| Summary: | 碩士 === 中央警察大學 === 鑑識科學研究所 === 107 === Gun-related criminal incidents often severely jeopardize the public security, so they are deeply valued by the law enforcement departments. According to the article 4 of the "The Control Act of Firearms, Ammunition, and Knives" in our country, guns that possess wounding potential shall be controlled. And the Secretary General of Judicial Yuan defines the " wounding potential criteria" of the firearms as： At the most powerful and appropriate distance, the kinetic energy possessed by the discharged projectile that is capable of perforating the human skin. In the judicial practice, the wounding potential of firearms and cartridges must be evaluated before the verdict was made. Test firing and muzzle energy determination is a common and direct method for the evaluation of wounding potential. However, sometimes we can only find the spent cartridge cases in the crime scene, and there are no firearms and cartridges available for firing tests. It is difficult to identify whether the gun involved in the shooting incident possesses wounding potential or not only basing on the observation of the appearance of the spent cartridge cases. We utilize the principles and knowledges concerning the fields of interior ballistics and plastic deformation of metallic materials to design and carry out our experiments. The 9 19 mm caliber cartridges were used to perform firing tests. The deformation features and gunshot residues (GSR) of the spent cartridge cases correlated with the discharge of lethal firearms were examined and analyzed, respectively. Hopefully, the experimental results would be applicable to the evaluation of the wounding potential of the fired cartridges.
A Glock 17 pistol was used to discharge the 9 mm cartridges made by the 205 Arsenal (205A) in Taiwan. And a Beretta 92 FS pistol was employed to fire the 9 mm cartridges purchased by the National Police Agency (NPA) in 2017. The diameters of each cartridge case were measured at the rim and at 16 positions of the body at 1 mm intervals before and after firing tests. With the exception of the rim diameter, each diameter measured from the head to the mouth of the case was numbered from 1 to 16. A total of seventeen diameters corresponding to the varied positions were obtained for each cartridge case. The inorganic and organic gunshot residues collected in the spent cartridge cases were analysed using scanning electron microscopy/energy dispersive X-ray spectrometry (SEM/ESD) and micro Fourier transform infrared spectroscopy (micro-FTIR), respectively.
Firstly, the seventeen diameters of the tested cartridges were measured before and after removing the bullets from the cartridge cases using an inertia bullet puller. And the diameters measured before and after removing the bullets were statistically compared using a Student’s t test at a significant level of 0.01. The results indicated that only a few diameters measured near the case mouth corresponding to the seating depth of the bullets were significantly changed after the removement of the bullets.
Secondly, 30 rounds of cartridges of each brand of ammunition were test fired. The seventeen diameters of each cartridge case were measured before and after shooting. The mean and standard deviation of each diameter obtained from each position of 30 tested cartridge cases were calculated. And the diameters obtained before and after shooting were compared with each other using a Student’s t test at a significant level of 0.01. With the exception of the rim and position 1, the diameters of all positions of the spent cartridge cases were significantly expanded after shooting for the 205A’s ammunition. The maximum expansion of the cartridge cases happened at position 11 where the diameter was expanded by 1.5%. For the NPA’s ammunition, the diameters of the postions from 3 to 13 were significantly expanded after shooting. The maximum expansion of the case was observed at position 10 where the diameter was expanded by 1.43%.
Cartridges with reduced amount of propellant powder were used for the third set of firing tests. The amount of propellant powder for each round of cartridge was reduced to either 0.29 g or 0.20 g. For each brand of ammunition loaded with different amount of propellant powder, 15 rounds of cartridges were test fired. The seventeen diameters of each cartridge case were measured before and after shooting. The muzzle velocity of each firing test was measured using a ballistic chronograph. The means of diameters for each set of 15 tested cartridges obtained before and after shooting were compared with each other using a Student’s t test at a significant level of 0.01. For the 205A’s ammunition loaded with 0.29 g of propellant powder, with the exception of the rim diameter, the diameters of all positions of spent cartridge cases were significantly expanded after shooting. Among them, the diameter of position 11 has the greatest expansion rate of 1.59%. For the 205A’s ammunition loaded with 0.20 g of propellant powder, the diameters of positions from 5 to 15 were significantly increaseed after shooting. The expansion rate of the diameter at position 12, which is 1.5%, is the greatest one. For the NPA’s ammunition loaded with 0.29 g of propellant powder, the diameters of position 2 and positions from 5 tom15 were significantly expanded after shooting. The expansion rate of the diameter at position 9, which is 1.27%, is the greatest one. For the NPA’s ammunition loaded with 0.20 g of propellant powder, only the diameter of position 16 was significantly changed after shooting. The expansion of the diameters at the other positions was not significant after shooting. In general, significant increase of diameters after shooting was observed at more postions on cartridges loaded with 0.29 g of propellant powder than those loaded with 0.20 g of propellant powder. The expansion rates of diameters were also greater for the cartridges with more propellant powder than those with less propellant powder.
Besides, because the muzzle velocities measured from the discharge of NPA’s ammunition varied dramatically, the distribution of the muzzle velocities can be divided into 5 groups. The distribution of the muzzle velocities for the slowest group is between 131 m/s and 146 m/s. The results of a Student’s t test comparing the diameters of the cartridges measured before and after the shooting at a significant level of 0.01 for the slowest group were not significant. The distribution of muzzle energy density of the slowest group is between 107.9 and 134.0 J/cm2 which is much higher than the wounding potential criteria, i.e. 20.0 J/cm2, specified in Taiwan. The results demonstrate that if the diameters of a spent cartridge case were significantly expanded, the kinetic energy density of the discharged bullet would be much greater than 20.0 J/cm2.
Gunshot residue (GSR) samples were collected in the 205A’s spent cartridge cases and analyzed for inorganic and organic GSR using SEM/EDS and micro-FTIR, respectively. The results show that spherical micro particles with characteristic elemental profiling of lead, antimony and barium were detected in all GSR samples. The characteristic absorption peaks corresponding to nitrocellulose and nitroglycerin were also identified in the IR spectra of all GSR samples. The results reveal that non-corrosive primer mixtures and double-based smokeless powder are used in 205A’s cartridges. Postitive results obtained from GSR analysis can be employed to confirm that the examined spent cartridge cases were originated from shooting incidents rather than from the disassembling of cartridges.
The study found that using guns and cartridges of different brands causing varied expansion rates of the diameters of each position of the cartridge cases after shooting. But the trend of expansion was the same. The expanded positions and the expansion rates of the spent cartridge cases can provide internal ballistic information to determine that the cartridges have been fired in the gun and have produced extremely high chamber pressure to propell bullets out of the barrel with kinetic energy much higher than the wounding potential criteria. Combined with the analysis of GSR samples collected in the spent cartridge cases and the other trace evidence at the crime scene, the wounding potential of spent cartridge cases left at the crime scene can be estimated. The estimation of the wounding potential of the fired firearm and ammunition can provide highly helpful evidence in the trial.
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