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Firearms Examiner Training. Dillon JH, Jr. 2013. Firearms Examiner…

- - - - Common metal-forming techniques, other than machine tool operations include the following
        
        b. Casting
        
        c. Blanking
        
        d. Shearing
        
        e. Stamping
        
        f. Drawing
        
        g. Swaging
        
        h. Extrusion
        
        a. Forging
      - Typical variations of metal forging include the following
        
        a. Cold forging or cold heading, in which heavy force is applied from the outside to a workpiece contained in a die with no heat involved. The metal is forced to fill the volume of the die and assume its shape. Bolts and rivets may assume their rough dimensions using this process.
        
        b. Hot heading or upset forging, which is similar to cold heading except that the workpiece is heated.
        
        c. Drop forging, which is very similar to cold forging except that the metal is heated prior to the forging operation and hammered into a die.
        
        d. Casting, which is a form of metal shaping done by pouring molten metal into a mold. It is possible that mold marks may be repeated in each casting as subclass characteristics if each mold is made from an original master mold or if the same mold is used repeatedly.
        
        i. Sand casting, which is a traditional technique in which a wooden form is embedded in sand in a two-part frame (cope and drag). The cope and drag are carefully separated, the form removed, and the two sections rejoined. Molten metal is poured into the now hollow space through a sprue hole and the hardened metal shape is later removed.
        
        ii. Investment casting (lost wax process, precision casting), in which a heat resistant ceramic slurry is coated over an expendable form made of some substance that will melt at low temperatures (e.g., wax, plastic). Once the ceramic coating sets at room temperature, the interior form is melted away, leaving a mold that will not melt when molten metal is poured into the mold.
        
        e. Blanking, which is cutting a large section of sheet metal stock into smaller pieces for another operation, such as drawing.
        
        f. Shearing, which is a form of metal separation accomplished partially by the slicing action of a dropping blade, followed by a clean fracture along with the cut.
        
        g. Stamping, which is a technique for transforming sheet metal into a three-dimensional object in a large machine press.
        
        h. Drawing (deep and shallow), in which a blank of sheet metal is restrained at the edges and the middle section is formed by a punch into a die to stretch the metal into a shape.
        
        i. Swaging, which is a process for reducing the diameter of a tube, rod, or other objects (e.g., a cartridge case in the manufacture of ammunition). The object is placed inside a die that exerts an exterior radial force on the object, gradually compressing it and reducing its diameter. It is often performed using a steel mandrel to control the shape and dimensions of the result.
        
        j. Extrusion, in which long straight metal parts can be produced or reduced in size by passing through successive dies. A wire is an excellent example.
    - - Some modern metal machining techniques
        
        b. Laser machining
        
        c. Electrochemical machining (ECM)
        
        d. Chemical machining (etching)
        
        e. Hydrodynamic machining (HDM)
        
        f. Ultrasonic machining
        
        g. Electron beam machining (EBM)
        
        a. Electrical discharge machining (EDM)
      - Some of the newest techniques
        
        a. Electrical discharge machining (EDM) is a process in which the tool (an electrode) and the workpiece are oppositely charged with a nonconductive dielectric fluid between them. A spark arcs across the fluid from the electrode to the workpiece, melting an area of the surface of the workpiece, which is then flushed away in the fluid.
        
        b. Laser machining involves the optical focusing of a laser beam, producing high temperatures, vaporizing the metal to be removed.
        
        c. Electrochemical machining (ECM) is similar to traditional electroplating but in reverse. The process uses an electrically conductive fluid and direct current to remove material.
        
        d. Chemical machining or etching involves the masking or protecting of areas not to be machined, then exposing the remaining areas to a chemical etchant.
        
        e. Hydrodynamic machining uses a water jet combined with abrasives that can cut complex patterns without the generation of heat.
        
        f. Ultrasonic machining is a technique that combines ultrasonic sound waves with a slurry of abrasive material for the controlled machining of a surface.
        
        g. Electron beam machining (EBM) is a technique used in the aerospace industry to cut any metal or nonmetal with a high level of precision. It requires a vacuum and shielding due to the x-rays produced by the electron beam.
    - - Common cutting operations using machine tools include these
        
        a. Turning
        
        i. Lathe operation
        
        b. Milling
        
        i. The three main categories
        
        • Face milling
        
        – The surface of the workpiece is parallel to the cutter face located at the end of a spindle or shaft. The cutters are located on the face and edge of the rotating cutter.
        
