Podcast
Questions and Answers
What distinguishes the broach cutter from the button process?
What distinguishes the broach cutter from the button process?
Which of the following terms describes the raised portion between the grooves in a rifled bore?
Which of the following terms describes the raised portion between the grooves in a rifled bore?
In a rifled firearm, how is the diameter commonly expressed?
In a rifled firearm, how is the diameter commonly expressed?
What method uses a mandrel to create rifling in firearm barrels?
What method uses a mandrel to create rifling in firearm barrels?
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What is the primary role of rifling in a firearm barrel?
What is the primary role of rifling in a firearm barrel?
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What is the main advantage of the button process compared to traditional cutting methods?
What is the main advantage of the button process compared to traditional cutting methods?
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What is a characteristic of the bore in a rifled firearm?
What is a characteristic of the bore in a rifled firearm?
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Which of the following describes the button in the button rifling process?
Which of the following describes the button in the button rifling process?
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What is the primary purpose of using an electrostatic unit with mylar film?
What is the primary purpose of using an electrostatic unit with mylar film?
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How long can floor surfaces be covered with a mylar sheet to search for dust impressions?
How long can floor surfaces be covered with a mylar sheet to search for dust impressions?
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What is the initial step in chemically developing snow impressions with Snow Impression Wax?
What is the initial step in chemically developing snow impressions with Snow Impression Wax?
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What is a key benefit of the electrostatic lifting technique?
What is a key benefit of the electrostatic lifting technique?
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What should be done after spraying the Snow Impression Wax?
What should be done after spraying the Snow Impression Wax?
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Which process is used to visualize faint or nearly invisible footwear impressions made with blood?
Which process is used to visualize faint or nearly invisible footwear impressions made with blood?
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What is applied after the Snow Impression Wax has dried?
What is applied after the Snow Impression Wax has dried?
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What is crucial for developing a charge difference in the electrostatic lifting technique?
What is crucial for developing a charge difference in the electrostatic lifting technique?
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What is the first step when examining evidence related to a shooting in a crime laboratory?
What is the first step when examining evidence related to a shooting in a crime laboratory?
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What characteristic features help in identifying gunpowder residue microscopically?
What characteristic features help in identifying gunpowder residue microscopically?
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What is a limitation when visually detecting gunpowder residue?
What is a limitation when visually detecting gunpowder residue?
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How can infrared photography assist in examining shooting evidence?
How can infrared photography assist in examining shooting evidence?
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Why might gunpowder residue not be visually detectable on a garment?
Why might gunpowder residue not be visually detectable on a garment?
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What may indicate the presence of gunpowder residue even if visual indicators are absent?
What may indicate the presence of gunpowder residue even if visual indicators are absent?
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Which of the following is NOT a reason why gunpowder residue might not be visually detected?
Which of the following is NOT a reason why gunpowder residue might not be visually detected?
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What is the consequence of visual inspection yielding no detectable gunpowder residue?
What is the consequence of visual inspection yielding no detectable gunpowder residue?
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What is a characteristic feature of semiautomatic pistols?
What is a characteristic feature of semiautomatic pistols?
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What is the primary difference between rifles and shotguns?
What is the primary difference between rifles and shotguns?
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Which type of revolver allows for swinging out the cylinder side for reloading?
Which type of revolver allows for swinging out the cylinder side for reloading?
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How does a break-top revolver operate?
How does a break-top revolver operate?
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What occurs when a semiautomatic pistol fires a cartridge?
What occurs when a semiautomatic pistol fires a cartridge?
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What distinguishes an automatic firearm from a semiautomatic firearm?
What distinguishes an automatic firearm from a semiautomatic firearm?
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What is the mechanism for firing chambers in solid-frame revolvers?
What is the mechanism for firing chambers in solid-frame revolvers?
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What is the type of ammunition found in shotguns?
What is the type of ammunition found in shotguns?
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What contributes to the individuality of each tool?
What contributes to the individuality of each tool?
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What is primarily modified on a cutting tool over its lifespan?
What is primarily modified on a cutting tool over its lifespan?
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How are tool markings compared in a forensic analysis?
How are tool markings compared in a forensic analysis?
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What type of evidence can enhance the evidential value of a tool?
What type of evidence can enhance the evidential value of a tool?
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Why is special care needed when handling evidence from a tool?
Why is special care needed when handling evidence from a tool?
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What happens when a tool scrapes against a softer surface?
What happens when a tool scrapes against a softer surface?
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Which of the following is a characteristic of striations left by tools during use?
Which of the following is a characteristic of striations left by tools during use?
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What process is utilized to analyze the markings made by tools?
What process is utilized to analyze the markings made by tools?
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What do the class characteristics of a weapon’s barrel include?
What do the class characteristics of a weapon’s barrel include?
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Why can't class characteristics impart individuality to a barrel?
Why can't class characteristics impart individuality to a barrel?
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What do striations on a barrel represent?
What do striations on a barrel represent?
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How are the unique striations on a bullet formed?
How are the unique striations on a bullet formed?
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What is necessary for a comparison between the markings on a fired bullet and those within a barrel?
What is necessary for a comparison between the markings on a fired bullet and those within a barrel?
