Forensic Emergency Medicine

Ballistics

Ballistics is the science of the motion of projectiles in flight. Ballistic physics is broken down into three parts, based on the material with which the bullet interacts:

• 1.

  Internal ballistics, which pertains to the projectile while in the firearm

• 2.

  External ballistics, or the path of the projectile from when it leaves the firearm until it reaches the target

• 3.

  Terminal (or wound) ballistics

Internal ballistics factors in the design of the firearm and how the combustion of gunpowder within the firearm generates the pressure that subsequently projects the missile. External ballistics takes into consideration the effects of gravity and drag on the missile and how they affect the distance travelled and the accuracy with which it hits the intended target.

Wound ballistics is the most clinically relevant because it is the study of the effects of penetrating projectiles on the body. Tissue disruption (wound severity) is directly related to the amount of kinetic energy (KE = ½mv2 [m = mass; v = velocity]) transferred to it, not to the total amount of kinetic energy possessed by the bullet. There are two broad categories of weapons based on the amount of potential kinetic energy that can be transferred from the missile to the tissue:

• 1.

  High-velocity weapons, which generate projectile velocities ranging from 1500 to 4000 ft/s (or >600 m/s). Examples of these are rifles or military weapons. Consequently, bullets fired from such weapons possess a higher kinetic energy and therefore theoretically have greater wounding capacity.

• 2.

  Low-velocity weapons, which generate projectile velocities ranging from 700 to 1600 ft/s (or <600 m/s). These weapons include handguns, shotguns, and air rifles (such as ball-bearing [BB] guns).

Rifle bullets have more kinetic energy and a theoretically higher wounding potential, but energy transfer (and, by extension, wound severity) is the result of multiple variables. The most important are bullet type (e.g., weight, deforming type, fragmenting type), bullet velocity, and the characteristics, location, and nature of the impacted tissue itself (tissue over bone or tissue over organs). The formula KE = ½m(V ₁ − V ₂ )2 better reflects the actual energy transfer occurring during tissue penetration and wounding.

The principal mechanism for tissue damage by bullets is crushing. A bullet traveling through tissue generates two cavities, one permanent and the other temporary. The size of the permanent cavity varies with the size, shape, and configuration of the bullet. A hollow point bullet that mushrooms can increase its diameter 2.5 times on impact and will increase the area of tissue crush 6.25 times compared with a non-deformed bullet. The temporary cavity results from a shock wave generated as the bullet enters the tissue, which results in a brief distortion or stretching of the tissue. This tissue distortion lasts for a brief amount of time, 5 to 10 milliseconds from its generation until its collapse, and leaves behind crushed tissue and the permanent cavity. The size of the temporary cavity is also dependent on bullet/missile mass and velocity, and the resulting effect depends on the elasticity of the tissue traversed. Solid organs such as the liver, bone, or kidneys do not tolerate this temporary deformation as well as more elastic tissue (such as lungs, skeletal muscle, skin), and therefore sustain more damage. Secondary wounding can occur in situations in which the bullet hits a target (usually bone) and fragments disperse, causing further tissue damage.

Forensic Evaluation of Handgun Injuries

Handgun Ammunition

The cartridge, or round, is composed of the primer, cartridge case, powder, and bullet ( Fig. e11.1 ). The bullet is the projectile that is propelled out of the muzzle. The primer is a small explosive charge in the base of the cartridge that ignites the gunpowder. The primer may contain lead, barium, or antimony. These materials may be deposited on the hands of the shooter, on the victim of a close-range assault, or on objects within a room in which the weapon was discharged.

The cartridge case is typically made of brass, although other materials may be used. The function of the cartridge case is to expand slightly, sealing the chamber against the escaping gases and propelling the bullet down the barrel. On detonation, a cartridge case is imprinted with unique microscopic marks that are valuable evidence and should be preserved for law enforcement.

The gunpowder found in all commercial cartridges except blanks a smokeless powder made with a single base (nitrocellulose) or a double base (nitrocellulose and nitroglycerin). When a weapon is discharged, not all the gunpowder is consumed. A percentage of the unburned or partially burned gunpowder will travel out of the end of the muzzle for a short distance (<48 inches or 60 cm). The distance depends on the physical characteristics and shape of the powder and the weapon’s barrel length.

Blank cartridges, muzzleloaders, and other antiques or replicas may use black powder. Black powder (a combination of potassium nitrate, charcoal, and sulfur) does not burn as efficiently as the smokeless powder and results in a large flame and white smoke. As such, black powder can produce powder burns by igniting clothing.

