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Examination of Gunshot Residue

Appearances of Gunpowder

All gunpowders are designed to burn quickly to produce rapid expansion of gas in a confined space. In an explosion something gets very big very fast. The burning rate of gunpowder can be classified in three categories:

  • Degressive (regressive) burning: gunpowder grains formed in flakes, balls, and sticks have a burning surface area that decreases continuously as the grains are consumed.

  • Neutral burning: gunpowder grains that are single perforated and the burning surface area remains relatively constant.

  • Progressive burning: gunpowder grains that are multiperforated and rosettes that have a burning surface area that increases continuously as the grains are consumed.

Unburned gunpowders can have recognizable shapes, colors, and sizes of grains. (Pun and Gallusser, 2007)

Composition of Gunshot Residue

Firing a weapon produces combustion of both the primer and powder of the cartridge. The residue of the combustion products, called gunshot residue (GSR), can consist of both burned and unburned primer or powder components, combined with additional residues from the surface of the bullet, surface of the cartridge case, and lubricants used on the firearm. Residues can be either inorganic or organic in nature. (Vachon and Martinez, 2019)

Residues most often derive from the primer cap, which typically contains a mixture of components: the shock sensitive explosive lead styphnate, oxidizer barium nitrate, and antimony sulfide fuel. Thus, the most commonly encountered residue metals are lead (Pb), barium (Ba), and antimony (Sb). (Vachon and Martinez, 2019) When all three are present, the findings are characteristic for GSR; when one or two are present, the findings are consistent with GSR. (Brozek-Mucha, 2017) Even a "blank" cartridge consisting of the casing, propellant, and primer cap, but without the bullet, will produce GSR when fired. (Vachon and Martinez, 2019)

Less common elements in GSR include aluminum (Al), sulfur (S), tin (Sn), calcium (Ca), potassium (K), chlorine (Cl), copper (Cu), strontium (Sr), zinc (Zn), titanium (Ti), or silicon (Si). A mercury-fulminant based primer may be found in ammunition manufactured in Eastern Europe, while sinoxid type primers are used in the West. (Blakey et al, 2018) Primer elements may be easier to detect in residues because they do not get as hot as the powder. So-called "lead free" ammunition may contain one or more elements including strontium (Sr), zinc (Zn), titanium (Ti), copper (Cu), antimony (Sb), aluminum (Al), or potassium (K). Both titanium and zinc are commonly used in paints and can be contaminants, but the appearance of particles containing them can be distinguished from gunshot residue by SEM. (Martiny et al, 2008) (Dalby et al, 2010)

The cartridge case, bullet, bullet coating, and metal jacket also contain specific elements that can be detected. Virtually all cartridge cases are made of brass (70% copper and 30% zinc). A few have a nickel coating. Primer cases are of similar composition (Cu-Zn). Bullet cores are most often lead and antimony, with a very few having a ferrous alloy core. Bullet jackets are usually brass (90% copper with 10% zinc), but some are a ferrous alloy and some are aluminum. Some bullet coatings may also contain nickel. (Ravreby, 1982).

Lead free primers were introduced in the 1980's, most specifically for indoor firing ranges, to avoid lead contamination with potential poisoning. The most common primer contains zinc peroxide, powdered titanium, tetrazine, diazodinirophenol, and nitrocellulose. The inorganic residue particles are likely to be zinc, titanium, and copper. Even lead, barium, and antimony may be retrieved when ammunition primers were presumably lead free, because of contamination by previous firing with lead-based primers. (Vachon and Martinez, 2019)

Organic GSR primarily comes from materials derived from the propellant powder and are compounds classified either as explosives or additives based on their chemical composition. These organic residues derive from the so-called "smokeless powder" typically utilized in small arms ammunition. The explosives can include a single-basse of nitrocellulose or a dual bse with both nitrocellulose and nitroglycerine. Additives include stabilisers, plasticisers, deterrents, coolants, flash inhibitors and other components intended to improve the performance of the powder. Stabilizers most often detected include diphenylamine (DPA), ethyl centralite (EC) and/or methyl centralite (MC). Organic GSR can be detected on the hands up to several hours after discharge despite losses due to evaporation and skin permeation. However, skin oils and lotions can interfere with organic GSR detection. The complexities, time, and cost of methods for organic GSR detection generally preclude routine usage. (Taudte et al, 2016) (Vachon and Martinez, 2019)

