Ballistics

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Ballistics
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Bullet Comparisons
The inner surface of the barrel of a gun leaves
its markings on a bullet.
These marking are unique and can link a bullet to
a suspects gun.
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Barrel Manufacture
A solid bar of steel is hollowed out by drilling
to produce a gun barrel
Rifling process in which the inner surface of
the gun is impressed with spiral groves. Grooves
impart spin to the bullet.
Note Grooves are the cut or low-lying portions
between lands
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Barrel Manufacture Con't
The surfaces of the original bore remaining
between the grooves are called lands.
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Broach Cutter Process Cuts the groves into the
barrel
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Button Process Under high pressureit passes
through the barrel and impresses grooves and
lands on the barrel
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Caliber
Caliber is the diameter of the gun barrel,
measured between opposite lands. It is measured
in millimeters or hundreds of an inch.
Examples .22, .38 and 9mm
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Class Characteristics
The rifling process used for a particular gun
imparts a definite number of lands and groves,
with the same width and twist.
Example .32 Smith and Wesson revolvers all have
six lands and groves twisting to the left.
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Individual Characteristics
The inside of a gun has fine lines or striations
running the length of the barrel.
These striations are randomly impressed into the
metal due to manufacturing irregularities.
No two rifled barrels, even those manufactured in
succession, will have identical striations.
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Cartridge Cases
Spent cartridge or primer cases may be impressed
with random striation markings sufficient to
allow individualization to a single weapon.
Markings come from three places the breechblock,
firing pin, and extractor or ejector mechanism
Note The breechblock is the rear of a firearm
barrel
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Shotgun Shells
Shotguns have smooth barrels and therefore do not
impress marking on the shells

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A Shotgun Shell

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Shotgun Shells Con't
By weighing and measuring the diameter of the
shot found a determination of its size can be made
The size and shape of the recovered wad, may
reveal gauge and manufacturer of the fired
shot-shell
Note gauge is a measure of the diameter of the
shotgun barrel the higher the gauge number the
smaller the barrel
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Gunpowder Residues
Gunshot residues allow an investigator to
determine the distance interval in which a weapon
was fire
This information is useful in self-defense and
suicide cases
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General Characteristics

• One inch or less heavy concentration of smoke
  like vaporous lead, star shape and scorch mark on
  fabric

2. 12 to 18 inches or less a halo of smoke
around the bullet hole
3. Up to 25 inches scattered specks of powder
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4. Up to 36 inches occasional scattered specks
of gunpowder.
5. Greater than 3 feet no gunpowder residue,
only bullet wipe will be seen
Bullet wipe dark ring around perimeter of
entrance hole
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Gunpowder Residues Cont

• Color contrast between the powder and garment can
  be enhanced with infrared photography
• Greiss test is a chemical test that can be used
  to develop nitrate patterns found from the
  gunpowder (chemically treated photo-paper is
  pressed onto the target)

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Primer Residues on Hands

• Residues blown back onto the shooters hand can
  be detected
• With the exception of .22 caliber ammunition,
  primers contain lead, barium, and antimony
• Tests for barium and antimony include using tape
  to remove any residues or swabbing hands with
  nitric acid. Then using SEM, X ray analyzer, or
  neutron activation analysis
• Big problem is that the primers are only on the
  hands for a short period of timeeasily removed
  by washing or wiping of hands

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Collection and Preservation of Firearms

• Hold gun by trigger guard or checked portion of
  grip
• Safety is the most important thing
• Evidence must be marked and chain of custody must
  be kept
• Tag should be on the trigger guard with the
  following information
• A) serial information
• B) Make
• C) Model
• D) Investigators initials

Note Every gun has a serial number for ID it
can be restored
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Collection and Preservation of Ammunition

• Protection of class and individual markings on
  bullets and cases most important
• Extreme care must be taken when dislodging bullet
  from surface
• Bullet and shells must be scribed with
  investigators initials
• Need exact location shell was recovered

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Other Impressions Tool marks

• A tool mark is any impression, cut, gouge, or
  abrasion caused by a tool
• Most common in burglary offenses
• Typically an impression is left on a frame of a
  door or window with a screwdriver or crow bar
• Leave micro-impressions as evidence
• Plaster or silicone is used to make casts of the
  impressions
• Chips of paint on tool also give evidence

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Other Impressions Shoe Tire

• Photos are taken first
• Lifting devices used to move impressions
• Dusting, large tape or film
• Electrostatic, charged paper
• Casting is done to preserve foot or tire marks