Rifles and handguns have rifled barrels, that is,
spiral grooves have been cut the length of the interior or bore of the barrel.
Rifling consists of these grooves and the metal left between the grooves—the
lands. The purpose of rifling is to impart a rotational spin to the bullet
along its longitudinal axis. This gyroscopic effect stabilizes the bullet’s
flight through the air, preventing it from tumbling end over end. This spin
does not, however, stabilize the bullet after it enters the body due to the
greater density of tissue compared to air. The term “twist,” as it pertains to
rifling, refers to the number of inches or centimeters of bore required for one
complete rifling spiral. All modern weapons have a twist which is constant for
the entire length of the barrel. Some weapons manufactured in the beginning of
the 20th century, had a “gain” twist; in this type of rifling, the rate of
twist increases from breech to muzzle. Muzzle velocity is allegedly increased 5
to 6%. Ballistic comparison of bullets fired from weapons with polygonal boring
may be very difficult. Because of this, when one police department ordered
their Glock pistols, they specified that the barrels have the traditional
rifling rather than the polygonal rifling that is routinely used in Glocks. In a specific barrel, the direction of the rifling can
easily be determined by examining the upper half of the bore and observing
whether the rifling curves to the left (left twist, counterclockwise) or to the
right (right twist, clockwise) as it proceeds away from one’s view. A minority of foreign handgun manufacturers
and some U.S. manufacturers of cheap weapons also use a left-hand twist. The majority
of domestic and foreign handgun manufacturers, however, use a right-hand twist.
Polygonal rifling has a right-hand twist. The number of lands and grooves in a weapon can range
from 2 to 22. Most modern weapons have four, five, or six grooves. Colt
handguns traditionally have had six lands and grooves with a left-hand twist,
while Smith & Wesson has had five lands and grooves with a right-hand
twist. Most centerfire rifles have four or six grooves, with a right-hand
twist. Rifle barrels with two grooves were manufactured during World War II for
the M-1 Carbine, the .30–06 Springfield rifle, and the British .303 Enfield. A
commercial M-1 Carbine manufactured by Universal had a barrel with 12 grooves
and a right hand twist. A CDM derringer examined by the author had one barrel
with six grooves and the other with twelve. When a bullet is fired down a
rifled barrel, the rifling imparts a number of markings to the bullet that are
called “class characteristics.” These markings may indicate the make and model
of the gun from which the bullet has been fired. They result from the specifications
of the rifling, as laid down by the individual manufacturer. These
characteristics are:
1. Number of lands and grooves
2. Diameter of lands and grooves
3. Width of lands and grooves
4. Depth of grooves
5. Direction of rifling twist
6. Degree of twist
For lead bullets these individual characteristics are
more pronounced where the grooves score the bullet. In contrast, for jacketed
bullets, the land markings are the most pronounced. These individual
characteristics are peculiar to the particular firearm that fired the bullet
and not to any others. They are as individual as fingerprints. No two barrels,
even those made consecutively by the same tools, will produce the same markings
on a bullet. Thus, while the class characteristics may be identical on bullets
fired by two different weapons, the individual characteristics will be
different.
When a gun is discharged, the bullet is forced down
the barrel by the gases of combustion. Both class and individual
characteristics are imparted to the bullet, whether it is lead or jacketed.
Because lead is softer, one might postulate that bullet markings on lead
bullets are more distinctive than those found on jacketed bullets. In actual
practice, markings on the jacketed bullets are usually superior, because the
jacket of harder metal is less likely to have the rifling marks wiped off by
the target. In order to recover bullets for ballistic comparison, bullets were
traditionally fired into cotton waste. The individual characteristics that a
barrel imparts to a bullet may be destroyed by rust, corrosion, or the firing
of thousands of rounds of jacketed ammunition down a barrel. Accumulation of
large quantities of dirt and grease from multiple firings may also alter to
some degree markings imparted to a bullet. The material of which the bullet is
constructed and the velocity and pressures to which the cartridge is loaded have an effect on bullet markings.
