Comprehensive Study Notes on Firearms Evidence, Tool Marks, and Impression Analysis

Course Scope and Exam Administration

• Exam Coverage and Curriculum Structure

  • The upcoming examination covers material across several structured chapters:

    • Chapter 1: Introduction to Forensic Science

    • Chapter 2: Crime Scene Procedures

    • Chapter 3: Evidence Documentation and Management

    • Chapter 15: Fingerprint Analysis

    • Chapter 16: Firearms Evidence and Tool Marks

    • Chapter 17: Impression Evidence

    • Trace Evidence: Principles and overview

  • Exam administration details:

    • Formal announcement will be provided $1\,\text{week}$ prior to the opening date.

    • The examination window will remain open on Brightspace for a duration of $3\,\text{days}$.

    • The format consists of a local multiple-choice review covering all cumulative topics.

Firearms Evidence and Mechanical Principles

• Fundamental Forensic Questions

  • Forensic examination of firearms evidence focuses on characterizing, categorizing, and comparing questioned ($Q$) evidence recovered from crime scenes (bullets, cartridge cases) against known ($K$) evidence test-fired in the laboratory using seized suspect weapons.

• Mechanical Components of Firearms

  • Barrel: The cylindrical tube through which the projectile travels upon firing.

  • Firing Mechanism: Consists of the trigger, hammer, and firing pin.

    • Operation relies on small, controlled chemical explosions to propel projectiles (analogous to internal combustion engines in automobiles).

  • Frame/Receiver: The outer housing casing that holds the functional components together.

  • Ammunition Storage: Held within either a removable magazine or a revolving cylinder.

• Primary Firearm Categories

  • Pistols: Handheld firearms, including single-chamber autoloading designs.

  • Revolvers: Handguns containing a rotating cylinder with multiple chambers.

  • Long Guns: Shoulder-fired weapons encompassing both rifles and shotguns.

• Action Types

  • Single-Action: Requires manual cocking of the hammer prior to pulling the trigger to release the hammer onto the firing pin.

  • Double-Action: Pulling the trigger performs two actions simultaneously: cocking the hammer and releasing it to strike the firing pin.

• Firing Sequence and Pressure Dynamics

  • Pulling the trigger releases the hammer, forcing the firing pin to impact the primer located in the base of the cartridge case.

  • The primer impact creates a spark that ignites the main propellant (gunpowder) inside the cartridge casing.

  • Combustion generates rapidly expanding hot gases within the confined barrel space, causing a rapid increase in internal pressure.

  • The pressure forces the bullet away from the casing and down the length of the barrel.

• Mechanical Comparison: Revolvers vs. Semiautomatic Pistols

  • Revolvers:

    • Ammunition is housed in a revolving cylinder aligned sequentially with the barrel.

    • Upon firing, expanding gases force the cartridge base backward against the rear wall of the frame, leaving a breech face mark on the primer and cartridge head.

    • Revolvers lack an automatic ejection mechanism; spent cartridge cases remain in the cylinder until manually extracted.

  • Semiautomatic Pistols:

    • Ammunition is fed upward from a magazine via a internal spring into the breech chamber.

    • Firing forces the casing backward against the breech face, leaving a breech face mark.

    • Contains an extractor tool that grabs the rim lip of the cartridge case to pull it into the chamber.

    • Contains an ejector tool that strikes and forcibly expels the spent cartridge case out of the firearm.

    • Generates $2\,\text{additional tool marks}$ (extractor marks and ejector marks) on spent casings that are absent in revolvers.

Barrels, Rifling, and Ammunition Class Characteristics

• Historical Evolution of Barrel Design

  • Smoothbore Barrels: Utilized in $1700\,\text{s}$ and early $1800\,\text{s}$ muskets.

    • Required manual loading of black powder charges and spherical lead musket balls down the muzzle.

    • Lacked internal rifling; projectiles were severely deflected by atmospheric factors, resulting in low accuracy and necessitating massed linear infantry formations.

  • Rifled Barrels: Developed to increase aerodynamic stability and operational range by imparting rotational spin along the projectile's longitudinal axis.

• Rifling Anatomy: Lands and Grooves

  • Grooves: Low-lying, spiral channels cut or impressed into the interior metal wall of the barrel.

  • Lands: The raised portions of metal remaining between the spiral grooves.

  • Directional Twist: The helical spiral turns in either a left-hand or right-hand direction.

