Introduction

Bullet placement is the single most important factor influencing the capacity of a bullet to cause immediate or rapid physiological incapacitation. This is because incapacitation is determined by the anatomical structures the bullet penetrates and the severity of the damage caused. For example, even a small calibre bullet such as calibre 22 Rimfire can be lethal if it strikes certain regions of the body [6, 24, 38]. However, immediate incapacitation occurs only if a projectile strikes the upper portion of the central nervous system (CNS), comprising the brain and/or upper cervical spinal cord [19, 31, 36, 38]. It is also possible to cause indirect CNS tissue damage by bullets that do not strike the CNS, through the mechanism of temporary cavitation in the vicinity of the spine. In this instance the spine would be moved suddenly and violently within the spinal canal. This would have a similar effect to the spinal cord being cut and could cause immediate incapacitation to the muscles innervated by the nerves that emerge from the spinal cord at or below the level of the concussion [16].

The other cause of rapid incapacitation is massive tissue destruction, or collapse of the circulatory system from severe disruption of vital organs and blood vessels in the torso. This causes cerebral hypoxia from hemorrhage [26, 31, 36].

Physiological responses

The rate of blood loss will be determined by the size of the bullet wound through the relevant blood vessels, as well as the pressure in the vessel at that time [16]. For example, an average human male weighs approximately 80 kg and has a cardiac output of 5.5 L of blood per minute. The total blood volume of this person will be 60 mL per kg of body weight or a total of 4,800 mL. However, if a person is subjected to severe stress, cardiac output can double and aortic blood flow may reach 11.0 L per minute. If a gunshot wound severs the ascending aorta, it will take only 4.6 s to lose 20% of the blood volume, which is close to the maximum amount the body can lose before a person is rendered unconscious. A scenario of this type would be classed as one of the best examples of rapid incapacitation for a bullet wound outside the CNS, because although not immediate, incapacitation has occurred within a very short time frame [36].

The centre of the torso is one of the most vulnerable areas outside the CNS because of the abundance of vital structures such as the heart, aorta, other major arteries and veins as well as the liver. In addition, because it is a larger target area, there is an increased hit probability compared with the smaller head. It is for these reasons that soldiers, police and other law enforcement officers are trained to shoot at the “Centre of Mass.”

There are factors which can reduce the amount of blood loss a person sustains involving the “flight or fight response” which describes the process of increasing blood circulation, promoting energy production and restricting non essential physiological activities. While the body may be able to compensate for about 25% of blood loss or about 1 L of total volume in a healthy person, losses of amounts above this cannot be sustained and the brain and heart will become deprived of oxygen resulting in unconsciousness and eventually death, if left untreated [3, 33, 36, 45]. Endorphins are also released from the pituitary gland and hypothalamus into the blood, spinal cord and brain that help inhibit the perception of pain and allow a person to keep functioning for longer [5]. Therefore, it can be seen that the body’s compensatory mechanisms are designed to save a person’s life after sustaining a significant wound. In situations of armed combat this means that an adversary, although severely wounded, may still be capable of presenting a threat unless immediately incapacitated.

Tissue damage

There are two components to bullet wound trauma. The first is the permanent wound cavity, which is the path taken by the bullet while crushing and effectively destroying tissue [11, 23, 26, 29]. The mass of tissue damaged is proportional to the effective volume of the permanent wound cavity. The cross-sectional area of this cavity is also equal to the bullet’s cross-section at that same point modified by a shape factor derived from the bullet’s configuration and the amount of resistance offered to it by tissue [34]. The second is the radial stretching of tissue around the bullet’s wound track, which momentarily leaves an empty space called the temporary wound cavity [1, 11, 26, 30, 39]. This cavitation effect is caused by high pressures surrounding the projectile that accelerate material away from its path. Harvey [23], Janzon [27, 28], Sellier and Kneubuehl [40] state that the temporary cavity is the most important factor in wound ballistics of high velocity rifle bullets, and that almost all biological phenomena can be explained by it. While the importance of the temporary cavity is recognized by all other contemporary researchers, most do not place its importance above that of the permanent cavity. This is because temporary cavity tissue damage is far less predictable [17, 34, 36, 38]. The temporary cavity also has little or no wounding potential with handgun bullets because the amount of kinetic energy deposited in the tissue is insufficient to cause remote injuries [8, 34]. The size of the temporary cavity is approximately proportional to the kinetic energy of the striking bullet and also the amount of resistance the tissue has to stress. Therefore, the maximum volume of the temporary cavity represents the amount of stored strain energy, which is the potential energy stored in a body by virtue of elastic deformation and is equal to the work that must be done to produce this deformation. The potential for this energy to cause wound trauma depends upon the following factors:

  • Magnitude of the stored energy that results from the drag force affecting the bullet. This force arises from pressure differences at the bullet’s surface, caused by tissue streaming against it in flight.

  • Sensitivity of the tissue to strain.

  • The size of the tissue or organ structure.

  • The anatomical constraints on tissue movement due to adjacent structures. [34].

The temporary cavity may be as large as 11–12.5 times the diameter of the centre-fire rifle bullet, but lasts only 5–10 milliseconds from its initial rapid growth until its collapse [8]. Because much of the body’s tissue is elastic, the temporary cavity quickly subsides as the elastic recoil of the stretched tissue returns towards the permanent wound track. Flexible elastic soft tissues such as muscle, intestine, skin and blood vessels, are good energy absorbers and are highly resistant to the type of blunt trauma caused by tissue stretch [38]. Therefore, depending on the tissue that was subject to these stresses, there may be no damage or a reduced amount of tissue damage. Again, this is an example of why the temporary cavity is not always a reliable wounding mechanism [36]. However, certain organs within the body are more susceptible to damage than others, such as the liver, kidneys, spleen, pancreas and completely fluid filled organs such as the bladder. This is because of their relatively low tensile strength and the high fluid content that deforms in a plastic manner [46]. These organs are therefore highly susceptible to splitting, tearing or rupturing as a result of temporary cavitation. Tests conducted by Amato et al. [2] found that the same amount of bullet energy caused greater damage to liver than to muscle. If enough energy was transferred, the liver may in fact disintegrate [27]. In addition, the displacement of tissue as a result of the temporary cavity can disrupt blood vessels, particularly arteries, or fracture bones some distance from the projectile path [20, 27]. Bone fragments can also function as secondary projectiles causing further tissue disruption [7, 8].

Patrick [37] indicates that the outward velocity of tissue forced aside by temporary cavitation forms no more than one tenth of the velocity of the bullet. This may be an over simplification, because the dynamics at the projectile-tissue contact surface involve explosive radial displacement of the tissues. The velocity of the tissue that is forced aside will vary depending on the type of bullet involved i.e. pointed rifle bullets (Spitzer shaped) versus a bullet with a large contact face such as a flat nose or expanding style of hunting bullet, as well as the tissue type in question. In addition, the velocity of tissue will decrease with increased radial distance from the shot line.

Penetration is one of the most important functions of a bullet and a minimum penetration of 300 mm in soft tissue is required to reach vital organs from any angle. This became the accepted standard within the USA following the first Wound Ballistics Workshop at the Federal Bureau of Investigation (FBI) Academy in September 1987. This was set up to assess the need to better arm FBI agents following an incident in Miami in 1986, where two agents were killed and five others wounded by two armed offenders who had already sustained multiple gunshot wounds that had failed to incapacitate them [10, 14, 34]. While it is acknowledged that many vital organs are far less than 300 mm from the anterior chest wall, this minimum standard allows for unexpected bullet paths such as laterally through an upper limb or through unusually fat or muscular bodies where vital organs are behind large amounts of tissue.

Shock wave injuries

A shock wave can be broadly described as a powerful propagating disturbance caused by a sudden change in pressure, density or temperature, which travels through a medium faster than sound is able to. Shock waves carry significant energy but this energy will dissipate with increased distance [32, 41].