GSR can be used to detect a fired cartridge. Gunshot residue may be found on the skin or clothing of the person who fired the gun, on an entrance wound of a victim, or on other target materials at the scene. The discharge of a firearm, particularly a revolver, can deposit residues even to persons at close proximity, so interpretations as to who fired the weapon should be made with caution. (Dalby et al, 2010)

In the physical examination of the scene or body for evidence of gunshot residue, it must be remembered that lead residues may mimic gunshot residue. Lead residues may be found up to 30 feet from the muzzle, and are always present on the opposite side of a penetrated target. Such a situation has been reported when an intermediate target (glass) was present. (Messler and Armstrong, 1978)

Patterns of Gunshot Residue

Gunshot residue (GSR) may be deposited by two mechanisms: (1) impact deposition from particles propelled by the force of the blast, and (2) fallout deposition of drifting particles that settle on a surface. Persons close to the blast, specifically the shooter, will likely have GSR from impact. Bystanders are likely to have GSR particles from fallout. Shooters are more likely to have a greater number of particles than bystanders, but not always. Settling of airborne GSR may take up to 10 minutes following firearm discharge. The depostion of GSR particles following initial firearm discharge is primary transfer. However, secondary transfer of these particles to other surfaces can occur from contact with the surfaces or persons on whom the particles have deposited, as with handshaking or contact with clothing. Movement of persons following the shooting, or even scene investigation by forensic scientists, may alter GSR distribution. Further tertiary or even quaternary transfer is possible. Law enforcement personnel may carry particles from prior shooting events. (Blakey et al, 2018)

The amount and pattern of GSR deposited may vary by the gun used to fire the bullet. Most GSR emanates from the ejection port of a semiautomatic pistol. GSR is expelled from the gap between cylinder and frame of a revolver. There is greater particle number with revolvers than with automatic rifles. Particle numbers are greater with nonjacketed bullets, mainly due to an increase in particles composed of lead. A faster burning rate of propellant powder reduces the distance of GSR particles travelled. (Blakey et al, 2018) (Vachon and Martinez, 2019)

GSR may be expelled ahead of the bullet, along with the bullet, and following after the bullet. Though the amount of residue deposited tends to decrease with increasing range of fire, the actual deposits can be highly variable for ranges up to 20 cm.(Brown, Cauchi, et al, 1999) GSR has been reported to be found at distances from 6 to 18 meters forward of the shooter, and up to 6 meters laterally. However, climatic conditions significantly influence recovery rates for GSR. (Dalby et al, 2010) Use of atomic force microscopy (AFM) for detection of particle size in relation to range of fire has been described. (Mou, Lakadwar, and Rabalais, 2008)

Detection of Gunshot Residue

The major methods for detection of primer residues are analytical and qualitative. Analytical methods include neutron activation analysis (NAA) as well as atomic absorption spectrophotometry (AAS) and inductively coupled plasma mass spectroscopy (ICP-MS). Scanning electron microscopy with energy dispersive analysis by x-ray detector (SEM-EDX) and atomic force microscopy (AFM) are used to identify the primer residue qualitatively. An X-ray analyzer can be beamed directly onto the particles visualized with SEM, so that the energy dispersive pattern can be generated, giving the elemental composition of the particles. For these methods, samples must be obtained from the skin surfaces of a victim or from objects at the scene. Delay in obtaining samples, movement of bodies or objects, or washing of the body prior to autopsy will diminish or destroy gunshot residues. (Molina et al, 2007) A rapid loss in numbers of GSR particles occurs from 1 to 3 hours post firearm discharge, though maximum recovery times of 1 to 48 hours have been reported. (Dalby et al, 2010). Detectable GSR particles may be identified 5 days after firearm discharge (Blakey et al, 2018)

