Ballistics

Jason Imperato and Leon D. Sanchez

Ballistics is the scientific study of the characteristics of projectiles, how they move in flight, and how they impart tissue damage. The majority of projectiles that cause significant injury in the United States are in the form of firearm injuries. According to 2011 data, there were 32,163 deaths involving firearms with even higher numbers involving serious injury (1). These injuries can be classified as violence related, accidental, or self-inflicted.

Ballistic science is typically divided into three stages: internal, external, and terminal (or wound) ballistics. Internal ballistics describes the forces that apply to a projectile from the time the propellant is ignited to the time the projectile leaves the barrel. External ballistics refers to the forces that act on the bullet in flight. Terminal ballistics is the behavior of the bullet once it impacts the target (2).

A firearm is a weapon designed to expel a projectile by the action of highly combustible, gas-generating gunpowder. Firearms are generally classified as handguns (i.e., pistols and revolvers), rifles, and shotguns. Most rifled firearms (i.e., handgun or rifle) have barrels with internal grooves, referred to as rifling, that impart spin to the bullet, enhancing inflight stability and accuracy. When the cartridge is fired, the burning of the powder generates gas in a contained space and the pressure generated by the gas propels the bullet forward. The bullet accelerates while inside the barrel and reaches its maximum speed, or muzzle velocity, upon exit.

Bullets are rotated around their long axes by the rifling of the barrel, which, once in the air, gives them sufficient gyroscopic stability to maintain the point-forward position while in flight. Without gyroscopic stabilization, bullets would preferentially travel in the more stable base-forward orientation where the greater mass of the bullet is. Bullets fired from a properly designed rifled barrel yaw no more than a few degrees during flight. The yaw angle is the angle between the long axis of the bullet and the line of flight. This angle is greatest as the bullet exits the barrel and is reduced as the gyroscopic spin stabilizes the bullet’s flight through the air.

Once a bullet enters a denser medium, the gyroscopic spin is insufficient to stabilize the bullet. As a result, when a bullet enters tissue, it will begin to tumble and deform. Depending on the makeup of the bullet and the properties of the tissue through which it travels, it may expand, fragment, or remain in one piece (3–13).

Bullet mass, which is related to the diameter and length of the bullet, is a major determinant of how deeply the bullet will penetrate the tissue. Bullet construction, such as whether the bullet is solid lead with no bullet jacket, is partially jacketed, or has a full metal jacket, is a primary determinant of how the bullet will deform or fragment once in tissue. The majority of bullets in use today are at least partially jacketed. The jacket is a layer of hard metal that surrounds what is typically a lead core. Without a jacket, lead bullets fired at speeds of >600 m/s would have some of the lead stripped by the rifling of the barrel (5). Jacketing allows bullets to reach greater muzzle velocities. Though military bullets are fully jacketed (14), civilian bullets usually have the lead core exposed at the tip (soft point). The tip is often further modified into different varieties of hollow-point bullets. The exposure of lead at the tip will cause the softer lead to deform much more than a fully jacketed bullet upon entry into tissue. These hollow-point bullets are designed to expand and deform as they penetrate tissue. A soft- or hollow-point bullet will deform earlier in its travel through tissue than a comparable fully jacketed projectile. As a result, the injury patterns will be different, although the claims that soft- or hollow-point bullets cause more tissue destruction are not consistently borne out.

Shotguns differ from rifles and pistols in that they have a smooth barrel that discharges hot gases, wad, and either multiple projectiles or a single projectile (rifled slug). Shot charges containing multiple projectiles spread out from the muzzle in a cone-like pattern. The distance from the muzzle of the shotgun to the point of impact of the projectiles is a key determinant of the magnitude of injury. At short range (<6 m), the shot charge containing multiple projectiles results predominantly in a single-hole wound (diameter ≤6 cm) that communicates with a deep underlying wound with massive tissue destruction. At this short range, soft tissue impact deforms the individual pellets, increasing their original cross section with a concomitant increase in tissue crush or hole size. The multiple pellets result in severe disruption between the multiple wound channels. A gradual decrease in the amount of pellet deformation and tissue destruction occurs as the distance of the impact range increases. When the impact range exceeds 7 m, the multiple projectiles result in numerous discrete wounds that are not associated with underlying massive tissue destruction (5).

Shotguns also can discharge rifled slugs that are designed for killing larger animals. The muzzle velocity of rifled slugs (487 m/s) is approximately half that of nonexpanding, fully jacketed rifle projectiles. The rifled slug does not hold the point orientation that it has as it is propelled from the muzzle of the gun but drifts toward a sideways orientation as it moves toward the target. The rifled slugs experience a 25% decrease in velocity as the impact range increases from 5 to 45 m. At short range (≤45 m), the slug deforms on striking the tissue, thereby enhancing the size of the cavity.

CLINICAL PRESENTATION

The nature of a projectile wound is dependent on both missile and tissue characteristics. As discussed earlier, missile characteristics are partly inherent and partly conferred by the weapon firing the projectile. Tissue characteristics also strongly affect the nature of the wound.

Tissue is injured by a bullet via two mechanisms: tissue crush and tissue stretch. These two mechanisms correspond to the permanent and temporary cavities created by the passage of the projectile through the tissue (6–9). As the bullet travels through tissue, it will crush tissue that is directly in its path. This is the primary method of injury from gunshot wounds. The most important determinant of injury is the properties of the tissue that the bullet crushes. The tissue that is crushed corresponds to the permanent cavity formed by the bullet. If the bullet is traveling with its pointed end forward and its long axis parallel to the longitudinal axis of flight (0 degrees of yaw), it crushes a tube of tissue approximately equal to its diameter. When the bullet yaws to 90 degrees, the entire long axis of the bullet strikes the tissue, amplifying the amount of tissue crushed.

