Firearms, Ballistics, and Toolmark Examination¶
Summary¶
This chapter covers the full ballistic pathway from chamber to target, and the microscopic examination techniques used to link projectiles and cartridge cases to specific firearms. Internal ballistics covers the rifling process, lands and grooves, breech face markings, firing pin impressions, and ejector marks as class and individual characteristics. External ballistics addresses trajectory calculation, and terminal ballistics examines entrance versus exit wounds, wound morphology, and ricochet analysis. Gunshot residue (GSR) and the Modified Griess test are covered as chemical evidence. The chapter extends to toolmark analysis — compression, sliding, and cutting marks — and the specialized procedures for serial number restoration via acid etching.
Learning Objectives¶
By the end of this chapter, investigators will be able to:
- Explain how rifling imprints class and individual characteristics on fired bullets and cartridge cases.
- Identify the key physical evidence from a firearm discharge: bullet, cartridge case, and GSR.
- Distinguish between entrance and exit wounds and explain the wound morphology differences.
- Describe how the comparison microscope is used to link a recovered bullet to a specific firearm.
- Explain the acid etching technique for serial number restoration.
Concepts Covered¶
This chapter covers the following 20 concepts from the learning graph:
- Internal Ballistics
- Rifling in Firearms
- Lands and Grooves
- Breech Face Markings
- Firing Pin Impressions
- Ejector Marks
- External Ballistics
- Bullet Trajectory Calculation
- Terminal Ballistics
- Entrance vs Exit Wounds
- Wound Morphology
- Ricochet Analysis
- Gunshot Residue (GSR)
- Modified Griess Test
- Toolmark Analysis
- Compression Marks
- Sliding Marks
- Serial Number Restoration
- Acid Etching Technique
- Comparison Microscope
Prerequisites¶
This chapter builds on concepts from:
Welcome, Investigators!
Every firearm is a machine — and every machine leaves its own microscopic signature on everything it touches. A fired bullet carries the marks of the barrel that launched it; a spent cartridge case carries the marks of the breech, firing pin, and ejector. Firearms evidence connects projectiles and cases to specific weapons with remarkable precision. Follow the evidence — right down to the microscopic striations.
The Three Ballistic Phases¶
The science of ballistics is divided into three phases, each covering a different part of the bullet's journey.
Internal Ballistics¶
Internal ballistics describes the events inside the firearm from trigger pull to bullet exit at the muzzle.
When the trigger is pulled, the firing pin strikes the primer — a small explosive compound in the base of the cartridge. The primer ignites the propellant powder (smokeless powder in modern firearms), which undergoes rapid deflagration (combustion), generating a rapidly expanding gas that propels the bullet forward through the barrel.
Rifling is the system of spiral grooves cut into the interior of a firearm's barrel. The raised portions between the grooves are called lands; the cut channels are the grooves. As the bullet travels down the barrel, the lands engrave a spiral pattern of striations on the bullet's softer metal surface. This imparted spin stabilizes the bullet aerodynamically after it exits the muzzle — a bullet without spin would tumble unpredictably in flight.
Rifling characteristics include: - Twist direction: clockwise (right-hand twist, most common) or counterclockwise (left-hand twist) - Rate of twist: how quickly the spiral completes one full rotation (e.g., 1:10 = one complete turn in 10 inches of barrel length) - Number of lands and grooves: typically 4–8 depending on the manufacturer and caliber - Width and depth of lands and grooves
These measurements are class characteristics — they identify the make and model of firearm but are shared by all firearms of that type. The individual characteristics (the specific random microscopic striations) arise from the manufacturing process and from normal wear, making each barrel unique over time.
Breech face markings are impressions transferred onto the cartridge case base from the breech face (the metal surface the cartridge presses against when the chamber is closed). Any machining marks, tool marks, or imperfections on the breech face are transferred to the soft brass cartridge case primer area.
Firing pin impressions are indentations left on the primer by the firing pin strike. The shape, depth, and any surface texture of the firing pin are transferred to the primer.
Ejector marks are scratches or impressions left on the case rim by the ejector (the mechanism that pushes the spent cartridge out of the chamber after firing).
External Ballistics¶
External ballistics describes the bullet's flight from muzzle exit to target. Key factors affecting bullet trajectory include:
- Muzzle velocity — determined by the propellant charge and barrel length; typically 200–1,200 m/s depending on the caliber
- Gravity — causes the bullet to drop over distance; longer distances require more aim elevation
- Air resistance (drag) — decelerates the bullet over distance; depends on bullet mass, diameter, and shape
- Wind — causes lateral drift
Bullet trajectory calculation in forensic investigation reconstructs the path of the bullet through the scene — important for determining shooting distance, shooter position, and the number of shots fired. Investigators use trajectory rods (rigid rods inserted into bullet holes or wound tracks) and laser instruments to establish the angle of impact at intermediate targets (walls, doors, furniture) and extend the trajectory backward to the shooter's position.