Therefore, it is good practice to use the same brand of ammunition as that
fired from the suspect’s gun when trying to make a comparison. In fact, it is
best to use other cartridges taken from the gun or from the same box of
ammunition for comparison testing. The reason for these suggestions is that
ammunition may vary greatly from one lot to another. The bullets used in one lot
may be slightly different in composition from those of another lot. The powder
used may be completely different, and the cartridges may be loaded to a higher
or lower pressure. Skidding occurs when the bullet jumps the gap between the
cylinder and the barrel and strikes the lands. The bullet resists the attempt
of the lands to impart a spin and “skids.” Bullets fired from an automatic may
show skid marks when the bullet is slightly smaller than the desired diameter
for a particular bore. This discrepancy causes the bullet to skid as it enters
the rifling before settling down. As a general rule, however, skidding rarely
occurs in automatic pistols, as the bullet is in contact with the lands before
firing and follows them from the start. The presence of skid marks on an
automatic pistol bullet may be of significance, as it may indicate that the
bullet was fired in a revolver rather than in an automatic. Shaving one surface
of a bullet fired from a revolver is sometimes encountered. This happens
because the cylinder of the revolver is improperly aligned with the bore of the
barrel (so-called poor indexing) and thus lead is shaved from the bullet as it
jumps the gap from the cylinder to barrel. Both cheaply made revolvers and
revolvers of quality that are badly worn may cause shaving. In some instances,
bullets may appear distorted when recovered from a body due to the fact that
they were fired in weapons not chambered for them. Ward et al. reported two
cases, a homicide and a suicide, in which .38 Special wadcutter cartridges were
fired in .30–30 rifles. Bullets recovered from decomposed bodies may show
partial or complete loss of individual rifling striations depending upon the
tissue from which the bullet was recovered and the construction of the bullet.
There were bullets inserted of various construction, into different areas of a
human body and let it decompose for 66 days, there is an outcome of what has
been found:
1. Nylon-clad bullets were uniformly unaffected by
decomposition.
2. Aluminum jacketed bullets were mildly affected but
there was no loss of striations.
3. Lead bullets from the brain, chest cavity, and
abdominal cavity showed mild tarnishing but were matchable while those from fat
and muscle showed dissolution and oxidation to the point of impairing a match.
4. Bullets with copper alloy jacketing, including those
with nickel-wash, were not matchable except for copper alloy bullets recovered
from the chest cavity which were borderline.
Examination of a fired cartridge case may make
possible the identification of a weapon in terms of type, make, and model. The
presence of magazine markings, the type of breech-block mark, and the size,
shape, and location of ejector and extractor marks are important imprints in
making such identification. The size, shape, and location of the firing pin on
fired rimfire cartridge cases can also be used to determine the make of the
weapon. During manufacture, small parallel grooves have been cut into the wall
of the chamber permitting powder gases to surround the cartridge case to allow
the neck of the cartridge case to “float” on gas, thus aiding extraction. They
are found in rifles, pistols, submachine guns, and machineguns. Flute marks may
be present only on the neck or shoulder area of cases or along most of its
length. Different forms of powder
produce different marks: spherical (true) ball powder produces numerous deep
circular pits; disk powder produces shallow, circular imprints as well as
linear markings (powder flakes striking on edge); black powder produces a
characteristic peppered appearance. If a
bullet passes through a body or intermediary target, or ricochets off a hard
surface, fragments of tissue or target material may adhere to or be imbedded in
the bullet. If the bullet is a hollow point, a relatively large wad of this
material may be deposited in the cavity. Recovery and identification of foreign
material from a bullet may identify the organs or intermediary object
perforated or prove that the bullet was a ricochet. Tissue recovered from
low-velocity bullets was better preserved and more abundant. Adipose tissue,
fragments of small vessels, clumps of spindle cells were most commonly found;
skeletal and cardiac muscle, occasionally. Visceral organ fragments were not
necessarily found even when the organs were perforated. Skin was the least
commonly encountered. In regard to gunshot wounds of the head, bone chips,
skeletal muscle, connective tissue, and strips of small vessels were commonly
identified. Fragments of brain were present but were not readily recognizable as
neural in origin. While blood from the victim is often searched for on the
clothing of the alleged perpetrator, it is not appreciated that in contact
wounds of the head, brain tissue may be blown back on the perpetrator’s
clothing. The stains produced do not resemble blood but rather coffee or soft
drink stains. If these areas are soaked in saline solution, cellular material
may be extracted from the cloth. Brain tissue can then be identified by
cytological methods. A bullet found at a scene may be linked to the specific
individual through which the bullet had passed by examining tissue deposited on
the bullet. This is possible even if no tissue is visible on macroscopic
examination of the bullet and it is full metal-jacketed. As a bullet passes
through a body, tissue is deposited on its surface even if the tissue is not
visible.