        • Peripheral milling (slab milling)
        
        – The surface being machined is parallel to the (often helical) cutting teeth in the periphery of the tool.
        
        • End milling
        
        – Fluted cutting edges are arranged on the circumference and end of a rotating shaft that is vertical to the workpiece. The cutting teeth are found on the end of the cutter and the circumference of the cutter body.
        
        ii. Two distinct methods
        
        • Conventional or up-milling
        
        – The workpiece is fed against the rotating cutting teeth
        
        • Climb or down-milling
        
        – The workpiece is fed in the same direction as the rotating cutter, producing a finer finish and an extended tool life.
        
        c. Boring
        
        i. Lathe operation
        
        d. Shaping
        
        i. A single-point cutting machine cuts in a straight line by repeated parallel strokes, gradually moving across a flat surface.
        
        e. Planing
        
        i. Similar to shaping, but the workpiece moves under a stationary cutting tool.
        
        f. Drilling
        
        i. A cylindrical cutting tool with multiple fluted cutting edges arrayed in a spiral (e.g., the interior of the barrel of a firearm).
        
        g. Reaming
        
        i. A cutting technique that it is sometimes used after drilling to produce accurate finely finished holes.
        
        h. Broaching
        
        i. Multi-tooth cutting tool that moves against a stationary workpiece. It can remove metal on both interior and exterior surfaces. Each tooth cuts slightly deeper than the previous one and removes only a small thickness of metal. A cylindrical broach is one type of tool used to put rifling in the barrel of a firearm.
        
        i. Sawing
        
        i. A multi-toothed cutting tool in which the cutting edges are arranged in line.
        
        j. Filing
        
        i. Material is formed and removed using teeth that are shallow and broad (the reverse of a saw blade).
        
        k. Abrasive machining
        
        i. Thousands of tiny cutting edges made up of individual abrasive grains and used as a finishing operation. Each abrasive grain removes a minute amount of material. Abrasive machining includes simple grinding, lapping, tumbling, and numerous other specialized applications of abrasives.
      - Types of Metal Chip
        
        a. Continuous chips (long ribbons of metal)
        
        i. Typically found in machining a ductile metal with a sharp cutting tool, resulting in a fine surface
        
        b. Discontinuous chips (segmented chips)
        
        i. Break up into particles; associated with brittle metals and a coarser result on the surface of the workpiece
        
        c. Continuous chips with a built-up edge
        
        i. Accumulate tiny particles that weld to the workpiece, producing coarse surfaces on the workpiece. This often occurs if the cutting tool begins to dull
      - Variables controlled by machinists
        
        a. Brittleness or ductility of the workpiece (e.g., cast iron is brittle, brass is very ductile)
        
        b. Quality of the cutting tool
        
        c. Adjustment of the cutting tool for the depth of the cut
        
        d. Sharpness of the cutting tool
        
        e. Speed at which the workpiece moves relative to the cutting tool (feed rate)
        
        f. Angle of the cutting tool relative to the workpiece
        
        g. Speed of the cutting tool
        
        h. Condition of the edge of the cutting tool
        
        i. Temperature of the workpiece and cutting tool (usually controlled using coolant fluids)
      - Lathe Operation
        
        a. Turning
        
        i. Rotates the exterior of a workpiece against a single-point cutting tool
        
        b. Facing
        
        i. The cutting tool is used on the surfaces perpendicular to the rotating spindle
        
        c. Boring
        
        i. A single-point cutting tool enlarges a hole beyond the capability of a drill or reamer and works the interior of a piece
  - - - Characteristics that may be class, subclass, or individual
      - Pattern identification
      - AFTE Theory of Identification as it Relates to Toolmarks
      - Principle of sufficient agreement
      - Principle of consecutive matching striae
      - Principle of consecutive matching striae
    - - Cartridge case marks
        
        a. Holding tools on a reloading press which grips the base of cartridge cases
        
        b. Resizing tools (e.g., sizing die) which squeeze fired cases back to their original specifications
        