Terminal Ballistics¶
Terminal ballistics describes the bullet's behavior on impact with the target. At close range, the three key wound morphology features are:
Entrance vs. Exit Wounds
Entrance wounds from handguns and rifles typically show: - A round or oval hole with an abrasion ring (friction burn from the bullet's passage) around the margin - At contact and near-contact range: stippling (gunpowder particle tattooing of the skin) and soot (unburned carbon deposition) may be present - Relatively contained wound margins
Exit wounds typically show: - Irregular, larger, stellate (star-shaped) or ragged margins - Lack of abrasion ring - Everted (outward-projecting) wound edges - No stippling or soot (the muzzle gases have dissipated by exit)
The distinction matters because the entrance wound establishes the direction of fire; the relationship between entrance and exit wounds (if both are present) establishes the trajectory through the target.
Wound morphology — the specific characteristics of a wound — provides information about the caliber of the projectile, the approximate shooting distance, and whether clothing or other intermediate targets were present.
Ricochet Analysis — when a bullet strikes a hard surface at an oblique angle and deflects rather than penetrating, it is a ricochet. Ricocheted bullets lose velocity, deform, and may leave characteristic lead wipe marks on the deflection surface. The angle of deflection depends on the bullet's velocity, angle of incidence, and the surface hardness.
Gunshot Residue (GSR)¶
Gunshot residue (GSR) consists of microscopic particles ejected from the firearm during discharge — from the muzzle, the gap between the cylinder and barrel (revolvers), or from the action (automatic/semi-automatic firearms). GSR is a mixture of primer combustion products, partially burned or unburned powder particles, and metallic fragments from the bullet, cartridge case, and barrel.
The most forensically significant GSR components are the primer residue particles — primarily lead (Pb), barium (Ba), and antimony (Sb) compounds. These elements are found in the priming mixture (lead styphnate, barium nitrate, antimony sulfide) and their specific elemental combination is characteristic of firearm discharge.
GSR deposits on: - The hands and wrists of the shooter (particularly the web between thumb and index finger) - The face and hair of the shooter - Clothing and surfaces near the discharge point
GSR is collected using adhesive stubs (SEM/EDX stubs) that are pressed against the skin to lift GSR particles. Analysis is performed by Scanning Electron Microscopy with Energy-Dispersive X-ray Spectroscopy (SEM-EDX), which simultaneously images the particles and performs elemental analysis — confirming the characteristic Pb/Ba/Sb signature.
The Modified Griess Test (also called the Dermal Nitrate Test) is a colorimetric chemical test that detects nitrite residues from gunpowder on surfaces (such as the wound area on skin or clothing). Desensitized photographic paper is treated with diphenylamine solution and pressed against the test surface; nitrite residues react to produce orange spots. The Modified Griess test is a presumptive test — it screens for gunpowder residue proximity and can help establish shooting distance, but it is not specific to firearms discharge.
Toolmark Analysis¶
Toolmark analysis extends the same comparison principles used in firearms examination to marks made by any tool — hammers, bolt cutters, wire strippers, screwdrivers, crowbars. Every tool has a unique combination of manufacturing marks, wear patterns, and damage that can transfer to softer materials on contact.
Types of Toolmarks¶
Compression marks (also called impressed marks) are made when a tool is pressed perpendicular to a surface, leaving an impression. A hammer blow, a crowbar pressed into a door frame, or a plier jaw gripping a wire all produce compression marks. The cross-section shape of the tool's contact face is preserved in the mark.
Sliding marks (also called striated marks) are made when a tool moves along a surface while pressing against it, creating parallel striations. Wire cutter blades sliding across a cut wire, or a screwdriver dragged along a surface, produce striated marks. The width, spacing, and individual irregularities in the striated marks can link them to a specific tool.
Comparison Microscope¶
The comparison microscope is the instrument central to both firearms and toolmark examination. It consists of two separate microscopes joined by an optical bridge that allows the examiner to view two separate objects side by side in a split-field view — with the left side showing one specimen and the right side showing the other.
In firearms examination, a questioned bullet or cartridge case from a crime scene is placed on one stage, and a test-fired bullet or cartridge case from the suspect firearm is placed on the other stage. The examiner rotates both specimens and adjusts the alignment until the microscopic striations on the two items are aligned. If the striations match (the patterns are continuous across the dividing line between the two fields), this provides evidence that both specimens were fired from the same barrel.
Serial Number Restoration¶
Many firearms recovered at crime scenes have had their serial numbers obliterated — ground off or stamped over — to prevent identification. Serial number restoration recovers the original numbers through a metallurgical technique based on the physical properties of stamped metal.
When a serial number is stamped into a metal surface, the die applies enormous pressure that compresses the metal beneath each character. This work-hardened zone extends approximately 1–3 mm below the stamped surface. Even if the surface metal is ground away, the crystalline structure and residual stress in the work-hardened zone remain different from the surrounding unstressed metal.