SEM-EDX has the advantage of providing visual recognition of GSR particles. The particles are characterized by a molten metal appearance with spherical shapes. Angularity as typifies crystalline structures should be absent from GSR. Particles range from 0.5 to 5 micrometers in size.(Vachon and Martinez, 2019)

The method of collection for residue is quite simple and easily carried out in the field. The two widely used methods incude collection onto a carbon-coated adhesive stub or with an alcohol swab. Of the two, the stub has fewer false negatives from greater collection efficiency. The swab method may have usefulness when the surface to be tested is smooth, or if propellant analysis is required. The stub can be directly applied to the surface (skin or other material) to be tested. The stub, with the residue on the surface, can be directly prepared for examination in the SEM device. A major advantage of SEM is that it can reveal the actual surface details of the particles examined, for comparison with known examples of gunshot residue, and pictures can be taken. The large particles of partially burned powder and the spheres of residue can be distinguished from contaminant materials. (Reid et al, 2010)

Scanning Electron Micrograph of GSR

Diagram of the SEM-EDX pattern of GSR

Any hand or body part or object that was close to the fired weapon may have residue appearing consistent with having fired the weapon. GSR released within an enclosed area, such as a vehicle interior or room, may become more readily deposited upon objects within such a space. Soiled, dirty, or moist surfaces impede sampling with detection of GSR. Clothing should always be retained on the body up to autopsy, as this may modify entrance wounds, need examination for gunshot residues, or aid in interpretation of the scene. GSR may even be detected following laundering of cloth. However, proximity does not always correlate with detection. When a suicide occurs via a firearm, the GSR detection rate is only 50%. (Dalby et al, 2010) (Vachon and Martinez, 2019)

The type of weapon can influence the distribution of GSR. For handguns, variables include: barrel length and caliber affecting the plume or cone of gases emitted with their GSR particle; nature of the ejection port of pistols; and barrel-cylinder-frame gap of revolvers. (Ditrich, 2012)

Gunshot residue analysis requires careful evaluation. False positives may be caused by contamination or transfer of GSR to the body by mishandling, or when the body is heavily contaminated by GSR from previous shooting. However, the number of particles from secondary environmental contamination is low. (Berk et al, 2007) False positives from neutron activation analysis or from atomic absorption spectroscopy assays can be avoided with SEM because of the ability to identify the morphology of particles. False negatives result from washing of the hands (when this area is sampled) or by victim wearing gloves. A rifle or shotgun may not deposit GSR on hands, bur more likely in the crook of the support arm. (Dalby et al, 2010)

SEM may also have usefulness for examination of bullets, as embedded materials from the target such as bone fragments may aid in reconstruction of the scene (DiMaio VJ et al, 1987). SEM has been used to study tool marks made by the firing pin impressions in the primers of spent cartridges. Such findings could be useful to determine which gun was used to fire the cartridge. Grove et al (1972) found that SEM could reveal clearly all surface detail in the impression and that 50% of shotgun impressions and 75% of rifle impressions could be positively identified on the basis of four or more individual characteristics, given similar class characteristics.

Another technique for analysis of GSR is inductively coupled plasma mass spectrometry (ICP-MS). In a study of wound samples microwave-digested and analyzed using ICP-MS to detect all elements present at measurable levels in GSR, shot versus unshot tissues could be distinguished. Additionally, jacketed and nonjacketed bullet types could also be distinguished. (Udey et al, 2011)

The presence of GSR may vary from entrance to exit wounds, for the entrance wound will usually have more than the exit, or the exit will have none. At close range, macroscopic examination of the entrance wound is in concordance with microscopic appearance of GSR in all cases, but for distant range gross detection of GSR is negative in a third of cases, though microscopically present. A fifth of exit wounds, though lacking grossly detectable GSR, have microscopic evidence of GSR, thus confounding distinction of entrance and exit wounds by microscopy alone. (Perez and Molina, 2012)

Residue is lacking in entrance wounds with airguns (Denton et al, 2006) (Cohle et al, 1987). The alizarin red S stain can be utilized in microscopic tissue sections to determine the presence of barium as part of GSR (Tschirhart, Noguchi, Klatt, 1991).

