Introduction
- firearms include arms that can fire plastic and rubber bullets, industrial nail guns,
pistols, revolvers and
rifles
- This group of firearms is divided into:-
* hand arms and shoulder arms
and also
* according to whether the barrel
is rifled or not.
- Rifled
firearms have spiral grooves on the inner surface of the barrel
which gives the spinning movement to the projectile as it travels down the
barrel. This rotational movement or spin stabilizes the projectile in flight.
- Hand arms for example:-
* revolvers
and
* pistols, as well as
* shoulder arms such as
hunting rifles, and
* automatic and
semi-automatic machine guns, all have these grooves.
-Smooth-bore
firearms (eg shotguns)
do not have grooves.
What happens when
firing a firearm?
- When the striking
pin activated by the trigger strikes the striking cap, the spark generated ignites the
propellant powder in the bullet casing.
- This powder has chemical and physical characteristics peculiar
to the type of ammunition.
-The gases
which develop during the explosion have a potential volume many times greater
than that of the powder, generally in excess of 500 times under atmospheric
pressure.
- It is this gas
pressure which drives the projectile along the barrel. While passing along the
barrel the projectile is soiled by oil and gas in the barrel. When it leaves the muzzle there is
also a flash and a gas emission, which includes carbon monoxide,
as well as hot and partially unignited powder particles, all of which impart to
the recipient surface characteristics which assist in determining the firing
range for that particular weapon and its ammunition.
- The medical practitioner - the observations made in the case
of firearm injuries can be of considerable assistance:-
* in establishing the range
* as well as the direction of fire,
* the type and
caliber of weapon and
* the nature of
the wound, for example whether it points at suicide, homicide or an
accident,
* the manner of death as well as the
* period of survival and the
extent of immobilization of the victim.
- In order to measure up to the expectations of the court in
this regard, close attention must be paid to the size and shape
of the entrance and exit wound(s).
* Careful inspection of the skin around the entrance wound,
noting the presence, size and shape of any abrasion collar, smoke blackening, tattooing and singeing
of hair,
is an essential element of the medical investigation.
* Evidence should also be sought of any vital reaction,
such as erythema (a reddening of the skin) in the vicinity of hot-powder
tattooing.
* An X-ray examination of the body (both at the clinical
examination and the autopsy) can be of considerable assistance in retrieving projectiles
for ballistic examination, particularly in the case of multiple entrance
wounds.
* In siting both entrance and exit wounds it is helpful if
measurements are expressed as the perpendicular height above the base of the
heel line.
* Entrance and exit wounds must preferably be indicated on a
diagram. This can assist (when evidence is given) to relate the findings to
circumstantial evidence.
* It must be remembered that the direction of the wound track,
with the body in the anatomical position during the post-mortem examination, is
not necessarily representative of the position of the body when it was hit by the
projectile.
· In shotgun injuries,
especially those inflicted at close range, the total mass of the individual
projectiles is a very important factor in the kinetic energy equation.
· At close range the pellets act as a solitary missile because of tight clustering, and their total weight is much greater than that of individual
rifle or handgun bullets. Furthermore, the ballistically poor design of multiple small, round balls does not
favour maintenance of high kinetic energy at considerable distances from
the muzzle, nor does it
permit great tissue penetration.
· However, at
close ranges (ie up to about 3 m in the case of shotguns) the energy
release generally causes massive tissue disruption close to the point of
penetration. For this reason close-range shotgun injuries have many of the
characteristics of high-velocity missile war wounds.
· When a missile enters tissue a temporary pulsing cavity, in
which the tissues swell
and stretch and compress, is formed along the missile tract within
10 to 30 milliseconds after impact. This cavity can be as large as 30 to 40
times the diameter of the bullet. This phenomenon that has been seen in
muscle, liver, bone, brain, lungs, and arteries is Caused by energy released by
the missile, forming shock waves with pressures of up to 100 to 200 atmospheres
directly related to missile velocity, and imparting momentum to the tissues, both
forward and laterally.
· In addition there is mechanical damage due to the shearing
effect between tissues of differing heterogeneity and density. The actual cavity created by the
missile rapidly collapses behind the projectile as a result of tissue elasticity,
haemorrhage leaving a smaller permanent pathway usually marked by tissue loss, haemorrhage, bullet fragments or sometimes
bone fragments which can be seen during radiological examination shortly after
injury.
