Gunshot Wounds Forensic Pathology

Issues of Concern

While the examination of a recent gunshot wound to exposed parts of the body is often straightforward, the appearance of the wound may be affected by putrefaction, charring, presence of multiple projectiles, overlaying of the injury by clothing as well as by injuries sustained by ricochet or fragmentation of the projectile(s) or bones.  

Forensic pathologists are also required to retrieve the projectile during the autopsy to assist in identifying the weapon used. The projectiles in rifled firearms have unique markings that result from the spiraling (rifling) present in the muzzle and can be divided into class characteristics that can identify the type of firearm used. The projectiles also demonstrate individual characteristics unique to individual firearms and can be replicated and compared with projectiles recovered from controlled firing to identify the exact weapon used.[2]

Causes

Globally, in 2016, 251,000 deaths resulted from firearms injuries caused by events not related to war.[4] Gunshot injuries account for the most homicides in the United States of America, accounting for an estimated 46 deaths every day.[3] Other studies have estimated over 90 deaths every day caused by firearm-related injuries.[5] In 2016, the second leading cause of death in children and adolescents was firearm-related injuries accounting for 15.4% of all deaths; Almost two-thirds of these deaths (59%) resulted from homicide, with just over a third (35%) as a result of suicide. Homicide was more common among urban youth than suicide, which was more common in rural youth. The difference in the rates was approximately twice in both cases.[6] Between 1999 and 2013, firearm-related injuries caused the death of 462,043 Americans.[7] In addition to mortality, firearm injuries also cause a massive burden on the health infrastructure, with 282,542 emergency discharges recorded in the USA between 2009 and 2012.[5]

Biochemical and Genetic Pathology

One of the most common questions encountered by forensic scientists is the determination of a person firing the weapon. Identification of gunshot residue that is characteristic or consistent should take into account both the chemical composition as well as the morphology of the particles examined. The gold standard for gunshot residue analysis is an examination using a scanning electron microscope with an energy dispersive X-ray spectrometer (SEM-EDX). The examination confirms the presence of the primer mixture, which is labeled 'characteristic gunshot residue' if all three heavy metal compounds: those of lead - lead azide or styphnate, antimony - antimony trisulfide, and barium - barium nitrate. The presence of one or more compounds, but not all three, is labeled 'consistent with gunshot residue.' 

Recent advances in the field of gunshot residue analysis have led to the development of a synthesized reference sample that can help determine the prevalence, persistence, transfer, and contamination. This analysis becomes further complicated by the presence of modern primer compounds, including titanium, aluminum, calcium silicide, etc., modern oxidizers, including zinc peroxide, potassium nitrate, etc. 

The composition of gunshot residue had also been effective in defining 'signature elements' for determining the manufacturer. This residue was useful for creating a 'signature' for police ammunition - titanium and zinc were used to differentiate 9 mm ammunition of the German, Dutch and Belgian police forces. This practice has, however, seen adoption worldwide to produce heavy metals-free ammunition. At present, police ammunition is characteristic, with the presence of gallium or gadolinium as a signature. Gunshot residue from non-police ammunition is less characteristic and therefore requires the analysis of residue collected from the weapon to develop a definite conclusion. 

In addition to identifying gunshot residue, advances in analytical techniques allow for differentiation of periods of fire of fewer than 5 hours from those longer than 1 to 2 days.[8]

Morphology

Gunshot wounds typically consist of the permanent tract or cavity, surrounded by a perimeter of abraded tissue called the abrasion rim. The wound also contains a grease collar, resulting from the accumulation of lubricant on the tissue during penetration by the projectile.

In addition to the wound proper, other characteristics that may present include the presence of muzzle burns in contact wounds, flame burns in near-contact or close-range injuries, and soiling (enveloping of surrounding skin by smoke and gas) and tattooing (embedding of unburnt particles and metal scraps in surrounding skin) in mid-range or intermediate-range wounds.

Mechanisms

Gunshot wounds result from the discharge of projectiles by firearms. The mechanism of formation of the injury includes the compression of tissue by the projectile and their disruption when these compressive forces exceed the elasticity of the tissue. The disruption of tissue is directly proportional to the mass and velocity of the projectile. The projectile's physical characteristics also greatly affect the wound, including the type and composition, shape, and tendency to fragment and deform.

