Chapter 15: Firearms, Tool Marks, and Other Impressions

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

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

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

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

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

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

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

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

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)

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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

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)

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

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

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)

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

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

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

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

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

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

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

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

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

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

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

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

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