Wound Ballistics: Entry Wounds, Exit Wounds and Cavitation

A typical distant-range entry wound (more than 60 cm from muzzle to skin in most calibres) shows three distinct zones around a central perforation. The innermost zone is the perforation itself, approximately the calibre diameter or slightly smaller due to elastic skin recoil after the bullet passes. The second zone is the abrasion collar (also called the abrasion ring or contusion ring), a rim of reddish-brown dried abrasion surrounding the perforation. The third zone, seen only with bullets that have passed through a fouled barrel or that carry lubricant on their surface, is the grease ring (also called the dirt ring or fouling ring), a grey-to-black discolouration at the outer margin of the abrasion collar.

The abrasion collar forms because the bullet, as it pushes through the skin, stretches the skin inward in the direction of travel before perforation occurs. The skin, being elastic, stretches ahead of the bullet and then snaps back around it as it passes. The stretched skin that was in contact with the bullet's surface and shoulder is abraded, dried, and left as the collar. Its width and symmetry carry diagnostic information. A symmetrical collar of uniform width indicates perpendicular bullet impact. An asymmetric collar (wider on one side) indicates an oblique angle of impact, with the widest side pointing toward the direction from which the bullet came. This angular information is directly used in trajectory reconstruction. The range of firing can also be read from the wound's soot and stippling pattern.

The grease ring, when present, is caused by lubricant and metal fouling on the bullet's surface being wiped off as the bullet passes through the skin. It sits just outside or within the abrasion collar and appears as a diffuse grey-to-dark ring. In post-mortem examination, the grease ring must be distinguished from soot deposition, which appears as a heavier, more concentrated black deposit and is only present at close-range firing. DiMaio's Chapter 3 provides photographic standards for each, and the UK Forensic Science Regulator's guidance on gunshot wound examination references DiMaio as the interpretive authority.

In India, the CFSL Post-Mortem Protocol (incorporated into AIIMS forensic medicine teaching from 2018) specifies that all gunshot entry wounds must be photographed with a scale bar before and after cleaning, and that the abrasion collar dimensions must be recorded in millimetres in all four quadrants (12 o'clock, 3 o'clock, 6 o'clock, 9 o'clock). This protocol is modelled on the FBI scene protocol and the UK Home Office Science Group (HOSDB) post-mortem documentation standard.

Exit wounds form where the bullet (or its secondary fragments) breaks out of the skin from the inside. The bullet at exit carries residual kinetic energy that pushes the skin outward, stretching it until it tears. Because the skin has less support from underlying tissue at the point of exit than at the point of entry, and because the skin may be pushed outward for several millimetres before tearing, the resulting wound edge is typically irregular, stellate, or slit-like rather than the round punched perforation of a typical entry wound. Classically, exit wounds are described as having everted (outward-turned) edges, though in practice the distinction from entry wounds in mobile skin is not always obvious on gross inspection alone.

Shored exit wounds represent the critical exception to this classical description. When the exiting portion of skin is pressed firmly against a hard surface at the moment of exit (a car seat, a wall, a belt, a shoe sole, a concrete floor), the skin cannot evert outward. The surface acts as a brace (a "shore"). The result is an exit wound that has an abrasion rim caused by the supporting surface, mimicking the abrasion collar of an entry wound. DiMaio addresses shored exit wounds in Chapter 4, noting that they have led to incorrect entry/exit wound assignments in court testimony when the pathologist did not attend the scene and did not know the decedent's position at the time of death. The medico-legal autopsy procedure specifies the documentation steps required to capture all contextual information needed to make this distinction reliably.

Shored exit wounds have led to incorrect entry/exit wound assignments in court testimony in multiple documented proceedings when the pathologist did not attend the scene and did not know the decedent's position at the time of death, as DiMaio notes in Chapter 4 as an instructive example of the medicolegal consequence of failure to correlate wound morphology with positional evidence from the scene.