Passage of the projectile at a high rate of speed also displaces other tissue away from the path of the projectile. As the tissue stretches away from the path of the projectile, a temporary cavity that exists for a brief time (measured in milliseconds) is formed. The temporary cavity is comparable to the splash formed by throwing a stone into a pool of water. By definition, no tissue is present in the temporary cavity. The stretch of tissue away from the temporary cavity can injure tissue to different degrees depending on the tissue elasticity defined as the resistance of tissue to stretch. The same stretch that causes moderate contusion and functional changes in the relatively elastic muscle, skin, lung, and bowel wall can cause catastrophic disruption of the inelastic liver, heart, and brain. Muscle, skin, and lung are flexible and elastic, having the physical characteristics of good energy absorbers. Though temporary cavitation stretches these tissues, their elastic properties allow them to maintain their structure and function. For example, muscle, when disrupted, will have an area of devitalized nonfunctioning tissue surrounding the permanent cavity whereas tissue farther away, which was stretched by the temporary cavity, is still structurally sound and maintains its contractile function. In comparison, the inelastic liver is fractured and rendered nonviable to a much greater degree by the temporary cavity (15). The brain is a special case of inelastic tissue. Not only is brain tissue very sensitive to structural disruption but this is magnified by the fact that the brain is encased in a solid container (the skull) that prevents tissue displacement. As a bullet penetrates the skull, temporary cavitation will increase pressure within the skull, and this pressure will be relieved either through the entrance or exit wound.

The size of the temporary cavity depends on the energy of the projectile, the surface area the projectile presents to the tissue as it moves through it, and the mass of tissue displaced. As the bullet tumbles through tissue from the point-forward orientation to the final base-forward orientation, it will present its greatest surface area at 90 degrees of rotation. It is at this point where maximal tissue displacement will occur and the size of the temporary cavity is greatest.

These permanent and temporary cavities are the sole wounding mechanisms of missiles. The sonic shock wave generated by supersonic missiles does not cause either tissue displacement or detectable damage. Projectile fragmentation can greatly enhance the effects of the temporary cavity by providing points of weakness on which the stretching effects of cavitation are focused, rather than being absorbed evenly by the tissue mass. Fragmentation can refer to the projectile’s forming multiple parts or to secondary fragments such as clothing, bone, or teeth that develop from being struck by the projectile.

ED EVALUATION

Gunshot injuries frequently do not occur in isolation. As a result, the emergency physician must evaluate the patient for not only the penetrating injury resulting from the gunshot wound but the blunt injury that may result from the victim’s falling to the ground after being struck by the projectile.

The initial evaluation of the gunshot victim is similar to that of any multiple trauma patient and should focus on rapidly identifying life-threatening conditions. Patients should be evaluated by the Advanced Trauma Life Support (ATLS) Guidelines (14). This includes an assessment of the primary survey (airway, breathing, circulation, disability) with rapid attention to immediately life-threatening injuries, followed by the secondary survey, ongoing monitoring of vital signs and pulse oximetry, and initial stabilizing interventions as indicated.

A careful history is important in obtaining useful information about the number and types of injuries sustained. Patients who are awake should be asked how many shots they heard and any other information they have relating to the event such as their distance from the weapon and the type of weapon utilized.

The physical examination should focus on the detection of penetrating or perforating injuries. A careful examination of the patient including the back, under the hair, axillae, and gluteal folds will help identify injuries. The location of the external wounds may suggest a track for the projectile, but the physical examination is generally not sufficient to determine the direction and extent of the penetrating projectiles due to the cavitation effects of the projectile through body tissues. Penetrating projectiles may ricochet off of bony structures altering the trajectory of the permanent cavity produced. A small external wound may belie massive internal injuries that are not immediately evident on examination (4). The identification of an initial wound should not negate the need for a complete survey of the patient to avoid missing other injuries.

Radiographs of the areas the bullet is thought to have traversed are indicated to identify the position of the projectiles. It is best to obtain at least two radiographs in planes separated by 90 degrees to determine a more accurate position. If the patient is clinically stable, a computed tomography (CT) scan may aid in the assessment. Laboratory studies should be ordered as clinically indicated and guided by ATLS Guidelines. The evaluation and management of injuries is best conducted in conjunction with a trauma surgeon and should follow the algorithms presented in previous chapters. (See Chapters 18, 32, 34, and 36.)

Gunshot wounds must be reported to the appropriate authorities. In most cases, shootings become the subject of a criminal investigation by law enforcement authorities. Therefore, the medical record and patient property should be preserved as evidence. If a patient’s clothing is cut, care should be taken to avoid cutting through the portions a bullet might have traversed. Any material that needs to be saved for law enforcement should be placed in paper bags, as plastic bags will trap moisture and thereby degrade evidence.

The location of all wounds identified should be recorded in the medical record, including comments on the presence of any powder residue observed surrounding the wound. It is often difficult to differentiate entrance from exit wounds. Although it is often thought that the smaller wound must be the entrance wound, this is not always true. It is best to record the size and location of the wounds, rather than using the terms entrance and exit. If a bullet is recovered, the emergency physician should avoid making any markings on the sides of the bullet because this will interfere with the forensic evaluation of rifling patterns (5). Any marking of recovered bullets that may be necessary for the preservation of chain of custody should therefore be done at the nose or base.