The injuries are also dependant on the type of firearm used, the distance of fire, and the direction of fire. The type of firearm determines the size (mass) and velocity of the projectile. The distance of fire determines the effects of accompanying components and, therefore, the appearance of skin and tissue surrounding and inside the injury. The direction of fire determines whether the injury is an entry wound or exit wound and therefore affects the characteristics of the injury.[1]

Clinical Significance

Type of Firearm

These devices can classify into two basic categories, based on the type of firearm – namely rifled firearms and smooth-bore firearms (shotguns). A third category includes country-made firearms that use unusual projectiles and is seen more commonly in less developed countries.

Gunshot wounds can also classify depending on the muzzle velocity of the projectile and fall into low-velocity and high-velocity firearm injuries. British researchers have generally used the speed of sound in air, i.e., 1,100 feet/second (335 m/s), to classify these differences. American researchers meanwhile have used arbitrary classification, using 2,000, 2,500, or 3,000 m/s to classify firearms into low-velocity and high-velocity.[9][10][11] These are also classified as small and large arms, respectively.

In addition to the direct effects of the projectile and accompanying components, gunshot wounds, especially of large-arms, may be accompanied by a sonic wave; this is usually not of great consequence, except when involving hollow viscera, like intestines, that could suffer disruption. The secondary effects in a gunshot wound are more severe due to a phenomenon known as temporary cavitation. These temporary forces include radial acceleration, shear, stretch, and compression and result in disruption of the tissue away from the main injury tract. They are more extensive in solid, uncompressible organs.

Entry and Exit Wound

The distance and direction of shooting also affect the characteristics of the wound, including the shape, invagination of tissue, the effects of accompanying components including flame, smoke, gas, unburnt particles, metal scraps, and grease. Gunshot wounds can produce two types of wounds, depending on the direction of travel of the projectile. These are entry wounds and exit wounds – entry wounds are generally smaller and more regular than exit wounds. Entry wounds show invagination of tissue into the wound, while exit wounds show outward beveling of tissue. The skin surrounding the entry wound will show the above characteristics, depending on the distance as described later; this includes the presence of flame burns and singeing, soiling and redding from gas, as well as tattooing from unburnt particle and metal scraps. Similarly, the presence of an abrasion rim and grease collar around the wound also indicate that it is an entry wound. (See Figure 1) However, an abrasion rim may also be present on the exit wound, where the skin is shored against a hard surface.

Exit wounds, in comparison, are larger and more irregular. They show outward beveling of the soft tissue and the margin. Exit wounds will be free of characteristics, including flame burns and singeing, soiling and redding from gas, as well as tattooing from unburnt particles and metal scraps. They do not have an abrasion rim or grease collar except when the skin is shored against a hard surface. (See Figure 2)

Distance of Fire

Depending on the distance of fire, the wound can also classify as contact wounds – again divided into firm-contact and loose contact – near-contact or close-range wounds, mid-range or intermediate-range wounds, distant or far-range wounds, and indeterminate wounds.

Contact wounds are self-explanatory and are diagnosable by the presence of muzzle-imprint on the skin. Near-contact or close-range wounds are identifiable by the presence of flame burns and singeing of hair. Mid-range or intermediate-range wounds do not have flame burns or singeing of hair but will show the presence of smoke, gas, and unburnt particles. Distant or far-range wounds will be embedded with any accompanying components other than the lubricant forming a grease collar. Indiscriminate wounds are atypical wounds beyond the range of the weapon with an irregular shape due to yawing (changing in the axis and direction of flight of the projectile).

Rifled Firearms

It is evident from the explanation above that the determination of the distance of fire is dependent on the type of firearm. Typically, small-arms, including revolvers and pistols, eject the flame for a distance of 15 cm while large arms, including most automatic 'assault' rifles, propel the flame for a distance of 30 cm. The accompanying components also demonstrate dramatic differences in the distance traveled depending on the type of weapon and can range from 15 to 30 cm for smoke and gas, 30 to 60 cm for unburnt particles and metal scraps in small arms, which may be propelled much greater distances in large arms. While these distances are arbitrary and provide a general outlook on the distance of fire, each weapon would have individual variations and, therefore, should be tested for confirmation.