There is still some debate as to whether shock waves generated from high velocity rifle bullets cause damage to tissues far removed from the impact site. Harvey [22] first raised this in 1947 and referred to it as secondary damage resulting from pressure changes accompanying the passage of these bullets. Sellier and Kneubuehl [40] went further and claimed the remote effects in tissues were the result of shock waves rather than temporary cavitation. They believed that while shock waves may not cause any obvious damage, cellular damage can be caused by the steep increase, followed by a sudden decrease, in pressure. Fackler [11, 13, 15] disputes the shock wave theory claiming there is no physical evidence to support it. Since that time, however, other authors have suggested that there is increasing evidence to support the theory that shock waves can cause tissue-related damage, and injury to the nervous system. Some of these opinions have resulted from experiments using simulant models [35, 42–44]. One of the most interesting is a study by Courtney and Courtney [4] which showed a link between traumatic brain injury and pressure waves originating in the thoracic cavity and extremities.

Bullet design

The design and construction of a bullet are also important elements in determining its wounding capacity. The bullet’s mass and striking velocity establish its potential to destroy tissue, while the shape and construction determine how much tissue is actually disrupted [12]. Spitzer shaped bullets are designed to have low drag characteristics, which refers to the amount of air resistance in flight. Bullet yawing upon target strike (deviation of the long axis of the bullet from its line of flight) and tumbling (complete loss of gyroscopic stability) will markedly increase this drag effect and therefore substantially increase temporary cavitation [35, 38].

During firing, the helical rifling within a rifle barrel imparts gyroscopic spin to the bullet. This is sufficient to stabilize the bullet during its flight in air, but tissue is denser than air and the increased drag on the bullet overcomes its rotational stability. While it begins its passage through tissue traveling point first, the bullet soon begins to yaw [28, 38, 39]. The changes in both density and elasticity within biological tissues such as fascia overlying skeletal muscle, causes a rapid increase in yaw [5]. When the yaw angle exceeds approximately 20°, there is generally no recovery and the projectile proceeds towards 90° yaw, thereby presenting its maximum frontal area to the direction of travel. As a result, maximum energy is deposited and “explosive” temporary cavitation is achieved. This generally occurs at a penetration depth of between 100 and 200 mm, depending on the bullet and the nature of any intervening materiel [17].

The maximum amount of temporary cavitation can also occur with maximum deformation or fragmentation of a bullet [38, 39]. Fragmentation in tissue can substantially increase the size of the permanent wound cavity as each of the multiple fragments spreads out radially from the main wound track and cuts their own path through tissue. This occurs at the same time as the temporary cavity is formed. The perforated tissue within that cavity loses its elasticity and tissue can tear or become detached because it is unable to absorb the amount of stretch that it normally would be able to [9, 19, 20]. Therefore, it logically follows that a fragmenting bullet will make a larger wound than a non-fragmenting one [5, 12].

Military rifle bullet design

Military bullet designs are constrained by the Hague Convention of 1899, which prohibits bullets which have been specifically designed to expand or flatten easily in the human body and do not have a full hard envelope (jacket). The Hague Convention IV of 1907, Article 23(e), forbids the use of arms, projectiles or material calculated to cause unnecessary suffering [20, 25, 30, 35, 39]. Therefore military bullet design is limited in comparison to the civilian or law enforcement market, where soft lead nose hunting bullets or bullets with polymer tips are common place and are generally capable of greater wound trauma and incapacitation because they are designed to expand. The USA was not a signatory to the Hague Convention of 1907.

Calibre 5.56 × 45 mm NATO

The 5.56 × 45 mm NATO bullet is often described as a varmint cartridge due to its small calibre. Nevertheless, it became NATO’s second standard small arms calibre for military forces in 1979 [27]. This second standard was always intended to complement NATO’s first standard small arms ammunition i.e. 7.62 × 51 mm NATO. The current SS109 design contained in STANAG 4172 replaced the former M193 bullet design after 1975. This new bullet was developed by the Belgian company Fabrique Nationale and was slightly heavier and longer.

