Forensic ballistics and explosion injuries

Forensic ballistics and explosion injuries focus on identifying weapons and reconstructing traumatic events through the analysis of injury patterns. Firearms are classified by their internal barrel architecture into rifled weapons, which feature spiral grooves to stabilize a single bullet, or smooth-bored shotguns, which disperse multiple lead shots . Internal dimensions are measured by caliber for rifled weapons or gauge for smooth-bored ones, while the dispersion of lead shots is often controlled by "choking," or narrowing the barrel end . Distance determination depends on identifying discharge artifacts: flame causes scorching or burning, smoke results in wipeable blackening, and unburnt powder produces permanent tattooing or stippling on the skin . Forensic reconstruction distinguishes inverted entry wounds, marked by diagnostic grease collars (bullet wipe), from everted exit wounds where tissue protrudes . Projectile trajectory is confirmed through skull bevelling, where the inner table of the bone is more widely fractured in entry wounds and the outer table in exit wounds . Pathologists must also recognize atypical projectiles like ricochets, which often produce keyhole-shaped wounds, or tandem shots where two bullets exit the barrel back-to-back . Explosion injuries are categorized by the blast medium into airblasts, which cause primary barotrauma like "blast lung," underwater blasts involving gastrointestinal damage, or solid blasts resulting in complex fractures . Marshal’s triad—the presence of abrasions, contusions, and lacerations on the same body surface—is diagnostic of secondary injuries caused by flying debris .

Forensic ballistics is the study of firearms and ammunition to reconstruct shooting incidents and identify the weapons involved. Firearms are primarily classified based on the characteristics of the inner surface of their barrels.

Classification of Firearms

The inner surface of a firearm's barrel is analyzed using a helixometer to determine its classification and the expected trajectory of the projectile.

• Choking: The intentional narrowing of the terminal end of a shotgun barrel to reduce the dispersion of lead shots.
  • Full Choke: Provides the least dispersion and maximum range.
  • Cylinder Bore: Features no narrowing, resulting in maximum dispersion.
• Paradox Gun: A smooth-bore gun that features terminal rifling at the muzzle end to impart spin while maintaining dispersion control.

Ammunition and Gunpowder

The structure of ammunition varies based on the type of firearm and the desired effect on the target.

• Shotgun Cartridge Components:
  • Wad: A specialized component that travels approximately 2–5 meters; it functions to lubricate the barrel, seal air pressure (obturation), and separate the gunpowder from the lead shots.
  • Lead Shots: Multiple projectiles that disperse upon exiting the barrel.
• Rifle Bullet Components:
  • Bullet: A single projectile placed directly over the gunpowder; wads are absent in rifle ammunition.
• Primer: The initiating explosive that ignites the gunpowder.
  • Components: Barium nitrate, Lead peroxide, Antimony sulfide, Styphnate (lead), and Tetrazine.

For the components of a primer, remember, BLAST: Barium nitrate, Lead peroxide, Antimony sulfide, Styphnate (lead), and Tetrazine.

Gunpowder Types

Gunpowder provides the energy required to propel the projectile out of the barrel and leaves characteristic residue at close ranges.

• Fineness: Gunpowder is graded from FG (coarse) to FFFFG (fine). Increased fineness leads to a higher burning rate and greater energy release.

The appearance of a gunshot wound is dictated by the distance between the weapon and the victim, as well as the specific discharge products emitted during firing. Forensic evaluation utilizes these artifacts to categorize the range of fire.

Discharge Products

The firing of a weapon results in the sequential emission of several products, each leaving a characteristic forensic signature.

Comparative Ranges of Fire

The distance at which specific discharge products affect the skin varies between rifles and shotguns.

Wound Characteristics by Range

Range determination is classified based on the presence or absence of specific discharge markers.

