbed spent cartridge casing or a group of cartridge casings.
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