Review: Determining the Effect of Cartridge Case Coatings on GSR Using Post-Fire Priming Cup Residue

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Emily C. Lennert

 

Category

Chemistry

Keywords

gunshot residue, GSR, priming cup, cartridge, case, scanning electron microscopy, SEM, energy dispersive x-ray spectrometry, EDX, EDS, SEM-EDX, SEM-EDS, lead, Pb, barium, Ba, antimony, Sb, copper, Cu, zinc, Zn, nickel, Ni, aluminum, Al, titanium, Ti, sulfur, S

Article Reviewed

  1. Terry, M.; Fookes, B.; Bridge, C. M. Determining the effect of cartridge case coatings on GSR using post-fire priming cup residue. Forensic Science International. 2017, 276, 51-63.

Disclaimer

The opinions expressed in this review are an interpretation of the research presented in the article. These opinions are those of the summation author and do not necessarily represent the position of the University of Central Florida or of the authors of the original article.

Summary

Gunshot residue (GSR) is the residue that result from the discharge of a firearm. GSR in the form of muzzle discharge, i.e. the plume of residue that originates from the muzzle of the weapon, may settle on the shooter, victim, and surrounding areas. Additional residues remain on the cartridge of the ammunition. GSR may be used to link a suspect to a victim or to a crime scene. Therefore, it is important to understand the composition of GSR and the origin of the residues relative to the ammunition itself.

Modern ammunitions, available to consumers, may be divided into two compositional categories: lead-free and lead-based. This refers to the composition of the primer in the ammunition. Ammunition is comprised of 4 main parts: the projectile, cartridge case, propellant, and primer. When a round of ammunition is fired, the firing pin of the weapon strikes the ammunition primer. When the primer is struck, the anvil and priming cup squeeze together and detonate the shock sensitive priming mixture, which contains either lead-based or lead-free components. After the priming mixture detonates, gasses and flames from the priming mixture detonation travel through the flash hole at the base of the cartridge case and ignite the propellant, most often smokeless powder. The propellant burns and produces high pressure within the cartridge case, which then expels the projectile, i.e. bullet, from the cartridge case and out of the muzzle of the weapon.

In this study, post-fire primer samples were analyzed to study primer composition and the differentiation of lead-free and lead-based primers in ammunitions. Residue particles were physically examined by scanning electron microscopy (SEM) and elemental analysis was conducted via energy dispersive x-ray spectroscopy (EDS). SEM uses a beam of electrons to generate a high-resolution image of the sample. Carbon coating is necessary to allow proper imaging; uncoated samples will not conduct energy and will result in bright white spots due to “charging”. EDS uses the x-rays produced by the electron bombardment to identify elements; each element’s x-rays will have characteristic energies that allow for elemental identification. Determination of the elemental composition of the residues allowed for characterization of the residues, which were then statistically evaluated to determine differences between ammunitions with lead-free and lead-based primers.

To begin, a classification model was developed using ammunitions from 4 manufacturers: Sellier & Bellot, Winchester, Liberty, and Federal. The ammunitions were selected so that one lead-based primer (LBP) ammunition and one lead-free primer (LFP) ammunition from each manufacturer, for a total of 8 ammunitions, could be included. For each ammunition, five cartridges were taken apart to leave only the cartridge and primer assembly, i.e. primed only (PO), while the remainder were left fully intact, i.e. full cartridge (FC). Samples were then fired by an officer of the University of Central Florida Police Department and collected. 9mm ammunitions were used in this study. After firing, samples were returned to the lab, where the primer assembly was removed from the cartridge using a de-priming tool. Once the primer had been removed, the priming cup and anvil were separated and placed on aluminum stubs using colloidal graphite. Samples were then carbon coated and analyzed via SEM-EDS. Particles were manually selected for visual and elemental analysis. Per sample, 5 PO anvils, 5 PO cups, 5 FC anvils, and 5 FC cups were examined. Per cup or anvil, 3 spectra were obtained for a total of 30 spectra per FC or PO ammunition sample.

In visual analysis, PO samples were observed to contain fewer residues than FC samples. The authors cite the likely cause as the lack of back-pressure from the propellant and projectile. In FC samples, the propellant and projectile create a back-pressure, which forces residues to remain in the priming cup assembly rather than exit through the flash hole. Additionally, the authors observed greater amounts of residue on the anvils compared to the primer cup, likely due to the anvil being in the direct path of the priming mixture as it travels to the flash hole. Combined with the issue of the high wall of the priming cup, which prevented x-rays from reaching the detector efficiently, the authors chose to focus on the anvil residues. Images of PO and FC anvils for LBP and LFP ammunitions can be seen in Figure 1 within the study.

