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