• Contact Shot:
  • Tight Contact: The muzzle is pressed firmly against the skin. All discharge products enter the wound.
  • Bony Prominence: On areas like the skull, gas expansion between the skin and bone causes a stellate or cruciate (star-shaped) tearing of the margins.
• Close Shot: Characterized by the presence of burning, blackening, and tattooing.
• Near Shot: Burning is absent, but blackening and tattooing remain visible.
• Intermediate Shot: Burning and blackening are absent; only tattooing is present around the entry wound.
• Distant Shot: Only the bullet wound and its collars are present. No discharge artifacts are visible.

Shotgun Specifics: Lead Shot Dispersion

In shotgun injuries, the range is further refined by the degree of dispersion of the lead shots.

• Near range: Lead shots enter as a single mass, but the margins of the central hole show scalloping.
• Intermediate range: A central hole is present, surrounded by independent satellite holes created by dispersing pellets.
• Distant range: Complete dispersion occurs; each pellet creates an independent puncture wound with no central hole.

The forensic reconstruction of a shooting incident relies on the systematic evaluation of wound morphology to differentiate entry from exit points and to determine the trajectory of the projectile.

Entry vs. Exit Wounds

The primary objective of wound examination is to establish the direction of fire by identifying the characteristics of the entry and exit points.

• Exception: A contact shot on a bony prominence, such as the skull, may result in an entry wound with everted margins due to the rapid expansion of gases between the skin and the bone.

Reconstruction of Trajectory

The path taken by a bullet through the body is determined by analyzing the shape and location of the wounds.

• Abrasion Collar: The shape of the outer (abrasion) collar provides information on the trajectory of the bullet.
  • A circular collar suggests a perpendicular (90°) impact.
  • An oval or eccentric collar suggests an oblique angle of fire.
• Bevelling of the Skull: In cases of gunshot wounds to the head, the skull bones provide a definitive indicator of the direction of fire.
  • Entry wound: Characterized by inner table bevelling, where the inner surface of the skull is more widely fractured than the outer surface.
  • Exit wound: Characterized by outer table bevelling, where the outer surface of the skull is more widely fractured than the inner surface.

Specialized Ballistic Wound Patterns

• Tandem (Piggy-back) Wounds: Occur when two bullets exit the barrel simultaneously, often resulting in a single, large entry wound that does not correspond to the caliber of the suspected weapon.
• Keyhole-shaped Entry: Typical of a ricochet bullet that has lost its gyroscopic stability and strikes the body at an irregular angle.
• Balling/Welding of Shots: In shotguns, lead shots may fuse into a compact mass, creating a large, oval entry wound that mimics a rifled projectile, surrounded by several small puncture wounds.

Forensic ballistic investigation utilizes specialized laboratory techniques to identify weapons, detect discharge residues, and reconstruct atypical shooting scenarios.

Bullet Fingerprinting

Bullet fingerprinting is the process of identifying a specific firearm by analyzing the markings left on a fired projectile.

• Primary Markings: Macroscopic markings produced by the rifling (lands and grooves) in the barrel. These indicate the make and model of the firearm.
• Secondary Markings: Microscopic markings caused by unique irregularities, imperfections, or wear within the individual barrel. These are considered the individual fingerprint of the gun and are unique to each weapon.
• Comparison Microscopy: The primary tool used to compare a crime bullet (recovered from the scene or victim) with a test bullet (fired from the suspected weapon into a recovery tank).

Gunshot Residue (GSR) Investigation

GSR consists of chemical discharge products deposited on the hands, face, or clothing of the individual who fired the weapon.

• Detection Methods (Mnemonic: HANDS):
  • Harrison Gilroy test
  • Atomic absorption spectrometry
  • Neutron activation analysis
  • Dermal nitrate test (often used for screening)
  • SEM–EDXA (most specific and definitive test)
• Collection: Swabs are typically collected from the areas of maximum deposition, such as the web space between the thumb and index finger.

Evidence Handling and Iatrogenic Alterations

The integrity of ballistic evidence is critical for the success of an investigation.

• Evidence Handling: Fired bullets must be handled using gloved hands or rubber-tipped tweezers. The use of standard toothed forceps is strictly contraindicated, as they may create additional microscopic markings that interfere with bullet fingerprinting.
• Kennedy Phenomenon: The iatrogenic (surgical) alteration of a gunshot wound during emergency medical care (e.g., placing a chest tube or debriding a wound margin). This can significantly complicate forensic range determination and trajectory reconstruction.

