Forensics Chapter 8

Several different forensic disciplines related to firearms evidence:

1. Firearms identification = A discipline mainly concerned with determining whether a bullet or cartridge was fired by a particular weapon 2. Ballistics = the study of the processes within a firearm as it is fired; the science of the motion of projectiles in flight 3. Identification of Gunshot Residues 4. Serial Number Restorations

Cartridge Casing Comparisons

• Act of pulling the trigger releases the weapon's firing pin, causing it to strike the primer, which in turn ignites the powder o Trigger = that part of a firearm mechanism that is moved manually to cause a firearm to discharge o Firing pin = the part of a firearm mechanism that strikes the primer of a cartridge to initiate ignition • Expanding gases generated by the burning gunpowder propel the projectile forward through the barrel and simultaneously pushes the now empty cartridge case (called a discharged cartridge case) back with equal force against the breechface o Breechface = rear part of a firearm barrel • Cartridge Case Markings o Firing pin impressions: Shape of the firing pin is impressed into the soft metal primer on the cartridge case Firing pin may have minute distortions sufficiently random to individualize the firing pin to a single firearm o Breeface impressions: Breechface is a machined surface populated with random striation markings Force of the cartridge case hitting the breechface may impart impressions on the case that may be individualizing o Extractor and ejector marks may also be impressed onto the surfaces of cartridge casings Extractor = the mechanism in a firearm by which a cartridge of a fired case is withdrawn from the chamber Ejector = the mechanism in a firearm that throws the cartridge or fired case from the firearm

Rifling Methods

• After the 1940s rifling is usually accomplished by one of several methods: 1) Cutting all grooves in one pass with a cutter known as a broach; 2) Pressing all the grooves at once onto the barrel with a tool known as a button (steel plug); 3) Hammer-forging the barrel over a mandrel containing the reverse image of the rifling. • Every firearms manufacturer will choose a rifling method that is best suited to their production standards and requirements of their product • Once they choose a method the class characteristics of each weapon's barrel will be consistent - each will have the same number of lands and grooves with the same approximate width and direction of twist

Rifling in the Barrel

• After the barrel is drilling out of the solid bar - the next step is impressing the inner surface of the barrel with spiral grooves, a step known as rifling • Rifling = the spiral grooves formed in the bore of a firearm barrel that impart spin to the projectile when it is fired • Why do we need rifling? o Grooves guide the bullet down the barrel giving it a rapid spin o Ensures accuracy - a spinning bullet does not tumble end over end when it leaves the barrel - remains on a straight course • Grooves = the cut or low-lying portions between the lands in a rifled bore • Lands = the raised portions between the grooves in a rifled bore o Lands are the surfaces of the original bore o The unique and random striations (drill marks) from when the barrel was hollowed out are present on these original surfaces, or lands • Caliber = the diameter of the bore of a rifled firearm o Measured between opposite lands o Usually expressed in hundredths of an inch or millimeters (ex: .22 caliber or 9 mm) o Not an exact measurement

Ammunition

• Ammunition = one or more loaded cartridges consisting of a primed case, propellant, and projectile(s) • Cartridge = a unit of ammunition, made up of a cartridge case, primer, powder, and bullet o A cartridge is sometimes incorrectly called a "bullet" • Bullet = the projectile • 4 Components of a Cartridge: 1. Projectile (the bullet) = an object propelled by the force of gases produced by rapidly burning gunpowder 2. Cartridge case = the container for all the other components that comprise a cartridge 3. Powder = commonly used term for the propellant in a cartridge or shotshell 4. Primer = an explosive substance that ignites when struck to detonate the powder in a cartridge

Class vs. Individual Characteristics in Bullet Comparisons

• As a bullet passes through the barrel, its surface is impressed with the rifled markings of the barrel • A fired bullet contains markings that are both class and individual characteristics: o Class characteristics = properties of evidence that can be associated with only a group and never a single source Class Characteristics on bullets - Rifling impressed onto a fired bullet • Number and width of lands and grooves • Direction of twist Firearms Examiner may be able to distinguish one type or brand of weapon from another based on the class characteristics of number of lands and grooves and direction of twist. o Individual characteristics = properties of evidence that can be attributed to a common source with an extremely high degree of certainty Individual Characteristics on bullets - Striations impressed onto a fired bullet • Striations = are fine lines found in the interior of the barrel running the length of the lands and grooves • Striations are impressed into the metal as the negatives of minute imperfections found on the rifling cutter's surface. Can be produced by minute chips of steel pushed against the barrel's inner surface by a moving broach cutter • No two rifled barrels, even those manufactured in succession, have identical striation markings Because the striations have a random distribution and the irregularities of the striations are impossible to duplicate exactly in any two barrels - Forensic Scientists believe the striations form individual characteristics which may allow a Firearms examiner to match a fired bullet to a firearm to the exclusion of all others.