GSR may be used to estimate distance between the fired weapon and the target. The skin surface of a victim, or an intermediate target such as clothing, may be analyzed by visual inspection for soot or singed fibers. Chemical analysis can be performed. Sodium rhodizonate solution applied to the target may aid identification of lead from a violet or purple color.. A dithiooxiamide (rubeanic acid) solution may detect copper in residue via dark greenish gray to black color. Incompletely burned gunpowder in a residue may contain nitrites detected by application of acetic acid, forming nitrous acid, then combined with alpha-naphthol forming nitrates detected by an orange-red color. Such visual and chemical tests have a "drop off" point when they are negative, but the weapon was fired at the target. For handguns, this drop-off is typically at a maximum 5 feet. Test firing of a weapon may reveal patterns of residue short of drop-off, providing a form of estimate of distance. (Vachon and Martinez, 2019)

It may be difficult to both find and determine the nature of gunshot wounds in a decomposed body. Determination of the range may be particularly difficult. Extreme care should be taken to avoid misinterpretation of the wounds and artefacts.

Other Examinations

Cases have been described in which suicide victims' hands were stained orange-brown from contact with gun barrels following death, presumably from perspiration coupled with a prolonged post-mortem interval of contact. (Norton et al, 1979)

Latent fingerprints may be detectable on cartridges and expended shell casings. Such fingerprints, called latent because they are transferred via a substance on the skin ridges to an object. On a gun, such substances could include cleaning solvents or gun oils. Usually, the substances consist of perspiration mixed with oils from sebaceous glands. Conditions of increased temperature and low humidity decrease the persistence of fingerprints. Brass retains the fingerprints better than nickel-plated materials. (Given, 1976)

Each firearm sold (other than black powder weapons) has a manufacturer's serial number stamped into it which may be used to identify the weapon. Registration of firearms provides a way of tracing gun ownership. However, attempts may be made to obliterate registration numbers by grinding or filing the metal. (Polk and Giessen, 1975)

Gas chromatography has been used to identify gun oils in targets, and was very sensitive, even with stored specimens (Kijewski and Jakel, 1986).

The DNA from cells of assailants can be identified on a firearm or clothing or other objects left at the scene. The cells may be present in blood or body fluids or from epithelium (skin) and left behind on objects recovered upon scene investigation. Sampling for recovery of these biological materials may involve: cutting a portion of material, swabbing a surface, and applying adhesive tape to a surface. Adhesive tape has the advantage of selective sampling to recover epithelial cells while reducing contamination of PCR inhibitors such as dyes in clothing. The technique commonly employed to detect DNA is the polymerase chain reaction (PCR), which detects minute amounts of DNA. (Barash et al, 2010) Short team repeat (STR) PCR-based assays. are the markers of choice for forensic DNA typing for individual identification. (Mulero and Hennessy, 2012)

So-called "backspatter" describes the deposition of blood from the victim onto the shooter at close range. The pattern of backspatter has been utilized to determine who fired the weapon. Spatter marks may provide clues to the sequence of events, point of origin, and direction. (Yen et al, 2003)

The "backspatter" phenomenon can affect the amount of DNA and RNA recovered from surfaces of the firearm following firing at a victim. In one study, DNA was recoverable up to 5 cm from the target with a revolver and up to 15 cm with pistols. RNA was recoverable up to 30 cm, but the recovery was variable and did not correlate with recovery of DNA. Both mRNA and miRNA can be recovered, but the latter is smaller and more stable for greater likelihood of recovery.. (Grabüller et al, 2016)

In a study using high-speed digital video imaging to visualize blood droplets, firearm muzzle gases, and ballistic shock waves with standard reflected light and shadowgraphy imaging techniques, a significant interaction between air currents, muzzle gases, and particulate material emanating from the firearms upon discharge with backspattered blood was observed. Blood droplets that initially spattered back toward the firearm and the shooter were observed to change direction under the influence of firearm-induced air currents and were blown forward toward and beyond their original source location. Hence, patterns of backspatter are complex and affected by multiple variables. (Taylor et al, 2011)

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