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 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 assist in determining the prevalence, persistence, as well as 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 the identification of gunshot residue, advances in analytical techniques allow for differentiation of period of fire of fewer than 5 hours from those longer than 1 to 2 days.[8]

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.

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 physical characteristics of the projectile also greatly affect the wound, including the type and composition, shape, as well as its’ 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]

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) An abrasion rim may, however, 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. Entry wounds will be free of characteristics, including flame burns and singeing, soiling and redding from gas as well as tattooing from unburnt particle and metal scraps. Also, 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 that are beyond the range of the weapon with an irregular shape as a result of 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 dramatical 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 to much larger 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 so 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 with the exception 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 back-splatter of soft-tissue and blood onto the firearm and fingers.

Similarly, the location of the wound 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 to 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 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 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 is dependant on the weapon as well as 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.