        c. Crimping tools used for some loads to press the mouth of cases slightly into a cannelure used as a crimping groove
      - Bullets bearing bullet-seating toolmarks
  - - - A flat-bladed tool held at 90 to a surface and drawn across that surface
    - - An opposed blade cutting tool, such as a pair of bolt cutters or diagonal wire cutters
    - - Shear cutters, the blades of which are offset to pass by each other in the cutting process, such as tin snips or scissors
    - - A single-bladed tool, such as a knife or axe
    - - A prying tool using leverage to force open a locked door or cover at one of its edges. May be a flat-bladed prying tool, (e.g., crowbar, screwdriver, tire iron, etc.)
    - - A gripping tool with opposing jaws, such as a pipe wrench, pliers, or a vise. Serrated jawed gripping tools add another dimension to the types of marks present
    - - An opposing jawed tool designed to press material together without cutting it
  - - - Measurable features of a tool that indicate a limited group source. They result from design factors and are therefore determined prior to manufacture. Common examples would be the width of the tip of a screwdriver, the width of the jaw of a gripping tool, or the kind of cutting action employed by a tool.
    - - Marks on an object produced by the random imperfections or irregularities on the surfaces of the tools used to manufacture the object. These marks are produced incidental to the manufacturing process and are typically at the microscopic level. Individual characteristics can also be produced on an object by use, abuse, and the effects of random corrosion. To use the example of a screwdriver, individual characteristics could be found on the tip of a precision screwdriver by the process of finishing it using a grinding wheel. As a screwdriver is used, misused, and corrodes, it becomes more unique.
        
        a. Accidental characteristics is a term formerly used to mean individual characteristics.
    - - Discernable surface features of an object that are more restrictive than class characteristics, but not as unique as individual characteristics. In the screwdriver example, subclass or family characteristics might be found on the tip of a few hundred inexpensive screwdrivers stamped out by a dedicated machine, with no further finishing. The class characteristics of this stamping machine would gradually change, but not before producing a potentially large number of screwdrivers with tips bearing similar subclass characteristics.
        
        a. Produced incidental to the manufacture of a tool
        
        b. Significant because they relate to a smaller group source, that is, a subset of the class to which they belong
        
        c. Identifiable within a time frame since manufacturing processes change over time
  - - - Establishing an identification as to a common source
      - Conveying results to a jury
      - Justifying results to a jury
  - - - The expectations of more sophisticated jurors
      - The need more objective identification criteria
      - The changing environment of the courtroom following Daubert v. Merrell Dow Pharmaceuticals
      - The potential for increased credibility for examiners in the courtroom
  - - - The theory of identification as it pertains to toolmarks enables opinions of common origin to be made when the unique surface contours of two toolmarks are in sufficient agreement.
      - This sufficient agreement is related to the significant duplication of random tool marks as evidenced by the correspondence of a pattern or combination of patterns of surface contours. Significance is determined by the comparative examination of two or more sets of surface contour patterns comprised of individual peaks, ridges, and furrows. Specifically, the relative height or depth, width, curvature, and spatial relationship of the individual peaks. Ridges and furrows within one set of surface contours are defined and compared to the corresponding features in the second set of contours. The agreement is significant when it exceeds the best agreement demonstrated between two toolmarks known to have been produced by different tools and is consistent with agreement demonstrated by toolmarks known to have been produced by the same tool. The statement that sufficient agreement exists between two toolmarks means that the likelihood of another tool could have made the mark can be considered a practical impossibility.
      - The current interpretation of individualization/identification is subjective in nature, founded on scientific principles and based on the examiner's training and experience.
    - - Sufficient agreement is the product of the examiners personal training, skills, and experience in
        
        a. Recognizing corresponding patterns of matching striations
        
        b. Recognizing corresponding patterns within impressed toolmarks
        
        c. Determining the best known non-match in their personal experience
        
        d. Comparing striated and impressed toolmarks
    - - Two broad categories
        
        a. An evidence toolmark from a crime scene is identified as having been made by a particular evidence tool.
        
        b. Two evidence toolmarks recovered from the same or separate crime scenes are identified as having been made by a single tool (no tool submitted).
      - Categories
        
        a. Identification
        
        b. Inconclusive
        
        i. Some agreement of individual characteristics and all discernable class characteristics, but insufficient for identification
        
        ii. Agreement of all discernable class characteristics without agreement or disagreement of individual characteristics due to an absence, insufficiency, or lack of reproducibility
        
        iii. Agreement of all discernable class characteristics and disagreement of individual characteristics, but insufficient for an elimination
        
        c. Elimination
        
        d. Unsuitable for comparison
        
        i. Examples
        
        • Saws and files generally leave toolmarks in which the particulate material falls away from the object that was cut. Each subsequent action removes the toolmark created by the previous action.
        
