Where KE = Kinetic energy, M = the mass of the bullet, and V = maximum velocity.
When a bullet strikes tissue (e.g. the human body), wounding energy is transferred to the body according to the following formula:
where V entry is the velocity on entry and V exit is the velocity on exit.
The barrel can be either smooth bore or rifled. The rifling comprises alternating grooves and raised areas (lands) along the length of the barrel. These give the bullets a gyroscopic stabilising motion as they spin along the barrel.
Bullets can be made from lead, or steel, and may be covered with an external, usually copper, jacket. The bullet can be designed to deform on impact, increasing its cross-sectional resistance and imparting more energy to what it hits, or can be designed to be unstable in flight so it can present a greater cross section on impact with the target. In principle, the heavier the bullet (the greater the mass), the more energy it will have. This is linear – twice the mass, twice the kinetic energy.
A shotgun is designed to fire multiple tiny balls, 2–4 mm in diameter, which collectively have a very large amount of energy, though individual projectiles have limited penetration. However these projectiles, when fired, can spread over a wide area of the target, causing significant damage.
Crowd control ammunition can be made with rubber or plastic or rubber-coated metal bullets. These special rounds are heavy, intended for minimal penetration and have a low velocity. They are supposed to be fired at a distance and preferably aimed at the ground, bouncing upwards to the torso. Close-range use of rubber bullets directly at individuals may be very harmful.
The initial velocity of the projectile is very important. All bullets will have a shock wave ahead of the bullet itself. This is related to the speed of the bullet, and the energy is exponential – twice the velocity, results in four times the energy delivered.
The behaviour of projectiles fired from a weapon can be classified as follows:
24.1.1 Internal Ballistics
It describes the movement of the projectile through the gun.
Once the firing pin is struck, it transmits the force to the primer which ignites the gunpowder that propels the projectile through the barrel of the gun, using the pressure generated from the flame and the gases produced. The length of the barrel and its spiral design will influence the behaviour and the speed of the projectile. The groves and the lands on the barrel will reflect on the bullet itself, leaving a ‘fingerprint’ characteristic of that individual weapon, and it is possible to identify the weapon used from these marks.
Different guns have specific properties that may be of use in determining from which gun a bullet was fired. Guns may have peculiar impressions in the barrel that are transferred to the bullet as it passes along the length of the barrel. A comparison microscope is used to compare the pattern from a test-fired bullet and the one found at the scene or recovered from the victim (Fig. 24.1a, b).
(a) The appearance of a gun barrel showing lands and grooves. (b) Deformed bullet retrieved for ballistic test. See clear markings that may assist in linking it to a suspected firearm
Where possible, bullets should be recovered. Care should be exercised to minimise damage to the retrieved bullet. Meticulous dissection and avoidance of contact with metal forceps will lessen artefactual marks on the bullet. This will help at the time of ballistic identification so as not to obscure potential class or specific gun marks left on the bullet. The bullet must be properly sealed, labelled and dated with the site of retrieval indicated and names of treating physician specified.
24.1.2 External Ballistics
It describes the travel of the projectile outside of the barrel through the air or a different medium, on its way to the target. Bullets are inherently unstable, as most of the mass lies at the rear of the bullet. The bullet spirals because of the rifling of the barrel, and the instability can be characterised as:
Yaw: Rocking from side to side.
Yawing represents deviation of the bullet in its longitudinal axis, making the projectile unstable. As the bullet travels, it may start to tumble.
Tumbling: The bullet tumbles in flight.
Tumbling represents forward rotation around the centre of the mass resulting in a greater surface area on impact. The more unstable the bullet is, the greater the tissue destruction.
Nutation and precision: The tip (leading edge of the bullet) rocks in a spiral fashion.
The bullet also has precision and nutation movements along the horizontal axis.
24.1.3 Terminal Ballistics
The effect and movement of the bullets at point of impact is referred to as terminal ballistics. If the track of the bullet is at precisely 90 ° to the surface of the target (body), the bullet tip may enter cleanly. However, if the bullet is tumbling, it presents a much greater surface area, with greater retardation of velocity and transfer of greater energy to the tissue.
24.1.4 Wound Ballistics
The injuries that result from bullet wounds are known as wound ballistics. This is the clinician’s actual point of care, which can only be competently exercised if there is an understanding of the potential injuries, also based on the type of weapon, the energy of the projectile and the nature of the bullet itself. It is no longer appropriate to talk of ‘high-velocity’ and ‘low-velocity’ injury. Injuries are better classified into ‘high-energy injuries’ and ‘low-energy injuries’.
The severity and extent of injuries resulting on the body at point of impact depends on a number of factors which include:
The velocity of the bullet on impact and the velocity on exit. The greater the retardation, the more energy has been transferred.
The range from which the shot was fired (bullets slow down with distance).
A high-velocity round fired from a longer distance may result in a low-energy wound.
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The type of missile or bullet used.
Bullets can be designed to mushroom or open on impact to increase their cross-sectional area and therefore retardation.
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