At high impact velocities, all of these bullets tend to fragment and the penetrator (located in the nose) may become separated. Another feature of these bullets is that they consistently exhibit curved trajectories during gelatine impact testing because their centre of gravity is located to the rear of the bullet’s centre of drag (centre of pressure). The centre of drag is defined as the theoretical point (usually in the forebody) at which all drag forces can be said to be acting, which causes the bullet to turn as it starts to yaw during tissue or gelatine penetration [5, 21]. These types of bullets typically have full metal jackets comprised of copper nickel gilding metal, as well as a lead-antimony alloy core. Antimony is added to the lead to increase its stiffness for superior penetration.

The 5.56 × 45 mm NATO bullet typically exits the muzzle of a rifle with up to 6 degrees of yaw at launch. As the bullet moves farther from the muzzle the gyroscopic stabilization dampens the yaw, which gradually decreases to approximately 2 degrees throughout the region of stable flight (pre-transonic—greater than Mach 1.2). This explains why close range wounds with this calibre are generally more destructive than long range wounds, because the bullet has become more stable over a greater range and at close range the bullet has far greater remaining kinetic energy and momentum. It also explains why these bullets penetrate deeper at 100 m than they do at 3 m [8, 18].

Another important feature of some high velocity bullets like the 5.56 × 45 mm NATO is their tendency to fragment. Fragmentation is the result of rapid yaw growth and is caused by a combination of forces that bend the projectile sufficiently to fracture the jacket [5, 27]. All types of STANAG 4172, 5.56 × 45 mm NATO bullets tend to flatten and break at the cannelure. This occurs because the cannelure is the weakest portion of the bullet jacket and stress forces focus on this area during maximum yaw. The bullet point tends to flatten but remains in one piece because of the structural integrity associated with the design (cone shape), but the rear section breaks into many fragments.

Fackler et al. [18] report that during ordnance gelatine block testing at various ranges involving the M855 bullet used in the M16 A2 service rifle, the bullet will fragment and the jacket behind the cannelure will break at a range of 50 m. At 150 m most of the bullet breaks at the cannelure and the jacket behind the cannelure is flattened but unbroken. At 250 m the bullet flattens behind the cannelure but does not break and at 350 m the bullet remains intact with a slightly flattened appearance. Again this is due to the bullet achieving greater stability as the target distance increases, as well as less residual energy to disassemble itself.

Reducing the barrel length of assault rifles such as the M16 in calibre 5.56 × 45 mm, will also reduce the velocity generated. This can potentially reduce the effectiveness of this calibre, depending on the target distance, by decreasing the likelihood the bullet will fragment [16].

Summary

Bullet placement is a critical factor in determining the incapacitation potential of a projectile. The major means of achieving immediate incapacitation is by disruption of the CNS (brain and cord). Rapid incapacitation can also be achieved by massive tissue destruction, or collapse of the circulatory system from severe hemorrhage.

There are two main mechanisms of trauma. The first is permanent cavitation, which is the path taken by a bullet while crushing and destroying tissue. This is the main method by which handgun bullets cause injury. The second is temporary cavitation, which is the radial stretching of tissues around a bullet track. This is caused by high pressures surrounding the projectile which accelerate material away from its path. This is particularly relevant to high velocity rifle bullets. There is still debate as to whether shock waves generated by high velocity bullets are responsible for remote injuries and cellular damage to tissues and the nervous system.

The other factors affecting the degree of wound trauma are related to the bullet itself. Apart from bullet placement, the shape, construction, mass and terminal velocity all determine how much tissue is actually disrupted.

Key points

  1. 1.

    Bullet placement is one of the most important factors in achieving incapacitation.

  2. 2.

    Tissue trauma consists of both the permanent wound cavity and also radial stretching of surrounding tissue resulting in temporary cavitation.

  3. 3.

    The extent of trauma depends on the type of bullet involved, the kinetic energy and mass of the bullet, the size of the organ or tissue, the presence or absence of anatomical constraints from adjacent structures, and the resistance of the tissue to strain.

  4. 4.

    Bullet yawing substantially increases temporary cavitation.

  5. 5.

    Bullet fragmentation can substantially increase permanent cavitation.