The tissue can be removed by swabbing the bullet; the
DNA replicated and DNA “fingerprinting” performed by short tandem repeat (STR)
analysis. Both the public and many police agencies do not realize, that
identifiable fingerprints may be obtained from fired cartridge cases. Pulling the trigger causes release of the
firing pin. This strikes the primer, crushing it, igniting the primer
composition, and producing an intense flame. The flame enters the main chamber
of the cartridge case through one or more vents, igniting the powder and
producing a large quantity of gas and heat. This gas, which may be heated to
5200°F, exerts pressure on the base of the bullet and sides of the cartridge
case. As the bullet travels down the barrel, some of the gas leaks past the
bullet, emerging from the muzzle ahead of it. The bulk of the gas and any
unburnt powder, however, emerge after the bullet. The amount of unburned or
partially burned powder exiting depends largely on the burning properties of
the powder and the length of the barrel. Smokeless powder does not explode;
rather, it burns. The rate of burning can be controlled by the manufacturer by
means of varying the size and shape of the powder grains, as well as by coating
them with substances that retard combustion. Ideally, the propellant should
start burning slowly, gradually increasing its rate of combustion until it is
completely consumed just as the bullet leaves the muzzle. Such ideal burning
powder is virtually never achieved because the same powder is used to propel
bullets of various calibers and weights down barrels of different lengths.
Bullet weight causes variations in burning by altering the pressure of the
gases in the firing chamber. When powder is ignited and gas forms, the bullet
does not begin to move immediately. There is a small interval of time necessary
for the gas to overcome the inertia of the bullet and the resistance of its
passing down the barrel. This interval increases with the weight of the bullet
if everything else remains constant. As the interval increases, the pressure
increases, causing the powder to burn faster and give off more heat. The heat
in turn raises the gas pressure. If there is an ideally progressive burning
powder for a specified bullet weight and that weight is increased, the interval
will increase and the powder will burn faster. Therefore, the powder will be
burned before the bullet leaves the barrel. Lightening the weight of the bullet
would cause the opposite effect; in this case, not all the powder will be
burned before the bullet emerges from the muzzle. The “ball of fire” emerging
from the muzzle consists of oxygen-deprived gases produced by ignition of
gunpowder. When they emerge from the barrel at extremely high temperatures,
they react with the oxygen in the atmosphere, producing what is commonly known
as the “muzzle flash.” This should not be confused with the flame.
To sum up it has been defined that a hangfire or delayed discharge
“as one that is of abnormal duration and perceptible to the shooter by means of
sight or sound”. It has also be noted other characteristics of hangfires:
reduced velocity; reduced report; substantial quantities of unburnt powder in
the cartridge case and/or bore of the gun; sooty deposits on the exterior wall
of the cartridge case and little or no expansion of the case.
Acknowledgements:
The Police Department;
www.politie.nl and a Chief Inspector – Mr. Erik Akerboom
©
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