    • Directional twist serves as a class exclusion parameter: a bullet with a left-hand twist cannot have been fired from a barrel machined with a right-hand twist.

• Dimensional Class Characteristics of Rifling

  • Number of lands and grooves contained within the barrel.

  • Width of the lands and grooves.

  • Direction of spiral twist (left or right).

  • Caliber: The nominal inner diameter of the barrel bore, measured between opposing lands.

    • Expressed in decimal fractions of an inch (e.g., $.45\,\text{caliber} = 0.45\,\text{inches}$, $.22\,\text{caliber}$, $30.06\,\text{caliber}$) or in millimeters (e.g., $9\,\text{mm}$).

• Shotgun Barrel Design and Gauge Determination

  • Shotguns generally feature smoothbore long barrels designed to fire shells containing multiple spherical pellets (shot) or single solid lead slugs.

  • Gauge: The unit of measure designating the interior diameter of a shotgun barrel.

    • Historically defined by the weight of a solid lead sphere that perfectly fits the bore, expressed as a fraction of a pound ($\text{lb}$).

    • $12\,\text{gauge}$: Bore diameter equals a lead sphere weighing $\frac{1}{12}\,\text{lb}$.

    • $20\,\text{gauge}$: Bore diameter equals a lead sphere weighing $\frac{1}{20}\,\text{lb}$.

  • Operational flexibility: Shotgun loads vary from small birdshot to heavy buckshot, as well as less-lethal projectiles such as beanbag rounds used for suspect incapacitation.

• Anatomy of Ammunition

  • Bullet: The metallic projectile expelled through the barrel.

  • Cartridge Case: The brass, steel, or aluminum container holding the internal assembly components.

  • Propellant: Smokeless gunpowder that generates high-pressure expanding gas upon ignition.

  • Primer: A small cup containing an impact-sensitive chemical compound located at the base of the cartridge.

  • Head Stamp: Stamped alphanumeric markings on the base of the cartridge case displaying caliber, manufacturer, and year of production (e.g., $9\,\text{mm Luger}$).

Tool Marks Generated by Firearm Operation

• Individualization of Rifling and Firearm Components

  • Although weapons produced under identical manufacturing specifications share class characteristics (make, model, land/groove count, caliber, twist direction), individual characteristics develop due to microscopic imperfections.

  • Manufacturing Imperfections: Machining tools used to cut or stamp rifling wear down continuously during factory production, causing minute structural variation from barrel to barrel even within a single production day.

  • Operational Wear: Repeated firing, corrosion, cleaning, and metallic friction introduce unique scratches and micro-defects over time.

• Types of Firearm Tool Marks for Comparison

  1. Firing Pin Impressions: Distinct indentation left on the primer cup when struck by the firing pin.

  2. Breech Face Marks: Microscopic transfer patterns impressed onto the cartridge base as internal gas pressure forces the case rearward against the breech block.

  3. Extractor Marks: Striations deposited on the cartridge rim lip by the mechanical arm that pulls the cartridge into the chamber.

  4. Ejector Marks: Impression marks deposited on the cartridge rim or base where the ejector pin forcefully strikes to throw the spent case out of the port.

  5. Bullet Striations: Fine parallel microscopic lines scratched along the soft metal exterior of a bullet as it forces its way past the harder steel lands and grooves inside the barrel.

Forensic Comparison Microscopy and Automated Identification Systems

• Comparison Microscopy

  • The standard instrument for ballistic comparison is the comparison microscope, consisting of two independent compound microscopes connected by an optical bridge.

  • Features and capabilities:

    • Displays questioned ($Q$) and known ($K$) evidence side by side under identical magnification.

    • Provides a split-screen optical field with a movable central dividing line.

    • Includes rotating mechanical stages to allow precise alignment of striations, lands, grooves, and tool mark contours.

• Analysis Outcomes and Error Classifications

  • True Positive (Match): High degree of structural consistency across individual characteristics; confirms common origin.

  • True Negative (Exclusion): Clear, non-matching individual and/or class characteristics; confirms different origins.

  • False Positive: Incorrectly matching two samples originating from different sources.

  • False Negative: Incorrectly excluding two samples that originated from the same source, often caused by bullet distortion, fragmentation, or ricochet damage.

  • Inconclusive: Insufficient surface detail or excessive physical degradation prevents definitive identification or exclusion.

• Automated Ballistics Identification Systems

  • IBIS (Integrated Ballistic Identification System): The underlying hardware and software platform used to capture digital images of bullet striations and cartridge case tool marks.

