Forensic ballistics_identification and interpretation

Forensic ballistics_ identification and

1. Chapter 15: Firearms, Tool
Marks, and Other Impressions

2. Introduction
•Crimes that involve firearms often require
analysis of bullets that are collected as
evidence
•As a bullet is shot from a firearm, each
gun creates an impression onto the bullet

3. Introduction
•By comparing the markings on the
unknown bullet from a crime scene with a
known bullet fired in the lab, forensic
scientists can determine if a certain bullet
was fired from a specific gun

4. Introduction
•Similarly, imperfections, wear marks, and
other unique characteristics of any type of
tool can provide information that allows
criminalists to determine if a certain tool
was used at a specific crime scene

5. Bullet Comparisons
•The inner surface of the barrel of a gun
leaves markings
•These markings are particular to each gun
•The gun barrel is made from a solid bar of
steel that has been hollowed out by drilling

6. Bullet Comparisons
•After the hole is drilled, the barrel is
imprinted with spiral grooves, which is
called rifling
•The machines that create the rifling
introduce tiny imperfections into the barrel
that are called striations

7. Bullet Comparisons
•As a bullet is fired it travels through the
barrel, it touches the grooves of the rifling,
and is imprinted with the shape of the
barrel
•Even guns manufactured consecutively
will not have identical patterns in their
rifling

8. Bullet Comparisons
•Therefore, it is possible to identify bullets
based on the pattern that the rifling was
made as it was traveling in the barrel
•The reason guns have rifling is to force the
bullet to spin (similar to the way a football
is thrown) and increase accuracy

9. Bullet Comparisons
•If a gun is suspected of being used in a
crime where a bullet was recovered, a test
bullet must be fired for comparison
•The test shot is usually done into a tank of
water to prevent damage to the bullet
•The two samples are analyzed using a
comparison microscope

10. Bullet Comparisons
•Another important feature of bullets is the
diameter of gun barrel, which is called the
caliber
•Caliber can be measured in hundredths of
an inch or in millimeters (i.e., .22 and
9mm)

11. Bullet Comparisons
•Another piece of information comes from
the direction of the spiral: they can turn to
the left or to the right (like a spiral
staircase)
•It is impossible to have a left spiral
impression on a bullet fired from a right
spiral gun

12. Bullet Comparisons
•If the comparison passed the initials tests
(caliber, right/left) then a closer
examination is done using a microscope
•A region that is undamaged is located and
a comparison is made to a similar region
of the test bullet

13. Bullet Comparisons
•By carefully and simultaneously comparing
the striations on each bullet a firearms
examiner can determine if they came from
the same gun
•Unfortunately, firearms examiners rarely
have samples that show a perfect match

14. Bullet Comparisons
•Bullets travel at very high speeds, and are
often damaged and mutilated upon impact,
which can make it very difficult to analyze
•The final opinion is based on the
judgment, experience, and knowledge of
the examiner

15. Bullet Analysis
•When a suspected gun is not available for
testing, the firearms examiner might be
given a bullet and asked for information
such as caliber and make of the weapon
•Sometimes the number of grooves can
indicate a certain manufacturer

16. Bullet Analysis
•In some cases this general information
can be helpful in a criminal investigation

17. Shotguns
•Shotguns do not have a rifled barrel,
rather they have a smooth barrel
•Also, shotguns do not fire a single bullet.
They fire small lead balls or pellets
contained within a shotgun shell

18. Shotguns
•Therefore it is very difficult to determine if
a crime was committed with a specific
shotgun
•Depending on what is recovered, the
analyst may be able to determine the
gauge which is the diameter of the
shotgun barrel

19. Cartridge Cases
•The act of pulling the trigger serves to
release the weapon’s firing pin
•The firing pin strikes the back of the bullet,
causing the gunpowder to ignite
•This explosion forces the bullet (or shot)
down the barrel

20. Cartridge Cases
•The cartridge case holds the gunpowder
and bullet before it is fired
•The case may be impressed with as it
comes into contact with the firing and
loading mechanisms

21. Cartridge Cases
•These imprints may be distinctive and
allow an analyst to match a casing to a
specific weapon
•The shape of the firing pin determines the
type of mark left on the casing, and can
provide information about the gun used

22. Cartridge Cases
•In some weapon designs the case is
ejected from after firing. This process can
leave a unique impression.