The acid etching technique exploits this difference. The area is polished smooth, then a chemical etchant (typically ferric chloride for steel, or acid copper sulfate solution for other metals) is applied. The etchant reacts differentially with the stressed and unstressed metal — the work-hardened zone under each stamped character dissolves at a different rate, causing the original number to reappear temporarily as a ghost image. Photographs must be taken immediately, as the image fades as etching continues.
What Does the Data Tell Us?
Serial number restoration does not always succeed — if the obliteration was deep enough (removed more than 1–3 mm of metal), the work-hardened zone may be completely destroyed. Newer techniques, including scanning electron microscopy of the polished surface to detect microstructural differences and magnetic particle inspection, can sometimes recover numbers when chemical etching fails. Technology advances; forensic techniques must keep pace.
Diagram: Firearms Ballistic Pathway Interactive Diagram¶
Firearms Ballistic Pathway Interactive Diagram
Type: infographic
sim-id: ballistic-pathway
Library: p5.js
Status: Specified
Learning Objective: Identify the three ballistic phases and the key evidence produced at each stage (Bloom Level 1 — Remember; verb: identify).
Bloom Level: Remember (L1) Bloom Verb: Identify
Purpose: Visualize the complete ballistic pathway from trigger pull to wound, with interactive identification of evidence at each stage.
Layout: - Horizontal diagram showing three labeled sections: Internal (inside firearm), External (flight), Terminal (impact) - Illustrated cross-section of the firearm barrel, a simplified bullet flight arc, and a target at the end - Evidence items labeled at each stage: rifling marks, breech face impression, GSR emission, trajectory angle, wound characteristics
Interactive controls: - Click any labeled evidence item to open a detail panel: what it is, when it is produced, and how it is collected and analyzed - Hover over the barrel cross-section to see animated rifling striations forming on the bullet - Click the "Entry wound" and "Exit wound" regions to see comparison descriptions
Data Visibility Requirements: - Show specific class characteristics (number of lands and grooves, twist direction) that can be determined from the bullet - Show GSR emission direction from the firearm during discharge - Show wound morphology differences between entry and exit
Instructional Rationale: A Remember objective (identify evidence at each ballistic stage) is best served by an annotated diagram where each evidence item is labeled and clickable for definition and collection procedure.
Color scheme: Gun metal gray for firearm, red for trajectory line, blue for evidence labels, tan for wound area.
Key Concepts Review¶
The following table summarizes the major concepts from this chapter:
| Concept | Definition |
|---|---|
| Rifling | Spiral grooves in barrel that spin the bullet and impart class/individual striation characteristics |
| Lands and Grooves | Lands = raised portions; grooves = cut channels; both imprint on bullet |
| Breech Face Markings | Unique impressions on cartridge case base from chamber's breech face |
| Firing Pin Impressions | Indentation on primer from firing pin; shape unique to specific firearm |
| Internal Ballistics | Events inside the firearm from trigger pull to muzzle exit |
| External Ballistics | Bullet flight governed by velocity, gravity, drag, and wind |
| Terminal Ballistics | Bullet-target interaction; entrance vs. exit wound morphology |
| GSR | Lead/barium/antimony primer combustion particles; collected by adhesive stub; analyzed by SEM-EDX |
| Comparison Microscope | Two-stage microscope for side-by-side striation comparison |
| Toolmark Compression | Tool pressed perpendicular to surface; preserves tool cross-section shape |
| Toolmark Striation | Tool moving along surface; creates parallel striations unique to tool |
| Acid Etching | Chemical technique to restore obliterated serial numbers via work-hardened metal differential |
Challenge: Class vs. Individual
An investigator recovers a bullet from a wall at a crime scene. Under the comparison microscope, the bullet shows six lands and grooves with a right-hand twist.
a) Are these class or individual characteristics? What do they tell you? b) What would you need to do to link this bullet to a specific firearm?
Answer: (a) Six lands and grooves with right-hand twist are class characteristics — they indicate the type/model of firearm this was fired from (many Glock pistols, for example, have hexagonal rifling; six-land right-hand twist is common in several manufacturers). This narrows the possible weapon types but does not identify a specific firearm. (b) To link the bullet to a specific firearm, you need a suspect weapon from which test shots can be fired. The test-fired bullets are then compared under the comparison microscope with the crime scene bullet. If the microscopic individual characteristics (the specific stria pattern arising from the barrel's unique surface) align continuously across the split-field view, a conclusion of "consistent with having been fired from this specific firearm" can be made.
Case Closed — For Now
From the microscopic striations on a bullet to the chemical signature of gunshot residue, firearms evidence links physical objects to specific events with remarkable precision. Chapter 14 shifts from weapons to words — the forensic examination of documents, handwriting, and forgery. Follow the evidence!