The bullet is the projectile that is propelled down the gun barrel at velocities ranging from 700 to 1600 ft/s (213 to 488 m/s). The higher velocities are achieved by the inclusion of supplemental gunpowder in the cartridge case (a magnum load; hence, a .357 magnum). The diameter of the bullet’s base is termed its caliber. Bullet caliber is described in hundredths of an inch or millimeters. Handgun bullets range from .22 caliber, or 5.56 mm, to .45 caliber, or 11.3 mm. A bullet’s weight is measured in grains, with 7000 grains/lb.

The most common bullet types are the round nose, full metal jacket, hollow point, wad cutter, and semi–wad cutter. Bullets generally are a solid core of lead or steel. If a jacket covers the bullet core, the jacket’s metal is usually copper or aluminum. If the jacket covers only a portion of the core, the bullet is said to be semijacketed. If the core is completely covered, it is said to have a full metal jacket. Some bullets have a hole in the tip and are called hollow points. The hole causes the bullet to expand on contact, which significantly increases the damage to tissue.

Forensic Aspects of Rifles

Rifles are shoulder-fired weapons designed to generate significant pressures in the muzzle and, as such, result in firing of ammunition at high speeds. Centerfire rifle bullets, .223 to .308 caliber, are similar in diameter to handgun ammunition but, based on the formula for kinetic energy (KE = ½mv2), their wounding potential is greatly enhanced by the higher velocity of the round. Injuries result from the transference of energy from the projectile to tissue, organs, and bony structures. With medium-velocity rounds (2000 to 3000 ft/s, approximately 600 to 900 m/s) and high-velocity rounds (>3000 ft/s), a temporary cavity is formed along the wound tract, which may be 11 or 12 times the diameter of the bullet. High-velocity rounds can also cause tissue damage away from the physical track taken by the projectile. Because of the amount of energy possessed and transferred to underlying tissue, exit wounds associated with centerfire rifles, in contrast to those associated with handguns, are generally larger than their corresponding entrance wounds ( Fig. e11.2 ).

Forensic Aspects of Shotguns

Shotguns are similar to rifles, but the missiles are multiple small projectiles, all of which have the potential to cause injury. Shotguns have the barrel length of rifles but can discharge pellets or single slugs down a smooth bore barrel. The caliber of a shotgun is defined by the term gauge. Historically, the gauge of a gun referred to the number of lead balls of the given bore diameter that make up a pound—for example, 12 lead balls to make 1 lb. A shotgun cartridge may contain only a single slug or may contain hundreds of pellets ( Fig. e11.3A ). Pellets range in diameter from 0.05 inch (#12 birdshot) to 0.36 inch (000 buckshot). A shotgun cartridge is made of several components that will be of forensic value and should be collected in the ED (see Fig. e11.3B ).

Shotgun slugs are projectiles that may weigh 200 times more than handgun ammunition; for example, a 12-gauge slug weighs 547 g, and a .22 long rifle bullet weighs 2.6 g (see Fig. e11.3B ). The velocities of shotgun slugs are in the range of 1500 to 1800 ft/s (457 to 549 m/s). The sabot is the structure that helps seat the shotgun slug and allows it to be fired at higher muzzle velocities. These are usually made of lightweight materials such as plastic, carbon fiber, or light metals (such as aluminum). Sabots themselves can cause injury if they make contact with tissue.

Forensic Aspects of Air Guns/Rifles

Air guns/rifles (such as BB guns and pellet air guns) work very similarly to traditional firearms and generate muzzle velocities comparable to low-velocity weapons. As such, they possess enough kinetic energy to cause injuries because they can penetrate tissue, and particularly because they are often fired at close range. In fact, recent studies have shown that these weapons pose a significant health concern, particularly in the pediatric population.

Errors of Interpretation and Terminology

Emergency clinicians are in the ideal position to evaluate and document the state of a gunshot wound because they see and explore it before it is disturbed, distorted, or destroyed by surgical intervention. Documentation of gunshot wounds should include the anatomic location of the wound as well as its size, shape, and distinguishing characteristics. Digital photographs of the wound should be taken whenever possible with a ruler for size reference. Wounds should be described according to the standard anatomic position, with the arms to the sides and palms facing forward.

Emergency clinicians should not describe wounds as “entrance” or “exit” but, using appropriate forensic terminology, should document a detailed description of the wound, including its appearance, characteristics, and location, without attempting to interpret the wound type or bullet caliber. Exit wounds are not always larger than entrance wounds, and wound size does not consistently correspond to bullet caliber.

The size of any wound (entrance or exit) is determined by five factors—the size, shape, configuration, or angle, and velocity of the projectile at the instant of impact with tissue and the physical characteristics of the impacted tissue itself. If the projectile is slowed and its shape unchanged on exiting the skin, the exit wound may be the same size as or smaller than the corresponding entrance wound. If the projectile increases its surface area by fragmenting or changing its configuration while maintaining a substantial velocity, the exit wound may be significantly larger than the entrance wound. If the bullet strikes bone, fragments may extrude from the exit wound and contribute to the size and shape of the wound. Tissue elasticity also affects wound size so that entrance or exit wounds may be smaller, equal to, or larger than the projectile that caused them. Wounds on the palm or sole may appear as slits and can be easily mistaken for stab wounds.