Barrel of a Firearm

• Barrel = the metal tube through which a projectile or shot charge is fired o May be rifled (rifles) or smooth (shotguns) o Barrel of a gun is produced from a solid bar of steel that has been hollowed out by drilling. Drill leaves behind microscopic marks on the inside of the barrel that are randomly irregular - this imparts a uniqueness to each barrel • Bore = the interior of a firearm barrel • Breech = the end of the barrel attached to the action • Muzzle = the end of the barrel out of which the projectile comes

Distance Determination

• Distance determination = the process of determining the distance between the firearm (muzzle) and a target, usually based on the distribution of powder patterns or the spread of a shot pattern o If the muzzle of the weapon is within a few feet - the GSR and smoke/soot is deposited onto the target o The distribution of GSR and other discharge residues around the bullet hole permits an assessment of the distance from which a handgun or rifle was fired • Using the suspected firearm - a series of test firings are made into targets at known distances • Visible patterns (vaporous lead, bullet wipe, soot etc.) and chemically developed patterns (using color tests for nitrites and for lead) observed around questioned bullet hole (ex: on victim's shirt) are compared to the patterns from the known test firings

Common Firearms Definitions

• Firearm = An assembly of a barrel and action from which a projectile(s) is discharged by means of a rapidly burning propellant • Barrel = the metal tube through which a projectile or shot charge is fired • Action = the part of a firearm that loads, fires, and ejects a cartridge • Cartridge = a unit of ammunition, made up of a cartridge case, primer, powder, and bullet

Comparison vs. Identification Analysis

• Firearms Examination can be considered both a Comparison discipline and an Identification discipline depending on what type of firearms evidence you are examining • Identification = the process of determining a substance's physical or chemical identity o What is this object/substance? o Example: identifying the components of gunshot residue like copper, lead, nitrites, and primer residues • Comparison = the process of ascertaining whether two or more objects have a common origin o Do two objects or patterns match? o Example: comparing striations on a bullet found at a crime scene or taken out of a body to striations on a known bullet (test fired from a known firearm) or to images of striations from other "unknown" bullets in a database to determine if they "match" or came from the same origin (firearm)

Analytical vs. Pattern Physical Evidence

• Firearms Examination is an analysis of both analytical evidence and pattern evidence depending on what type of firearms evidence you are examining • Analytical evidence = a laboratory-based analysis is used to examine this type of evidence o Chemical or serological tests Example: chemical color tests for copper, lead, and nitrites o Instrumental analysis Example: Scanning electron microscope (SEM) used to identify primer residues • Pattern evidence = an expert interprets or matches observed patterns (the patterns are the evidence) o Patterns/impressions are visually compared to each other to determine if they "match" o The pattern is the evidence Example: questioned GSR pattern on the shirt of a deceased person is compared to the GSR patterns on test firings done at different known distances to determine which known distance pattern most closely "matches" the pattern on the questioned item Example: Striations (the pattern) on a questioned bullet are compared to striations from test fired bullets fired from a known weapon to see if they "match"

Identification of Gunshot Residues (GSR)

• Gunshot residue = complex mixture of vapors and particles that are expelled from a firearm when it discharges • What are the components of the complex mixture that is gunshot residue? o Primer residue o Burnt, unburnt, and partially burnt gunpowder residue - Nitrites o Fragments of the cartridge (casing and projectile) o Lead (from the projectile) o Copper (from copper jacketed cartridges) o Soot (carbonaceous material) o Oil/lubricants o Stabilizers, plasticizers, deterrents, coolants, flash inhibitors and other components designed by the ammunition manufacturer to improve the performance of the product • Powder Residues o Gunpowder - Most ammunition contains smokeless powder or nitrocellulose inside of the cartridge o When a firearm is discharged, unburned and partially burned particles of gunpowder along with smoke are propelled out of the barrel along with the projectile o Gunpowder particles may be identifiable by their characteristic colors, sizes, and shapes • Primer Residues o When a firearm is discharged - GSR particles are not only propelled forwards at a target - they are also blown back at the person firing the weapon (or onto a person standing nearby) o With the exception of most .22 caliber ammunition - primers currently manufactured contain a blend of lead styphnate, barium nitrate, and antimony sulfide High barium and antimony levels strongly indicate a person fired or handled or was in the vicinity of a firearm being discharged. These elements are present in such small quantities (less than 10 micrograms) - only the most sensitive analytical techniques can detect them Collection stubs (from a suspect's hands or clothing) can be analyzed by a SEM (Scanning Electron Microscope) linked to an x-ray analyzer to conduct an elemental analysis for lead, barium, and antimony

2 General Categories of Firearms - Handguns & Long guns

• Handguns = firearms that are designed to be held and fired with one hand o Examples: Single-shot handguns, revolvers, semiautomatics • Long guns = firearms that are designed to be fired while resting on the shoulder o Long guns can be subdivided into 2 categories: 1) Rifles & 2) Shotguns

Serial Number Restorations (SNR)

• Many manufactured items are stamped with unique, sequential serial numbers (SN) for identification including automobile engine blocks and firearms o SN's are usually stamped on a metal body or frame with hard steel dies, which strikes the metal surface with enough force to allow each digit to sink into the metal to a prescribed depth • Criminals may attempt to obliterate the SN by grinding, rifling, punching, scratching, etching, or drilling the metal around the numbers o Restoration of serial numbers is possible through chemical etching because the metal crystals in the stamped zone are placed under a permanent strain that extends a short distance beneath the original numbers o An etching agent, such as an acid, is applied and dissolves the strained area faster than the unaltered metal - revealing the etched pattern in the form of the original numbers o If the zone of strain has been removed, or if the area has been impressed with a different strain pattern, the SN usually cannot be restored

Comparing Bullet Markings

• No practical way to directly compare striations on a bullet to striations inside of a barrel • To do a comparison the firearms examiner must fire a test bullet through the barrel in question and then compare the test fired bullet to a questioned bullet o Test firings are normally made into a box filled with cotton or a tank of water o Done to prevent damage to the bullet and to make it easier to recover the bullet • Striations found on test fired bullet are compared to a questioned bullet using a comparison microscope o Two bullets can be observed and compared simultaneously within the same field of view o Not only must the lands and grooves of the test and evidence bullet have identical widths, but the longitudinal striations on each must coincide.