EDX spectra were obtained and averaged to produce average spectra for each sample in the model dataset, as seen in Figure 3 within the study. LBP ammunition spectra were observed to contain lead (Pb), barium (Ba), and antimony (Sb), the characteristic metals known as traditional LBP GSR. In addition, cartridge case elements copper (Cu) and zinc (Zn), used to make brass, were observed. Cartridge case elements present in the residues were likely due to liquation of the outer layer of the priming cup during primer detonation. Finally, aluminum, common in frictionators, was observed. LFP ammunition spectra varied, and contained potassium (K), silicon (Si), and titanium (Ti). Cartridge case elements Cu, Zn, and nickel (Ni) were also observed.

Statistical analysis was performed to determine whether LBP and LFP ammunitions would be easily distinguished and to identify the most significant elements in the spectra. Principal component analysis (PCA) was performed and a plot of PC 1 vs PC 2 is given. In the plot, the separation of LBP and LFP is clear, as seen in Figure 4 within the study. This means that the composition of significant variables, i.e. elements, in LBP and LFP is different between the two groups, producing a distinct separation. This supports the idea that LBP and LFP can be readily distinguished by SEM-EDS analysis. Analysis of the PCs shows that, for LBP ammunitions, Pb, Ba, and Sb were the primary sources of differentiation. For LFP ammunitions, Cu, sulfur (S), Ti, K, and tin (Sn) were distinct LFP elements consistent with common toxic-metal replacements.

A statistical model was then created using linear discriminant analysis (LDA) using a 10x repeated, 5-fold cross validation technique, which resulted in 400 samples classified for each of the PO and FC datasets. A combined dataset was also prepared, which resulted in 800 samples. By this test, the model was determined to be between 97.5 and 100% accurate, dependent on the dataset. The results of the model set LDA validation are summarized in Figure 6 within the study.

In addition to the model set, an additional 17 ammunitions were selected for use as a test set: 14 LBP and 3 LFP ammunitions. A test set is used to evaluate the accuracy of a statistical model. These samples were prepared and processed in the same manner as the model set samples. The test set was then applied to the above described LDA models. The results are summarized in Figure 7 within the study. For the FC dataset, testing the external test set against the model, the model was found to be 64.7% accurate overall: 90% accurate in classifying LBP and 28.6% accurate in classifying LFP. For the PO model, the model was 41.1% accurate overall: 83.3% accurate in classifying LBP and 18.2% accurate in classifying LFP. For the combined dataset model, PO and FC, the model was 64.7 accurate overall: 100% accurate in classifying LBP and 33.3% accurate in classifying LFP. While the model did not perform as well on the external test set as with the model data alone, this may be attributed to the small size of the model dataset. It may be possible that, given a larger model dataset from which to form classification models, the classification accuracy of the models will increase. This is supported in that the classification accuracy of LBP ammunitions, of which there were 14 ammunitions, was much higher for each model than for LFP ammunitions, of which there were 3 ammunitions.

Finally, the authors applied this model to 4 muzzle discharge samples. Two samples of a LFP ammunition, PO and FC, and two samples of a LBP ammunition, PO and FC, were shot at a target at point-blank range, and the targets were returned to the lab. The residues were removed from each target using aluminum stubs with carbon adhesive tabs. The samples were then carbon coated and analyzed by SEM-EDS. Each muzzle discharge sample was found to reflect its respective primer composition, overall, suggesting that the research presented in this study may have potential applications in muzzle discharge residues, a common form of GSR.

Scientific Highlights

  • LBP and LFP were distinguished from one another based on SEM-EDS elemental data, which showed differences in elemental composition between LBP and LFP.
  • While LBPs were mostly comprised of Pb, Ba, and Sb, LFP compositions varied and included Ti, K, and Si.
  • The creation of a classification model supported the separation of LBP and LFP ammunitions. Additionally, the model accuracy was higher in larger sample populations, indicating that the model may become more accurate as the dataset grows.
  • Muzzle discharge samples corresponded with primer data.

Relevance

GSR is often used, but the source of GSR within the ammunition itself is rarely investigated, This study examined the primer as the source of GSR, and investigated the distinction between LBP and LFP ammunitions.

Potential Conclusions

  • LBP and LFP are readily distinguished from one another based on elemental composition.
  • Primer composition contributes to muzzle discharge GSR.