A treating emergency clinician may be requested to render factual testimony, expert testimony, or both in a criminal case. Expert forensic testimony rendered without an appropriate forensic examination or adequate forensic training may mislead participants in the criminal justice system (e.g., “the exit wound is always larger than the entrance wound”). Opinions related to entrance versus exit wounds or the range of fire can affect the determination of innocence or guilt and should not be used except by trained practitioners.

The subsequent sections highlight the clinical features of entrance and exit wounds that result from the use of different types of firearms. This information is provided to make emergency clinicians aware of the varying presentations, depending on the distance from the target. Clinicians should refrain from speculating on the type of wound and simply document the findings noted on examination.

Handgun Entrance Wounds

Range of fire is the distance from the muzzle to the victim and can be divided into four general categories: contact, near-contact or close range, intermediate or medium range, and indeterminate or distant range ( Table e11.1 ). The size of the entrance wound does not correlate with the caliber of the bullet because entrance wounds over elastic tissue will contract around the tissue defect and have a diameter much less than the caliber of the bullet.

Contact Wounds

There are three subcategories of contact wounds: (1) tight contact, in which the muzzle is pushed hard against the skin; (2) loose contact, in which the muzzle is incompletely or loosely held against the skin; and (3) contact through clothing.

In a tight contact wound, all materials—the bullet, gases, soot, the incompletely burned pieces of gunpowder, and metal fragments—are driven into the wound. These wounds can vary from a small hole with seared blackened edges from the discharge of hot gases and an actual flame to a gaping stellate wound ( Fig. e11.4 ). Large wounds occur when the wound is inflicted over thin, inelastic, or bony tissue, and the injected hot gases cause the skin to expand until it stretches and tears. These tears will have a triangular shape, with the base of the triangle overlying the entrance wound. Tears are generally associated with .32 caliber or greater, or with magnum loads. Large stellate contact wounds are easily misinterpreted as exit wounds if the determination is based solely on their size (see Fig. e11.4B ).

Stellate tears are not pathognomonic for contact wounds, however. Tangential wounds, ricochet or tumbling bullets, and some exit wounds may also be stellate. These wounds’ appearance differs from that of tight contact wounds by the absence of soot and powder within the wound and a lack of seared wound margins.

In some tight contact wounds, expanding skin is forced back against the muzzle of the gun, leaving a characteristic muzzle abrasion or muzzle contusion ( Fig. e11.5 ). Patterns such as these should be documented before wound débridement or surgery because they are helpful in determining the type of weapon used (revolver vs. semiautomatic).

When a gun’s muzzle or barrel is in loose contact with or is angled to the skin, the soot and gunpowder residue are present within and surrounding the wound. The angle between the muzzle and skin determines the soot pattern. A tangential, loose, or near-contact wound produces an elongated searing and soot deposit surrounding the wound. Discharge of a weapon in contact with clothing results in the gases and soot being deposited between the garment and skin. This results in a diffuse pattern of soot surrounding a wound, with seared margins (see Fig. e11.4C ).

Close-Range Wounds

Close range is the maximum range at which soot is deposited on the wound or clothing. The muzzle to target distance is usually less than 6 inches (15 cm) but may be as much as 12 inches (30 cm). Beyond 6 inches, most of the soot usually falls away and does not reach the skin or clothing. The concentration of soot varies inversely with the muzzle-to-target distance and is influenced by the type of gunpowder and ammunition used, length of the weapon’s barrel, and caliber and type of weapon itself ( Fig. e11.6 ). A precise range of fire (e.g., 2 vs. 5 inches) cannot be determined from examination of the wound alone. Because soot can be removed with débridement or wound cleansing, its presence and configuration around the wound should be noted and photographed before débridement unless the patient’s clinical condition precludes this.

Intermediate-Range Wounds

Tattooing, or stippling, is pathognomonic for an intermediate-range gunshot wound. Tattooing appears as punctate abrasions and is caused by contact with partially burned and wholly unburned pieces of gunpowder ( Fig. e11.7 ). Tattooing or stippling cannot be wiped away. The appearance differs from powder burns due to black powder because black powder burns rapidly and at high heat, resulting in a burn appearance more reminiscent of those from thermal injuries. Tattooing rarely occurs on the palms of the hands or soles of the feet because of the thickness of the epithelium.