  • NIBIN (National Integrated Ballistics Information Network): The national database maintained by the Bureau of Alcohol, Tobacco, Firearms and Explosives (ATF) allowing law enforcement agencies to compare ballistic digital images regionally and nationally.

• Three-Dimensional Surface Topography

  • Unlike two-dimensional ($2\text{D}$) fingerprint impressions, firearm tool marks exist in three dimensions ($3\text{D}$).

  • High-definition digital imaging captures surface height, depth, and spatial contours of lands, grooves, and impressions.

  • Topographic and Saliency Mapping: Generates detailed surface contour maps that enable mathematical algorithms to quantify structural similarities, establishing statistical probability models for source attribution.

General Tool Mark Evidence and 3D-Printed Firearms

• Principles of General Tool Mark Analysis

  • A tool mark is created when a harder material comes into mechanical contact with a softer substrate, leaving an indentation, scratch, or cut.

  • Three Primary Mechanics of Tool Marks:

    1. Compression Marks: Created when a tool is forcefully pressed into a surface without lateral movement (e.g., a wrench jaw clamped onto a pipe or a hammer striking a surface).

    2. Sliding/Friction Marks: Created when a tool slides laterally across a surface, producing parallel striations (e.g., a screwdriver blade dragged across a metal door frame).

    3. Cutting Marks: Created when a tool squeezes or shears through a substrate (e.g., scissors, wire cutters, or saws cutting through material).

  • The angle of application severely alters the dynamic profile of the resulting striations and impression widths.

• 3D-Printed Firearms ("Ghost Guns")

  • Ghost guns are homemade, un-serialized firearms fabricated using polymer materials or additive manufacturing tools, making them difficult to trace through traditional records.

  • Additive manufacturing (3D printing) heats thermoplastic filament inside a preheating unit and extrudes it through a computer-controlled nozzle along precise spatial pathways to build components layer by layer.

  • Consecutive Matching Striations (CMS): An analytical method evaluating sequential matching striation patterns formed by the continuous extrusion layers and nozzle pathing of a specific 3D printer.

  • Temporal Degradation of Tool Signatures:

    • Components of 3D printers (nozzles, drive gears) undergo mechanical wear rapidly compared to hardened steel firearms.

    • If a suspect prints a firearm frame and subsequently utilizes the 3D printer for several months (e.g., over a $6\,\text{to}\,8\,\text{month}$ interval) to print unrelated items, the CMS signature will evolve, potentially preventing an analytical match to a crime scene weapon.

Tool Mark Casting Techniques

• Forensic Casting Compounds

  • MicroSil: A specialized two-part forensic silicone putty consisting of a main base material and a liquid chemical catalyst (blue activator tube).

    • Procedure: The base putty and catalyst are manually kneaded together and pressed firmly over a tool mark or 3D-printed surface to form an accurate, flexible micro-impression cast.

  • Accutrans: An automated silicone dispensing system utilizing a dual-barrel cartridge mounted in a caulking-gun frame.

    • Procedure: A specialized mixing nozzle automatically blends the two-part polymer formula upon squeezing the trigger, allowing direct, clean application onto vertical or delicate substrates.

  • Purple Putty: Used for capturing impressions across larger surface areas.

• Investigative Application Case Study

  • In a series of commercial motel burglaries in Cincinnati, suspects utilized a custom-engineered mechanical tool specifically designed to pop vehicle trunk locks to steal laptops and tools. Forensic analysts applied casting putty to lock mechanisms across multiple crime scenes to recover matching striations, linking the burglaries to a single unique tool recovered from a suspect.

Distance Determination and Gunshot Residue Analysis

• Distance Determination Methodology

  • Distance determination estimates the physical range between a firearm muzzle and a target based on gunshot residue (GSR) deposition patterns.

  • Laboratory procedures require test-firing the suspect firearm using identical ammunition at measured incremental distances against test targets to match discharge patterns.

• Residue Patterns Relative to Muzzle-to-Target Distance

  • Contact / Heavy Press: Muzzle held directly against target. Produces a tight, dark burn hole, severe structural tearing of fabric, and heavy, localized carbon soot deposition.

  • Close Range ($6\,\text{cm}$): Thermal hot gases create concentrated circular stippling (unburned and partially burned gunpowder grains embedded into the substrate) immediately surrounding the bullet entrance defect.

