Forensic ballistics

Forensic ballistics focuses on identifying the weapons and ammunition used in traumatic events by analyzing injury patterns. Based on their internal barrel architecture, firearms are usually classified as rifled weapons, which have spiral grooves to stabilize a single bullet, or smoothbore shotguns, which disperse multiple lead shots. Internal dimensions are measured in caliber for rifled weapons and gauge for smoothbore ones. The dispersion of lead shots is often controlled by choking or narrowing the barrel end. Determining the distance between the weapon and the victim depends on identifying discharge artifacts: Flames cause scorching or burning, smoke results in wipeable blackening, and unburnt powder tattoos (stipples) the skin. The forensic reconstruction of a shooting incident relies on the systematic evaluation of wound morphology to differentiate entry wounds (inverted borders; marked by diagnostic grease collars that are also called "bullet wipe") from exit wounds (everted borders; involving tissue protrusion), and to determine the trajectory of the projectile. Atypical ballistic behaviors (e.g., ricochets, tandem shots) resulting from specialized bullet designs, flight instabilities, or interactions with intermediate surfaces may produce unique wound morphologies that differ from standard entry and exit wounds.

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

Using a helixometer, firearms are classified by the inner surface of the barrel 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: the least dispersion and maximum range
  • Modified choke: moderate narrowing of the barrel, balancing shot dispersion and range
  • Improved cylinder choke: slight narrowing of the barrel, causing relatively wide dispersion with a modest increase in range compared with a cylinder bore
  • Cylinder bore (unchoked): no narrowing, resulting in maximum dispersion
• Paradox gun: a smoothbore gun with broad and shallow rifling in the chokes of the barrels, retaining the ability to fire both shots and bullets

Ammunition and gunpowder

Ammunition

• Shotgun cartridge
  • Wad
    • A specialized component that travels approximately 2–5 meters
    • Lubricates the barrel, seals air pressure (obturation), and separates the gunpowder from the lead shots
  • Lead shots: multiple projectiles that disperse upon exiting the barrel
• Rifle bullet
  • A single projectile placed directly over the gunpowder
  • Rifle ammunition does not contain wads.
• Primer
  • The initiating explosive that ignites the gunpowder
  • Both rifle bullets and shotgun cartridges contain primer.
  • Components: barium nitrate, lead peroxide, antimony sulfide, styphnate (lead), and tetrazine

Gunpowder

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

Gunpowder types

Fineness

• Gunpowder is graded from FG (coarse) to FFFFG (fine).
• The finer the gunpowder, the higher the burning rate and the greater the energy release.

The appearance of a gunshot wound is determined by the range of fire (the distance between the weapon and the wound), 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 trace.

Wound characteristics by range

Rifle shot

Discharge markers are used to determine the range of fire.

• Contact shot
  • Tight contact
    • The muzzle is pressed firmly against the skin, causing muzzle imprinting.
    • All discharge products enter the wound.
  • Bony prominence: On areas such as the skull, gas expansion between the skin and bone causes a stellate or cruciate (star-shaped) tearing of the margins.
• Close shot: characterized by burning, blackening, and tattooing
• Near shot
  • No burning
  • Visible blackening and tattooing
• Intermediate shot
  • No burning or blackening
  • Tattooing around the entry wound
• Distant shot
  • Only the bullet wound and its collars are present.
  • No discharge artifacts are visible.

Shotgun shot

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

• Contact shot: similar to rifle shot, as all lead shots enter a single entry wound
• Close shot: similar to rifle shot, as all lead shots enter a single entry wound
• Near shot: Lead shots enter as a single mass, but there is scalloping of the central hole margins.
• Intermediate shot: a central hole surrounded by independent satellite holes created by dispersing pellets
• Distant range
  • No central hole
  • The lead shots are completely dispersed, with each pellet creating an independent wound.

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 skin and bone.