The opposite of shoring is a blow-out exit wound, which occurs when the exiting bullet carries high residual energy and the overlying skin is unsupported. The result is a large, ragged wound with radiating tears extending outward from a central perforation, sometimes removing a section of skin entirely. Blow-out morphology is most commonly seen with high-velocity rifle rounds and with contact-range shotgun discharges. In the 2008 Mumbai attacks, the terminal post-mortem reports from JJ Hospital Mumbai described blow-out exit wounds consistent with 7.62x39mm AK rifle rounds at close range, and this morphology supported the reconstruction of firing distances documented in the subsequent forensic analysis.

The permanent cavity is the wound channel remaining after the bullet has come to rest or exited the body. It is what the pathologist sees when dissecting the wound track: crushed and lacerated tissue forming a cylindrical or irregular tunnel along the bullet's path. At handgun velocities, the permanent cavity diameter approximates the expanded diameter of the projectile. This is what most forensic pathology teaching focuses on, because it is the directly observable finding.

The temporary cavity is an entirely different phenomenon. As a high-velocity bullet decelerates in tissue, it transfers energy radially as well as along its axis of travel. The surrounding tissue, being fluid-filled and viscoelastic, is displaced radially outward from the bullet's path at velocities that briefly but dramatically exceed the permanent channel radius. This temporary radial displacement can reach three to five times the projectile diameter in magnitude for rifle rounds above 750 m/s. The cavity then collapses back as the tissue's elastic properties restore approximately its original position.

The mechanical damage left by the temporary cavity is not the cavity itself (which has already closed by the time the pathologist examines the tissue) but the tearing at the margins of the zone of maximum displacement. Where blood vessels, fascial planes, and nerve trunks cross the margin of the temporary cavity, they are subjected to rapid stretch beyond their elastic limit and rupture. The resulting haemorrhage, at a distance from the permanent channel, is the diagnostic signature in tissues with high fluid content (liver, spleen, brain) but is less pronounced in tissues with high elasticity (lung, muscle).

DiMaio provides quantitative estimates of temporary-cavity diameter relative to permanent-cavity diameter across commonly encountered projectile types. At 9mm 124 gr FMJ velocities (approximately 370 m/s from a typical service pistol), the temporary cavity diameter is approximately 2-3x the permanent cavity. At 5.56mm M193 velocities (approximately 960 m/s from a 20-inch barrel), the temporary cavity at peak expansion reaches 10-12 cm in diameter in calibrated gelatin, while the permanent cavity diameter is approximately 0.9 cm before fragmentation. The ratio is the key: the forensic examiner who sees haemorrhage at a distance from the wound channel in a liver wound should consider a high-velocity rifle round even if the permanent channel appears relatively narrow at that depth.

In the UK, HM Coroner inquests involving military firearms deaths (such as inquest findings from the 2003-2011 Iraq deployments, published by the coroner for Oxfordshire) have required expert witnesses to address the temporary-cavity dimension explicitly, because witnesses or co-soldiers may have observed the immediate behaviour of the body at impact (a sudden backward displacement, a convulsive movement) that corresponds to the mechanical effect of the large temporary cavity rather than to permanent injury.

An intermediate target is any object through which a bullet passes before striking the victim. Intermediate targets include clothing (which reduces velocity and may initiate, partially inhibit, or prevent JHP expansion), vehicle glass and sheet metal (which can fragment the bullet and alter its trajectory), and biological targets (a limb or the body of another person). When a bullet passes through an intermediate target it typically loses velocity, may deform, and may change direction. The resulting entry wound in the primary victim may lack a clean abrasion collar (if the bullet is tumbling), may show an atypical shape (if the bullet has deformed), or may contain trace materials from the intermediate target (glass particles, fabric fibres, paint chips) that serve as physical evidence of the intermediate barrier.

