FM6.1-2,FM7.1-2,FM14.11 | Firearm, Blast & Regional Injuries — Graded Quiz

Contact range firing drives combustion gases (including carbon monoxide) deep into the wound track and cranial cavity. CO binds haemoglobin to form carboxyhaemoglobin, producing cherry-red discolouration — a pathognomonic contact-wound finding.

Contact gunshot wound hallmarks: stellate tearing (gas expansion), soot in wound depths, muzzle mark/imprint, cherry-red carboxyhaemoglobin in tissue. These features are absent at intermediate/distant range.

Cherry-red brain discolouration in a contact gunshot wound is caused by carboxyhaemoglobin (CO from propellant gases forced into the tissue). This is NOT seen at intermediate or distant range where gases dissipate before reaching the target.

A set trigger (or hair trigger) is a specialised mechanism that pre-tensions the sear so the trigger releases with minimal force — used in target shooting for precision. Forensically important when accidental discharge is claimed.

Trigger pull weight is forensically relevant. A light 'set trigger' supports accidental discharge defences; a stiff factory trigger makes accidental firing less credible. Always record trigger pull force at autopsy/scene examination.

A set trigger reduces trigger pull to a feather-light weight, meaning accidental discharge with minimal force is plausible. This has direct medico-legal relevance when assessing claims of unintentional firing.

Bilateral tympanic membrane ruptures and pulmonary contusions/haemorrhages without external penetrating wounds are the hallmark of primary blast injury — caused by the overpressure wave acting on air-filled cavities.

Primary blast lung (pulmonary barotrauma): bilateral contusions, haemorrhage, pneumothorax — without surface marks. Tympanic membrane perforation at <35 kPa is the most sensitive biological indicator of primary blast exposure.

Primary blast injury specifically targets air-filled organs: tympanic membranes (most sensitive), lungs (blast lung — the most lethal primary injury), sinuses, and bowel. The absence of penetrating wounds distinguishes it from secondary blast injury.

Entry is occipital-right, exit is frontal-left. The bullet therefore travelled right-to-left (laterally), posterior-to-anterior, with a slight upward angulation — reconstructed from entry-to-exit track direction.

Reconstruct bullet trajectory by connecting entry (smaller, abrasion collar) to exit (larger, irregular). Direction, laterality, and inclination help determine shooter position relative to victim — key in reconstruction of homicide/suicide.

Bullet trajectory: entry → exit gives direction. Occipital-right entry, frontal-left exit = right-to-left, posterior-to-anterior travel. Vertical component (up/down) depends on the precise heights of entry and exit wounds.

The 'bumper bruise' height on the victim's leg corresponds to the vehicle's bumper height at impact — critical for identifying vehicle type (car vs. truck) and confirming witness accounts in road traffic offence investigations.

Vehicular injury reconstruction: bumper height (identifies vehicle type), tyre-tread pattern (identifies specific tyre), paint transfer (vehicle matching), dragging injuries (horizontal abrasions), and secondary injuries from fall.

Bumper bruise level = bumper height at impact. This is used to identify the class of vehicle involved and to corroborate or refute witness/accused statements about the vehicle involved.

Traumatic aortic tear at the isthmus (just distal to the left subclavian artery, at the ligamentum arteriosum) is the classic deceleration injury. The mobile aortic arch decelerates rapidly while the descending aorta is tethered, creating shear at the isthmus.

Traumatic aortic isthmus tear: mechanism = deceleration shear; location = just distal to left subclavian artery at ligamentum arteriosum; accounts for ~90% of traumatic aortic injuries in RTA; immediate fatal haemorrhage in ~80% without intervention.

The isthmus tear is a deceleration-shear injury, not a direct compression injury. The ligamentum arteriosum tethers the aorta at this point; when the body decelerates suddenly, tensile/shear forces tear this transition zone.

Contre-coup injury is brain contusion/laceration at the site opposite to the point of impact. It results from cavitation: the skull accelerates away from the brain at impact, creating a negative pressure zone on the far side.

Coup-contrecoup: coup injury is at the impact site; contrecoup is opposite. Contrecoup is often worse than coup injury. Contre-coup predominates in falls where the head decelerates suddenly (occipital impact → frontal lobe contusion).

Contre-coup = injury opposite the impact site, due to cavitation (negative pressure) in the CSF as the skull moves away from the brain. Coup = injury at impact site. Both may coexist (coup-contrecoup pattern).

High-velocity bullets transfer energy rapidly, pushing tissue radially outward at high speed — creating a transient large cavity (temporary cavity) many times the bullet diameter. This crushes and tears tissue far beyond the narrow permanent track.

Wound ballistics: permanent cavity = tissue directly crushed by bullet; temporary cavity = transient radial displacement (high-velocity only). Wounding capacity depends on kinetic energy transferred (½mv²) — doubling velocity quadruples energy.

The temporary cavity (unique to high-velocity projectiles) is the radial expansion-collapse cycle of tissue around the bullet track. It destroys tissue well beyond the permanent cavity, explaining why high-velocity wounds are far more destructive than the bullet diameter suggests.

Histology confirms abrasion collar (grease collar) microscopically at entry. On the skull, entry bevels inward (inner table wider) and exit bevels outward (outer table wider) — the most reliable anatomical indicator when surface wounds are atypical.

Skull: internal bevelling = entry; external bevelling = exit. Soft tissue: abrasion collar (histology) = entry. These two methods are complementary for determining entry/exit when surface morphology is ambiguous.

When surface wounds are atypical (tangential, intermediate-range, defleshed), skull bevelling direction is the most reliable indicator: internal bevel = entry; external bevel = exit. Histology confirms abrasion collar at entry.

The triad of bilateral calcaneal fractures (foot-landing axial load), lumbar burst fracture (axial compression transmitted up the spine), and bilateral Colles' fractures (outstretched hands catching the fall) is the classic pattern of landing on feet after ejection.

Ejection injury pattern: calcaneal fractures (feet landing) + lumbar burst fracture (axial load up spine) + Colles' fracture (outstretched hands). Reconstruct the sequence of events from injury pattern for road accident reconstruction.

Bilateral calcaneal + spinal burst + bilateral wrist fractures = ejection-and-feet-landing injury pattern. Calcaneal fractures are pathognomonic of axial foot-landing force; the spine fails in axial compression; wrists fail in extension impact.