Reconstruction of trajectory

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

• Abrasion collar
  • The shape of the wound's 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.
• Beveling of the skull
  • In gunshot wounds to the head, the skull bones provide a definitive indicator of the direction of fire.
  • Entry wound: characterized by inner table beveling, in which the inner surface of the skull is more widely fractured than the outer surface
  • Exit wound: characterized by outer table beveling, in which the outer surface of the skull is more widely fractured than the inner surface

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

Bullet fingerprinting

• Bullet fingerprinting is used to identify a specific firearm by analyzing the markings left on a fired projectile.
• Comparison microscopy: the main tool used to compare a bullet recovered from a crime scene or victim with a test bullet fired from the suspected weapon into a recovery tank
  • Primary markings
    • Macroscopic markings produced by the rifling (lands and grooves) in the barrel
    • These markings indicate the make and model of the firearm.
  • Secondary markings
    • Microscopic markings caused by unique irregularities, imperfections, or wear within the individual barrel
    • These markings are considered the fingerprint of the gun because they are unique to each weapon.

Evidence handling and iatrogenic alterations

• The integrity of ballistic evidence is critical for the success of an investigation.
• Bullet handling
  • Fired bullets must be handled using gloved hands or rubber-tipped tweezers.
  • The use of standard toothed forceps is absolutely contraindicated, as they may create additional microscopic markings that interfere with bullet fingerprinting.
  • Kennedy phenomenon
    • The iatrogenic alteration of a gunshot wound during emergency medical care (e.g., placing a chest tube or debriding a wound margin)
    • Can significantly complicate forensic range determination and trajectory reconstruction

Gunshot residue investigation

• Gunshot residue (GSR) consists of chemical discharge products deposited on the hands, face, or clothing of the individual who fired the weapon.
• Swabs are typically collected from the areas of maximum deposition: the dorsum of the hand, particularly the metacarpophalangeal region of the index finger (trigger finger); the dorsum of the thumb; and the first interdigital web space.

Detection methods

• SEM–EDX (scanning electron microscopy with energy dispersive x-ray spectroscopy)
  • An imaging and analytical technique used to visualize surface topography and determine the elemental composition of materials
  • Utilizes an electron beam to generate high-resolution images and characteristic x-rays for elemental analysis
  • Detects characteristic GSR particles (lead, barium, antimony) by their morphology
  • Most specific and confirmatory test
• Neutron activation analysis: highly sensitive technique that identifies trace elements in GSR by measuring radiation emitted after neutron exposure
• Harrison-Gilroy test: colorimetric test to detect lead, barium, and antimony in GSR samples
• Atomic absorption spectrometry: quantitative detection of metallic elements in GSR samples using absorption of light by free gaseous atoms at element-specific wavelengths
• Dermal nitrate test
  • Screening test for nitrates on the skin (e.g., on the hands)
  • Low specificity (false positives from environmental sources)

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 bullets

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

Atypical projectile behaviors

Deflections or flight instability before impact alter the expected wound trajectory and shape.

• Ricochet
  • A bullet that strikes and deflects off an intermediate surface (e.g., bone, metal, or concrete) before entering the body
  • Wound morphology: typically does not have a traditional abrasion collar and often produces a characteristic keyhole-shaped entry wound
  • Range effects: Tattoing and the effects of flames and smoke 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.
• Tandem (piggy-back)
  • Two bullets are fired one after the other.
  • Results in a single, large, irregular entry wound that exceeds the suspected caliber
• Billiard ball ricochet effect: In shotgun injuries, pellets may strike an intermediate object or each other, resulting in a scattered dispersion pattern that mimics a distant-range shot even when fired from close
• 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.

Retained projectiles

• Trajectory: may lodge in deep tissues and follow unpredictable paths, including deflection by bone or migration along vascular channels
• Encapsulation: The body typically walls off retained bullets with a dense fibrous capsule, limiting systemic lead absorption.
• Complications: no encapsulation or location of retained material in biological fluids (e.g., synovial fluid, cerebrospinal fluid) leads to chronic lead poisoning
• Expulsion: In rare cases, projectiles may be naturally expelled via the respiratory or gastrointestinal tract (e.g., via coughing, vomiting, defecation).