Tattooing may occur as close as 1 cm to and as far away as 1.3 m from the weapon but is generally found at distances of 60 cm or less. The density of the tattooing and associated pattern depends on the length of the barrel, muzzle-to-skin distance, type of gunpowder, presence of intermediate objects, and caliber and type of ammunition. Clothing, hair, or other barriers may prevent tattooing from occurring. The presence of partially or entirely unburned pieces of gunpowder and gunpowder residue on clothing or skin aids in determining the range of fire. On rare occasions, pieces of gunpowder can penetrate thin clothing and leave punctate abrasions ( Fig. e11.8 ).

Long-Range Wounds

The distant or long-range wound is inflicted from far enough away that only the bullet makes contact with the skin. There is no tattooing or soot. As the bullet penetrates the skin, the skin is indented, resulting in the creation of an abrasion collar, also termed an abrasion margin , abrasion rim , or abrasion ring ( Fig. e11.9 ). This collar is an abraded area of tissue that surrounds an entry wound as the result of friction between the bullet and epithelium. The width of the abrasion collar varies with the angle of impact. Most entrance wounds will have an abrasion collar. Entrance wounds on the palms and soles are exceptions because these wounds usually appear slit-like.

The abrasion collar is secondary to friction and is not the result of thermal changes associated with a hot projectile. The abrasion collar of a contact or close-range wound may be undetectable because hot gases and a flame have seared the tissue. When an abrasion collar is the only visible superficial clinical finding present, the term indeterminate range describes the range of fire. A wound inflicted from 10 feet will appear the same as a wound inflicted from 100 feet. An exact range cannot be determined with a distant wound.

Determining the range of fire may be complicated when clothing prevents the deposition of soot and powder on the skin. Without the overlying clothing or without information regarding the crime scene, the wound may appear to be from a distant range of fire. In reality, the range may have been close or intermediate. Conversely, a projectile discharged from a distant range of fire may mimic an intermediate range if it strikes an object, such as glass, that fragments. As with unburned gunpowder, when the glass fragments strike the skin, they may also cause punctate abrasions, resulting in pseudotattooing ( Fig. e11.10 ).

Atypical Entrance Wounds

Atypical entrance wounds occur when a bullet encounters an intermediate object, such as a window, wall, or door, before striking the victim. The intermediate object can cause the bullet to tumble and may change the its size, shape, or path. Such changes can result in entrance wounds with large stellate configurations that mimic close-range or contact wounds ( Fig. e11.11 ). Ricochet bullets may also cause atypical entrance wounds. Graze wounds are atypical wounds from tangential contact with a passing bullet.

Handgun Exit Wounds

Exit wounds are the result of a bullet pushing and stretching the skin from the inside out. The skin edges generally are everted with sharp but irregular margins. The size of the exit wound is determined by the energy transferred from the bullet to underlying tissue and by the bullet’s size and configuration as it exits the skin ( Fig. e11.12 ). Once a bullet enters the skin, its configuration may change from its usual nose-first attitude owing to tumbling and yaw. A bullet that exits the skin sideways, or one that has increased its surface area by mushrooming or transferring its energy to underlying bone, will have an exit wound larger than its entrance wound.

Atypical Exit Wounds

A shored exit wound is a wound that has an associated false abrasion collar. If the skin is pressed against or supported by a firm object or surface at the moment the bullet exits, the skin can be compressed between the exiting bullet and supporting surface ( Fig. e11.13 ). Examples of supporting structures include belts, floors, walls, doors, chairs, and mattresses.

Centerfire Rifle Wounds

Projectiles discharged from centerfire rifles have the potential to inflict massive tissue damage (see Fig. e11.2 ). Entrance wounds associated with high-velocity, centerfire projectiles do not significantly differ from those of handguns. Entrance wounds will generally exhibit abrasion collars or microtears on the skin surface ( Fig. e11.14 ). Wounds will also have associated soot deposition and tattooing but, because of a number of variables such as muzzle length, amount of powder in a given cartridge, muzzle configuration, and type of gunpowder, the range of fire in rifle wounds is not as clearly defined as in handgun wounds. The determination of an exact range of fire for rifles and shotguns is best established through controlled testing performed by a firearms examiner at a crime laboratory.

High-velocity bullets with jackets and lead cores generally break up into hundreds of fragments, termed a lead snowstorm , on entering tissue, resulting in significant tissue damage ( Fig. e11.15 ). If the tissue penetrated is deep, the bullet fragments may fail to exit and remain embedded. It is therefore possible to sustain an injury from a high-velocity round and not exhibit an exit wound. High-velocity rounds with steel cores will almost uniformly exit intact. Steel core ammunition is not available for civilian use so such injuries will rarely be encountered by most ED providers.

Shotgun Wounds

The massive damage caused by slugs may obliterate the abrasion collar usually associated with entrance wounds. Shotgun slugs will almost uniformly exit the body with large exit wounds. Tissue penetration is more limited with shotgun pellets as the projectile velocities are much lower, particularly if fired at long range. If fired at close range, however, the damage is much more significant, again depending on the elasticity of the tissue encountered.