Atypical Bullets and Projectile Behavior

Atypical ballistic behaviors result from specialized bullet designs, flight instabilities, or interactions with intermediate surfaces. These patterns often produce unique wound morphologies that differ from standard entry and exit wounds.

Atypical Bullet Designs

Specialized projectiles are engineered to produce specific lethal or forensic effects.

Anomalous Flight and Impact Behavior

Instabilities in flight or deflections before impact alter the expected wound trajectory and shape.

• Tandem (Piggy-back): Occurs when two bullets are fired one after the other and exit the barrel simultaneously (typically due to a lodged projectile being struck by a subsequent shot). This often produces a single, large, irregular entry wound that exceeds the suspected caliber.
• Ricochet: A bullet that strikes and deflects off an intermediate surface (e.g., bone, metal, or concrete) before entering the body.
  • Wound Morphology: Typically lacks a traditional abrasion collar and often produces a characteristic keyhole-shaped entry wound.
  • Range Effects: Effects of flame, smoke, and tattooing are absent, regardless of the actual proximity of the firearm.
• Yawning and Tumbling:
  • Yawning: The bullet travels with its longitudinal axis at an angle to its flight path.
  • Tumbling: The bullet rotates end-over-end in flight or upon entering tissue.
  • Both result in large, irregular entry wounds and extensive internal lacerations.
• Billiard Ball Ricochet Effect: In shotgun injuries, pellets may strike an intermediate object or each other, causing a scattered dispersion pattern that mimics a distant-range shot even when fired from a closer range.

The forensic investigation of an explosion focuses on the reconstruction of the event, the determination of the victim's orientation relative to the blast, and the identification of the explosive medium.

Forensic Classification by Blast Medium

The nature of the surrounding environment significantly dictates the resulting pathological patterns.

• Airblast: The most common explosion type; energy is transmitted through the air as a supersonic overpressure wave.
  • Ear (Most common): Characterized by traumatic tympanic membrane rupture.
  • Lung (Most fatal): "Blast lung" syndrome involving alveolar hemorrhage and pulmonary edema.
• Underwater Blast: Due to the relative incompressibility of water, energy is transmitted with high efficiency directly to internal organs.
  • Characteristic injury: Massive damage to the gastrointestinal tract (GIT) and other gas-containing hollow viscera.
• Solid Blast: Energy is transmitted through solid structures (e.g., floors or hull of a vehicle).
  • Characteristic injury: Multiple, complex, often symmetrical skeletal fractures of the lower limbs and pelvis.

Injury Reconstruction and Eponyms

Forensic pathologists utilize specific injury patterns to establish the mechanism of trauma and the victim's proximity to the epicenter.

• Marshal’s Triad: The diagnostic presence of punctate abrasions, contusions, and lacerations on the same surface of the body. This triad is a hallmark of secondary blast injuries caused by flying missiles and debris.
• Orientation and Proximity:
  • Mapping the distribution of flash burns and debris embedded in the skin allows for the determination of which side of the victim was facing the explosion.
  • The degree of tympanic membrane damage serves as a biological indicator of the pressure level at the victim's location.
• Traumatic Asphyxia (Quaternary Injury): Often results from the structural collapse of buildings following an explosion, presenting with the characteristic masque ecchymotique (pale chest with a cyanosed face).

Forensic Evidence Collection

A critical component of the postmortem examination in explosion cases is the recovery of trace evidence for criminalistic analysis.

• Fragment Recovery: All foreign objects (e.g., metal fragments, wires, or building materials) recovered from the body must be carefully labeled and preserved to assist in identifying the composition and design of the explosive device.
• Residue Analysis: Swabs of the skin and clothing are collected to test for volatile chemical residues from the explosive agent.
• Postmortem Gas Levels: Analysis of carboxyhemoglobin (COHb) and cyanide levels is performed to determine if the victim inhaled toxic gases post-explosion, indicating they were alive in the immediate aftermath.