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Which of the following correctly describes lands and grooves in rifled barrels?
Which of the following correctly describes lands and grooves in rifled barrels?
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What typically prevents two rifled barrels from having identical striation markings?
What typically prevents two rifled barrels from having identical striation markings?
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What is the effect of the rifling markings on bullets as they pass through the barrel?
What is the effect of the rifling markings on bullets as they pass through the barrel?
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Study Notes
Criminalistics: An Introduction to Forensic Science
- This book, Criminalistics: An Introduction to Forensic Science, is in its thirteenth edition.
- The book is by Richard Saferstein and Tiffany Roy.
- Chapter 9 is dedicated to Firearms, Tool Marks, and Other Impressions.
Firearms Identification
- Firearms identification is a discipline focused on determining if a bullet or cartridge was fired by a specific weapon.
- It's different from ballistics, which studies projectile motion.
- Structural variations (scratches, nicks, breaks, wear) on a bullet or tool mark help link it to a specific weapon or tool.
- Firearms identification is often crucial to crime scenes due to high frequency of shooting cases.
- It involves investigating various weapons, restoring serial numbers, identifying gunpowder residues, determining muzzle-to-target distances, and detecting powder residues on hands.
- Two categories of firearms:
- Handguns (pistols): held and fired with one hand. Types include single-shot, revolvers, and semi-automatic. Single action or double action firearms.
- Long Guns: Rifles (grooved barrel) and Shotguns (smooth barrel). Different firing mechanisms (single shot, repeating, semi-automatic, automatic).
Handguns
- Single-shot pistols: Fire a single round per cycle, requiring manual loading of each round.
- Revolvers: Feature multiple firing chambers in a revolving cylinder.
- Semi-automatic pistols: Feature a removable magazine and one shot per trigger pull, utilizing the firing mechanism to eject and load the next round.
Long Guns
- Long guns are either rifles or shotguns, designed to be fired while resting on the shoulder.
- Differences lie in ammunition (bullets for rifles, shot for shotguns) and barrel configuration (smooth for shotguns).
- Rifles have rifled barrels.
- Types of long guns cover various loading mechanisms (single shot, repeating, lever action, pump, slide, bolt and semi-automatic).
Gun Barrel Markings
- The barrel's inner surface leaves its markings on a passing bullet, making it unique to each gun.
- These markings are generated during manufacture through processes, like cutting grooves (rifling) using a broach, pressing with a button, or hammering with a mandrel.
- Microscopic drill marks are irregular and add uniqueness.
- Class characteristics (number of lands and grooves, twist direction) are consistent within a given manufacture but not individually unique
- Individual characteristics (striations/irregularities) are unique to each gun barrel.
Bullet Examination
- No two rifled barrels have identical striation markings.
- Key comparison points include the number of lands and grooves, their twist direction, and related details during initial examination to distinguish between one weapon and another through comparing evidence bullets and test-fired bullets.
- Microscopes are used to compare these striations.
Bullet Examination: Modern Technique
- Modern firearms identification uses comparison microscopes, locating and comparing striations on evidence and test bullets.
- Automated firearm identification systems exist.
- The analysis looks at the unique characteristics, such as the number of lands and grooves, their twist direction, and the minutiae marks, present on the bullet.
- Using a comparison microscope to match characteristic marks from a test bullet to those on evidence helps to determine if they originated from the same gun.
Distance Determination
- Gunpowder residue distribution around a bullet hole can help determine the firing distance.
- Distribution patterns are influenced by firing distance.
- Distance affects the presence, distribution, and density of unburned and partially burned powder particles on the target.
Gunpowder Residue on Hands
- Firing a weapon produces gunpowder and primer residues, often deposited on the shooter's hands.
- Examiners detect barium and antimony to determine if someone recently fired, handled, or was near a discharged firearm.
- Chemical tests (e.g., Greiss test) may be used if visual detection fails, confirming the presence of gunpowder residue.
Serial Numbers
- Methods of restoring obliterated serial numbers involve chemical etching.
- Etching agent dissolves the strained metal around a serial number, revealing the original marking.
- Careful cleaning and polishing are essential for successful restoration.
Firearm Evidence Collection
- Important aspects in collecting firearms for evidence include safety and maintaining chain of custody.
- Packaging, documentation (e.g., photos, recordings) of surrounding conditions of firearms are necessary for preservation of evidence.
- Unloading guns and recording chamber positions and cartridge states are critical pre-examination procedures..
Tool Marks
- Tool marks are impressions, cuts, gouges, or abrasions from a tool on another object.
- The size and shape of the tool may be used in a crime.
- Minute imperfections, such as nicks, scrapes, or irregularities, identify individual tools.
- Comparison methods, akin to ballistic testing, match these marks on the tool with marks at the crime scene.
- Comparing impressions can link a tool used at a crime to a particular tool, aiding in determining the crime tool.
Other Impressions
- Impressions of various types (e.g., shoes, tires, fabrics) are similarly collected and preserved, focusing on photography, especially of the impression's details.
- Chemical and other methods, such as casting can be used to preserve impressions in crime scenes, especially when they involve traces in materials such as blood
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