In addition to the above findings, the distance of fire also affects the nature of the wound, especially in contact wounds. Contact wounds can thus divide into two different classes, firm contact and loose contact. In loose contact wounds, the gases, gunpowder, flame, and other material mostly enter the wound. However, due to the loose contact, a small portion of it may escape from the gap between the muffle and skin and may be found on the skin surrounding the injury. In the case of firm contact wounds, the nature of injuries is further dependant on the underlying tissue. In contact gunshot wounds with soft tissue underlying the injury, the injuries are similar to those caused by loosed contact, except that surrounding skin and tissue are free from flame burns, soot, tattooing, and other changes. These findings are present in the depth of the wound. Also, there is a distinct muzzle pattern around the injury.

In contact gunshot wounds with bony tissue underlying the injury, the injuries are typically called a stellate wound, where the gases exit the barrel before the projectile. As a result, the gas collides with the bone tissue, resulting in the reflection of the gases. This gas causes the expansion of subcutaneous space and pressure, which results in an explosive injury, tearing, and lacerating the skin and subcutaneous tissue, resulting in the stellate wound. Additionally, the expanding gases also cause a back-splatter of soft-tissue and blood onto the firearm and fingers.

Similarly, the wound's location on the body can affect the wound characteristics and, perhaps, more importantly, the outcome of the injury. It is abundantly evident that the involvement of a major organ or vessel can lead to life-threatening injuries. Solid organs are susceptible to more significant injuries as compared to hollow organs or elastic tissue. The location of the injury can also affect the wound characteristics by the effect of the intervening layers of clothing or other objects that can not only reduce the velocity of the projectile but also prevent the deposition of accompanying components.[11]

Smooth-Bore Firearms

Smooth-bore firearms or shotguns have a completely different profile of injuries as compared to rifled firearms. This difference is determined primarily by the type of projectiles in the shotgun. Shotgun cartridges consist of multiple projectiles that disperse a short distance from the muzzle, leading to more extensive damage. The penetrating power of each projectile is, however, reduced. 

Also, the significant difference between smooth-bored weapons and rifled weapons is the projectile. While a single projectile is ejected from the muzzle of a rifled firearm, the entire cartridge is released from the smooth-bored firearm. This cartridge consists of multiple tightly packed pellets that disperse as the cartridge moves towards the target. These pellets or projectiles are secured using plastic or cardboard wads that help separate the primer and gunpowder from the projectiles while also preventing the diffusion of pressure and gas upon their burning. Smooth-bore firearms, therefore, produce injuries that also show abrasions and contusion caused by the projectiles, cartridges, and wads. Also, the injury tract may contain pieces of cardboard or plastic, called wads,

Shotguns, in general, also propel the flame for a distance of 15 cm, smoke and gas for 30 cm, unburnt particles, and metal scraps for 60 cm. The contact injuries appear similar to a rifled firearm, in both pressed and loose, as well as with underlying bone and soft tissue. In pressed or firm contact injuries, the skin may present with a double muzzle imprint from the double barrels.

In general, gunshot injuries resulting from firing a smooth-bore weapon within 30 cm presents as a circular wound with smooth or crenated margins with no satellite pellets and presence of wads in the wound tract, in addition to the effects of accompanying components, including skin burns and singeing due to the flame, soiling and redding of tissue due to gas and smoke, tattooing from unburnt powder and metal scraps.

The pellets tend to show a pattern of dispersion beyond 60 cm, which presents as nibbling of a 'rat-hole" circular wound, with minimal satellite injuries and wads, that may be present. The flame burns and singeing are absent, and also, there is minimal or absent redding of the tissue from carbon monoxide. 

Beyond a distance of 1 meter, the pellets become sufficiently dispersed to demonstrate a central "rat-hole" wound with multiple satellite pellet holes, with no burning or singeing, no reddening of tissue, no soiling, no tattooing, and absence of wads in the wound tract. 

Beyond a distance of 10m, the pellets are completely dispersed, seen as multiple small punctate injuries with the absence of a central wound. Similarly, none of the accompanying components reaches the injury, and their effects are not seen. 

The dispersion of the pellets is extremely variable and depends on the weapon and the cartridge. This is further complicated by the practice of choking of the barrel, intentional narrowing of the muzzle, which increases the dispersion of pellets at a given range. As with rifled firearms, it is essential to test each weapon for individual variations.