  • Intermediate Range: As distance increases, thermal soot disperses while unburned gunpowder particles spread over a wider area, decreasing in density.

  • Extended Distance Threshold ($120\,\text{cm}$): Beyond approximately $120\,\text{cm}$, gaseous exhaust, soot, and particulate stippling fail to reach the target.

  • Bullet Wipe: At extended ranges, the entrance hole exhibits only a dark, thin ring around the margin created by dirty lead, primer residue, and barrel lubricants wiped off the bullet surface upon entry.

• Ohio Admissibility Rules for Chemical GSR Testing

  • Gunshot residue contains heavy metal particles (lead, barium, antimony) expelled during primer ignition.

  • Evidentiary Restriction: Courts in Ohio do not admit dermal swab chemical analysis for GSR from suspect hands due to low scientific reliability and high rates of false positives.

    • Common environmental contaminants causing false positives include soil nitrates (encountered during gardening) and organic nitrates found in cured meats (such as bologna).

  • Physical evidence of firing (photographed soot, thermal burns, or physical stippling on skin or clothing) remains admissible as circumstantial evidence.

Serial Number Restoration Procedures

• Mechanical Principles of Serial Number Stamping

  • Serial numbers are stamped into firearm metal frames using hardened steel dies.

  • The stamping process compresses the metallic crystal lattice structure beneath the visible digits, increasing metal density and altering local chemical reactivity.

• Acid Etching Restoration Method

  • When criminals file or grind away serial numbers, they typically remove only the visible surface digits, leaving the underlying compressed crystal structure intact.

  • Laboratory technicians apply acid etching reagents (acid chemical solutions) to the ground region.

  • The acid reacts and dissolves the altered, strain-hardened metal underneath at a faster rate than the surrounding matrix, causing the obliterated numbers to temporarily reappear ("pop up").

• Polymer and Hidden Restoration

  • Hidden Serial Numbers: Firearms manufacturers frequently stamp secondary, redundant serial numbers on concealed internal components unknown to average consumers.

  • Polymer Restorations: For modern plastic/polymer frame weapons, technicians utilize specialized chemical etching, low-power microscopy, and spectral reflectance spectroscopy to visualize obliterated numbers.

Introduction to Impression Evidence and Footwear Analysis

• Categories of Impression Evidence

  • Impression evidence belongs to pattern evidence, encompassing footwear prints, tire tread tracks, tool marks, and fingerprints.

  • Patent Impressions: Clearly visible $2\text{D}$ or $3\text{D}$ prints left in visible transfer media (e.g., blood, paint, ink, oil, or wet soil).

  • Latent Impressions: Invisible or barely visible impressions requiring physical or chemical enhancement (e.g., faint dust tracks on smooth flooring).

  • Plastic Impressions: Three-dimensional indentations pressed into soft, pliable substrates (e.g., mud, thick grease, snow, sand).

• Dimensionality

  • Two-Dimensional ($2\text{D}$): Prints deposited onto non-yielding, flat surfaces (e.g., dirty shoe soles leaving dust on clean tile).

  • Three-Dimensional ($3\text{D}$): Prints pressed into deformable materials (e.g., shoe tread forced into roadside mud or wet sand).

• Successive Classification Process

  • Class Characteristics: Features shared by manufacturing (brand, tread pattern design, shoe model, physical length, and width).

  • Individual Characteristics: Unique accidental characteristics accumulated through wear and use (e.g., localized tread erosion, unique cut marks, gouges, embedded stones, or twigs).

• Detection and Enhancement Techniques

  • Oblique Lighting: Positioning a handheld light source at a low, grazing angle across a floor surface to highlight latent dust prints via shadows.

  • Electrostatic Discharge Apparatus (ESDA / Electrostatic Dust Lifter):

    • Equipment consists of a high-voltage power supply, a conductive grounding plate, and a dark Mylar plastic lifting sheet.

    • Procedure: The Mylar film is placed over a suspected $2\text{D}$ dust impression on a flat surface. Applying an electrostatic charge draws fine dust particles upward onto the film, capturing the precise outline of the footwear tread.

• Casting Three-Dimensional Impressions

  • Dental Stone: A high-strength gypsum plaster used to cast $3\text{D}$ impression evidence pressed into soil, sand, or mud.

  • Procedure: All impression evidence must be photographed with a rigid scale prior to casting. Liquid dental stone mixture is poured carefully into the impression and allowed to cure into a rigid, solid block for lab comparison.