Ricochet wounds arise when a bullet deflects from a hard surface (concrete, asphalt, rock, steel) before striking a victim. The deflected bullet is typically deformed, moving at reduced velocity, and at a changed trajectory angle. The resulting wound has several characteristic features: the bullet is often mushroomed or fragmented on one side (the ricochet face), the wound entry may be elongated or irregular in shape, and the abrasion collar may be absent or atypical because the bullet is not spinning in its original orientation. Ricochet is discussed extensively in Hueske's Practical Analysis and Reconstruction of Shooting Incidents (CRC Press, 2015) and is a specific topic in the FBI firearms examiner certification curriculum. The UK Forensic Science Regulator's guidance on trajectory analysis addresses ricochet reconstruction as a distinct technical domain.

Bone fragmentation as secondary missile is one of the most clinically significant phenomena in gunshot wound pathology. When a bullet strikes a major bone (femur, pelvis, skull, humerus), the fracture propagates at high velocity and the resulting bone fragments are themselves projectiles, carrying the kinetic energy transferred from the bullet. These secondary bone-fragment missiles can lacerate blood vessels and nerves at distances from the primary wound channel where the bullet never travelled, creating satellite wound tracts that are invisible to a pathologist who has not followed each tract to its terminus.

The forensic implication is direct: a wound track that appears to end prematurely, or that shows multiple diverging damage channels beyond a bone, should trigger a full radiological survey before dissection. The Indian CFSL post-mortem protocol mandates full-body radiograph before dissection in all gunshot deaths, consistent with the FBI protocol and the UK Home Office Science Group standard (HOSDB 60/05). This step identifies secondary bone fragments and retained bullet components before they are disturbed by dissection.

DiMaio's Gunshot Wounds and Spitz and Fisher's Medicolegal Investigation of Death are the two most commonly cited reference texts in gunshot wound expert testimony internationally. DiMaio, a former Chief Medical Examiner for Bexar County, Texas, grounded his work in thousands of post-mortem examinations and in gelatin data from the Wound Ballistics Laboratory at the Army's Letterman Army Institute of Research, where Dr. Martin Fackler served as founder and director from 1981 to 1991 and developed the 10% ballistic gelatin standard. Spitz and Fisher's text, now in its fourth edition, is a multiauthor reference covering all aspects of medicolegal death investigation, with the gunshot wound chapters representing the consensus view of the US medical examiner community.

The UK's analogous reference is the Forensic Pathology chapter of Knight's Forensic Pathology (fourth edition, CRC Press, 2016, edited by Saukko and Knight), which explicitly cross-references DiMaio and adapts the US classification system for UK coronial and Crown Court contexts. The German-language reference used in EU continental forensic medicine is Sellier and Kneubuehl's Wundballistik und ihre ballistischen Grundlagen, with the English translation serving as the secondary source in Germany, Austria, Switzerland, and the Netherlands.

In India, the Textbook of Medical Jurisprudence and Toxicology by Jaising Modi (thirteenth edition) and the Handbook of Forensic Medicine and Toxicology by Parikh are the primary teaching references, but both cite DiMaio and Spitz-Fisher on wound ballistics specifically. CFSL expert witnesses typically cite both Indian references for legal context and DiMaio for wound-ballistics technical detail, a convention that reflects the international consensus that DiMaio is the discipline's primary technical authority regardless of jurisdiction.

The reconstruction workflow that DiMaio, Knight, and CFSL expert-witness guidelines all describe follows a common sequence: (1) document all wounds with photography and dimensional measurements before any procedure; (2) conduct full-body radiological survey; (3) probe wounds with a flexible probe to establish track direction before dissection; (4) dissect tracks methodically, mapping all satellite tracts; (5) recover all projectile material with non-ferrous instruments (to preserve GSR and prevent artifactual marking); (6) map wound tracks onto a body diagram with entry angles noted; (7) correlate wound tracks with scene evidence to reconstruct the firing position. This protocol is identical in its essential structure across the FBI guidelines, the UK Forensic Science Regulator's Codes of Practice, and the CFSL post-mortem protocol, reflecting a genuine international convergence in medicolegal gunshot wound practice.