· Bullet fragmentation is a major cause of tissue disruption and there is a close connection between large exit wounds and
bullet fragmentation. The critical velocity at which bone is fractured is
approximately 66 m/s. Kinetic energy imparted to the tissue causes bone to
fracture. Although some bone fragments may be contained within the surrounding
framework of the periosteum (bone membrane), muscle and connective tissue,
others may have sufficient energy to act as secondary missiles, creating
further destruction and multiple
exit-wounds.
· The medical examiner may thus be faced with a discrepancy
between the number of entrance and exit wounds. In low-velocity gunshot wounds, defined as
those inflicted by missiles travelling at a velocity of less than 360 m/s (ie most handguns),
the energy transferred to the tissues is considerably less than that of
high-velocity missiles, resulting in less tissue destruction.
Cutaneous (skin) entrance
wounds caused by rifled firearms
- Entrance wounds are usually round
or ovoid (oval) with an abrasion ring where the bullet abraded the
skin during entrance.
- This abrasion
ring is caused by
the force exerted by the projectile at entry, when the outermost layers of
the epidermis are stripped away. It is not caused by the rotational movement
of the projectile, as it is sometimes described in the literature, but by the
tangential force exerted on the skin.
- Similar abrasions
are sometimes seen surrounding the exit wound when the skin is pressed against a hard object,
for instance a belt buckle.
- After the bullet perforated the skin the elasticity of the
cutaneous tissue restores the skin's previous elasticity and the skin defect
contracts.
- When bullets strike skin at a 90 degree angle
(ie perpendicular to the skin surface) they produce round entrance wounds with a more or less
symmetrical collar or abrasion ring.
- Oval
or elliptical entrance wounds with asymmetrical abrasion rings occur when
bullets strike the skin at
acute angles. The wider area of abrasion is then located on the side of
the entrance wound closest to the gun.
-
Entrance wound shape
(ie round, oval, star-shaped or cross-shaped) and size must be described precisely.
-
Cutaneous injury and its
associated features, such as soiling
and rim abrasion ring, should be accurately measured and the dimensions recorded.
-
A circular wound
requires only the diameter
measurement, whereas an elliptical (oval) wound is measured across its
widest and narrowest diameters and variations in width of the marginal abrasion
are recorded.
-
Marginal dotting due to unburnt powder residues should be
specifically noted.
Tattooing
due to burnt, unburnt and partially burnt gun powder particles
Smoke (soot) deposits
Singeing of hair due
to heat
Abrasion ring
surrounds central entrance defect. The abrasion ring is the result of trauma to
the skin due to radial forces which act on the skin as it is pierced by the
projectile/bullet Sometimes oil, etc is also seen on the inner aspect
Central defect. This can have a smaller diameter than the bullet, due to
stretching of the skin
Contact
wounds
- The margins of the skin perforation are charred by the flame
coming from the muzzle, and the abraded border is usually soiled with powder residue
which can also be clearly visible in the subcutaneous and deeper tissues.
- The surrounding abrasion ring is of uniform width, and around this there can be an additional abrasion
ring from the gun barrel and gun sight.
- These muzzle imprints result from expansion of the dermal
tissue when gas is blown into the tissue, elevating the skin surface and
pressing it tightly against the muzzle.
- The dermal tissues in and immediately adjacent to the bullet
track may have a cherry
red colour due
to localised carboxyhaemoglobin formation from carbon monoxide in the muzzle
gases. If the muzzle is at an angle with the skin, the distribution is
more elliptical.
- Contact-range entrance wounds in the scalp look slightly different because stellate
(star-shaped) lacerations radiate from the central defect. This is due to
expansion of the explosion gases between scalp and skull and within the cranial
cavity. Explosion skull fractures may result, producing bone fragments which
can act as secondary missiles.
Intermediate or medium
distance entrance wounds
- Entrance gunshot wounds are classified as intermediary when muzzle-to-target distances are
such that the target surface is still within range of the muzzle blast although
target and muzzle are not in contact. Maximum muzzle-to-target
distances at which flame and muzzle blast create dermal damage and soiling vary
according to the type of gun and ammunition.
- Handguns and ammunition encountered in civilian homicides do not ordinarily deposit powder on
the target at ranges greater than 50 to 75 cm.
- Close-range entrance wounds in unclothed areas may be
surrounded by a radial arrangement of soot and be singed by flame. If the wound
is in a hairy area,
hair may be singed and examination with a hand lens may reveal clubbing of the
hair ends due to melting of the keratin.
