Forensic ballistics_identification and interpretation
- 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