        • Abrasive tools (e.g., grinding wheel or sandpaper) create individual marks on a surface. The tool changes (wears) with each rotation or stroke, eliminating any chance for identification.
  - - - Comparison microscope
      - Stereomicroscope
      - Microscope lighting options
      - Imaging equipment
  - - - A tool will bear unique microscopic characteristics due to the manufacturing processes it undergoes, as well as through use and abuse.
      - Tools will generally mark surfaces with which they come in contact with class and individual characteristics.
      - These class and individual characteristics are typically reproducible and identifiable with a particular tool.
    - - Test marks from an evidence tool compared to toolmarks on evidence items recovered from a crime scene to determine if the evidence tool marked the evidence item
      - Evidence toolmarks recovered from multiple crime scenes to determine if the scenes are related to a single tool
  - - - Chain of custody
      - Marking of evidence
      - Laboratory case identifiers
      - Investigative file identifier
      - Examiner identity
      - Quality assurance
    - - Paint
      - Glass
      - Blood
      - Bone
      - Soft tissue
      - Hair
      - Fibers
      - Wood
      - Metal smears
      - Masonry
      - Stone
    - - Paint may be removed by soaking the tool in alcohol or acetone.
      - Plaster may be removed by soaking the tool in 15% acetic acid.
      - Loose material may be removed by rinsing the tool in water or methanol.
      - Blood and other biohazards may be removed by soaking for at least one minute in a 10% bleach solution.
  - - - Remove the questioned marks from the evidence item. Before proceeding, obtain appropriate permission to modify the evidence.
      - Use plasticene (a plastic paste) or clay as a foundation to secure the questioned area in an appropriate orientation.
      - Fabricate a specimen mount.
      - Cast the marks and compare these casts with casts of the test marks.
    - - If the class characteristics are different, it will be possible to exclude the questioned tool.
      - If the class characteristics are consistent, the examination should proceed.
    - - If an area of sufficient agreement is found, a corresponding index mark should be placed on the evidence using the same color permanent marker as was used to index the test mark.
  - - - The toolmarks were made by two different tools.
      - Damage occurred that caused distortion, deformation, or elimination of microscopic detail in the toolmark.
      - The tool was damaged, extensively used, abused, or remachined (i.e., sharpened) between the time the two marks were made.
      - Numerous subclass characteristics may be present.
    - - Remount both test and evidence marks end to end on their respective mounts and reexamine them.
      - Smoke the evidence toolmarks with magnesium smoke to enhance detail. This technique is beyond the scope of this training and should be performed only by an experienced examiner.
      - Prepare additional test marks in a different material, possibly a softer material or material more like the evidence item.
      - Change the amount, direction, and angle of force employed to create the test marks.
  - - - Remove the questioned marks from the evidence item. Before proceeding, obtain appropriate permission to modify the evidence.
      - Use plasticene or clay as a foundation to secure the questioned area in an appropriate orientation.
      - Fabricate a specimen mount.
      - Cast the marks and compare these casts with casts of the test marks.
    - - If the class characteristics are different, the examination should end.
      - If the class characteristics are the same, the examination should proceed.
    - - The toolmarks were made by two different tools.
      - Damage occurred that caused distortion, deformation, or elimination of microscopic detail in the toolmark.
      - The tool was damaged, extensively used, abused, or remachined (i.e., sharpened) between the time the two marks were made.
      - Numerous subclass characteristics may be present.
    - - With several evidence items with the same class characteristics and with varying degrees of quality, it may be necessary to use more than one evidence item or multiple marks on a particular item to establish a chain of identifications within a set of submitted items.
  - - - The end result of the placement of the specimens on their respective stages will be two questioned fractured objects prepared for comparison using reverse lighting.