Distant wounds
A distant wound
may show a surrounding faint
zone of discoloration due to dermal bleeding. Only the defect where the
bullet entered the skin, with a surrounding abrasion ring, is evident.
Cutaneous gunshot exit
wounds caused by rifled firearms
- These are produced by a stretching force applied to the skin from its undersurface with
perforation of the dermis and epidermis when their limits of elasticity are
exceeded. The surface defects vary widely in shape and configuration and may be stellate, cruciate,
round, elliptical, crescentic or linear cutaneous-lacerations.
- The greater
the missile's velocity when it exits, the larger and more jagged the exit
wound. Skin edges are
often everted (inside-out) and shreds of contused (bruised), haemorrhagic subcutaneous fat may
extrude through the defect. Characteristic features of dermal entrance wounds such as
the marginal abrasion ring, soiling and burning, are absent from exit wounds, with the exception of those located over a part of the
body which lies beneath or is pressed against a firm surface such as a leather
belt, a wall or the ground. In these cases a marginal abrasion ring may
surround the exit wound.
- Exit wounds are usually larger than entrance wounds, provided the latter had not been
affected by expanding explosion gases. Two factors, acting either individually
or together, are usually responsible for the greater size of exit wounds. The first factor is tumbling of the missile,
which increases the chances of it leaving the body with an oblique surface
presented at the striking area. The missile is thus turned sideways, and does
not move forward head first. The second factor is bullet
deformation due to it striking some hard object such as bone, which
causes consequent flattening of and an increase in missile diameter, a type of
change especially common in soft-nosed and hollow-nosed
projectiles.
- When the examiner is confronted with a single entrance wound and multiple exits, the possibility of bullet and/or bone
fragmentation should be considered. If the shot was fired from beyond
the range at which tattooing would occur it can be difficult to differentiate
between an entrance and exit wound. If the projectile passes through bone, and
particularly the skull, the direction
can be readily determined, since the entrance into the bone will be sharply
cut and about the same diameter as the projectile. On the inner table the bone
will exhibit a bevelling (sloping) effect and the defect in the bone will be
generally larger than that at the entrance. The bone breaking away from the
inner table will produce secondary missiles which may cause more extensive
tissue damage than the missile itself. As the projectile leaves the skull the
converse will apply, that is the wound on the outer table will be bevelled.
Gunshot wounds caused by smooth-bore
firearms (shotguns)
These wounds merit a separate discussion because shotguns and their ammunition
differ so markedly from rifled weapons and their ammunition. As mentioned
above, range, gauge (barrel diameter), degree of ``choke'' and size and number
of pellets in the shell play a role in determining the characteristics of
shotgun injuries.
-The explosive effect and the burns created by the muzzle flame
can cause additional damage to the damage created by the shot if gun and victim
are sufficiently close.
- Additional
trauma may result from the padding striking the victim, a feature not
seen in other types of firearm injuries.
Contact and close-range
cutaneous shotgun entrance wounds are round or elliptical,
depending on the angle between muzzle and skin. Smoke and powder residue with scorching, surround
the entrance wound, and marginal
abrasion similar to a single-bullet entrance is present. Occasionally, stellate lacerations due
to the muzzle blast are seen as gases expand beneath the skin and lacerate the
wound edges as they exit through the original entrance.
Contact shotgun wounds of
the head commonly cause extreme mutilation.
With close-range
shots of up to 120 to 150 mm the shotgun charge enters the body as a single conglomerate mass
producing a round defect somewhat larger in diameter than the bore of the
barrel. Wound margins in such cases show abrasion as well as scalloped defects.
The linear abrasions occasionally seen in the immediate vicinity of these
wounds are due to the impact of clothing against the stretched skin during penetration of the
shot.
Beyond this range, as muzzle-to-target distance increases, the major central defect becomes
progressively smaller and individual pellet wounds become more numerous as the
pellets begin to fan out. However, the minimal distance at which this occurs varies
considerably, ranging from 90 to 120 mm with sawn-off shotguns, 180 mm with
cylinder-bore guns and up to 540 mm with
full-choke guns. Wadding and
plastic casing usually enter the wound when muzzle-to-victim distance is less
than 150 to 180 mm. As
a rough estimate it is usually accepted that for every meter the pellets will disperse
2,5 to 3,0 cm. If the diameter of the wound (the maximum distance between the
most remote pellet wounds on the skin) is therefore 30 cm, the firing distance was
approximately 10 meters.
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