23. Automated Firearm Search
Systems
•Due to the increased use of automatic
weapons in crimes, databases have been
created to catalog the impressions of
bullet and cartridge cases
•This has made identifying firearm related
evidence a much more efficient process

24. Automated Firearm Search
Systems
•The Integrated Ballistic Identification
System (IBIS) is an extensive database of
images of known casings and bullets.
•The IBIS uses image recognition software
to analyze pictures of unknown samples
and determine what type of gun was used

25. Automated Firearm Search
Systems
•While automated search systems are
becoming increasingly sophisticated, they
do not replace the skills of human firearm
examiner

26. Gunpowder residues
•In incidents involving gunshot wounds, it is
important to determine the distance from
which the weapon was fired = distance
determination
•This is particularly important for claims of
self defense

27. Gunpowder residues
•Distance determination is also important in
determining if a gunshot wound was self
inflicted or not (i.e. suicide vs. homicide as
cause of death)

28. Gunpowder residues
•The gunpowder that explodes to force the
bullet out of the gun is not entirely
consumed
•If a target is close enough to the end of the
barrel of the gun, some of the unburned
gunpowder will be deposited and tested =
gunpowder residue

29. Gunpowder residues
•Accurate determinations of distance are
made by comparing the pattern of
gunpowder residue at the crime scene to
known test samples
•The tests must be conducted with a similar
weapon and ammunition for the test to be
admissible

30. Gunpowder residues
•If the type of suspected gun is not known,
then only approximations can be made
•The Greiss Test is used to identify the
presence of nitrites on a sample. (Nitrites
are found in gunpowder)

31. Gunpowder residues
•In addition to looking for GPR on the
victim, firearms analysts also can test
suspects.
•When a person shoots a gun, gunpowder
and primer residues also blow backward,
landing on the shooters hands

32. Serial Number Restoration
•Serial numbers can be used to trace the
origins of a gun
•Serial numbers are pressed into the metal
of guns using a hard steel die
•The die strikes the metal with enough
force to sink each digit a certain depth,
leaving an imprint of the number

33. Serial Number Restoration
•The metal below the impression is altered
in a way that is not visible to the human
eye
•Therefore, if the serial number on a gun
has been scratched or grinded away, an
etching solution can be applied to the area
to reveal the serial number

34. Collection and Preservation of
Firearms Evidence
•The primary concern in handling a weapon
is safety
•All precautions must be taken to prevent
accidental discharge
•In many cases, it is necessary to unload
the weapon before it is transported

35. Collection and Preservation of
Firearms Evidence
•An identification tag should be attached to
the trigger guard
•If a firearm is discovered underwater, it
should remain submerged in the same
type of water it was found in, and
transported in that water

36. Collection and Preservation of
Firearms Evidence
•This prevents rust from forming on the
weapon
•When removing a bullet from an object,
extreme caution must be used to prevent
unnecessary damage to the bullet

37. Collection and Preservation of
Firearms Evidence
•It is best to remove the surrounding
material, and avoiding direct contact with
the bullet
•Bullets should be wrapped in tissue paper
for transport and storage

38. Tool Marks
•A tool mark is considered to be any
impression, cut, gouge, or abrasion
caused by a tool coming into contact with
another object
•The most common place for tool marks
are burglary crime scenes that involve
forcible entry into a building or safe

39. Tool Marks
•Typically an impression is left on the frame
of a door or window as a result of prying
action of a screwdriver or crowbar
•As with gun barrels, the manufacturing
process for metal tools introduces tiny
imperfections that sometimes can be used
to identify individual tools

40. Tool Marks
•One difficulty matching a suspected tool to
a crime scene is the ability to reproduce
the tool mark in the lab
•Under no circumstances should an
investigator attempt to fit a tool into an
impression

41. Tool Marks
•This may alter the impression and/or the
tool, as well as raise concerns about the
integrity of the investigation

42. Other Impressions
•Shoeprints, tire tracks, fabric impressions,
etc. can all be used as evidence to identify
a suspect
•As with all physical evidence, they should
be photographed before being moved

43. Other Impressions
•Once the impression or photograph reach
the lab, they are compared to known
samples for identification