Forensic Ballistics - Week 2 Notes - lect 3
Jury Duty Reminder
Complete jury duty availability on the ballistics page before Wednesday.
Attend official assigned session; swapping is required if unable to attend.
Check timetables for clashes with lab sessions.
Lecture Overview
Recap of key terminologies and processes from last year.
Forensic application of ballistics in police intelligence and wider operations.
Aims
Build on understanding of cartridges, bullets, and their structures.
Expand knowledge of class and individual characteristics.
Review markings on bullets and cartridges, their interpretation, and evidential value.
Live Ammunition
Defined as functional units referred to as rounds of ammunition.
Key defining values: calibre and chambering.
Calibre
Diameter of the bullet fired from a firearm.
Also the internal diameter of the gun barrel; measured between two grooves diametrically opposite inside the barrel.
Example: 7.62mm calibre means the bullet diameter is approximately 7.62mm.
It's a ballpark figure, not always precise, which can be frustrating.
Bullets of the same calibre can have different profiles, shapes, and lengths, leading to varied performance and compatibility.
Chambering
Length of the cartridge case, which sits in the chamber of the firearm.
Example: 7.62 x 39mm ammunition; 7.62mm is the calibre, and 39mm is the chambering length.
Chambering values tend to be accurate.
Caliber Examples
7.62 x 39mm: Classic AK-47 ammunition.
7.62 x 51mm: Standard NATO military ammunition.
7.62 x 54R: Older style Russian military ammunition with a protruding rim.
7.62 x 25mm Tokarev: Much smaller ammunition with the same bullet diameter.
Caliber Unit Systems and Conversion
Metric (millimeters) and Imperial (inches) systems are used.
5.56mm (military) is approximately equivalent to 0.223 inches (civilian).
Caliber system: Decimal component removed from imperial value (e.g., .223 becomes 223 cal).
Conversions aren't always perfect, requiring awareness of general conventions.
Ammunition Variety
A wide range of ammunition exists for given calibers.
Chambers are specific to the firearm and cartridge length; incorrect ammunition can cause damage or malfunction.
Safety: Always use the correct ammunition.
Consequences of Using Incorrect Ammunition
Too Big:
May not fit; weapon won't close.
If it's a close fit, it may jam in battery after firing.
Cartridge case expands and gets stuck in the chamber.
Too Small:
The firing pin may miss the primer.
If it activates, the cartridge expands excessively, leading to rupture and potential explosion due to unusual pressure conditions on the propellant.
Propellant may detonate instead of burning, causing catastrophic failure of the firearm and potential harm to the shooter.
Head Stamps
Located on the base of the cartridge case; provides useful information.
Can identify manufacturer, calibre, and other details.
Can assist in tracing ammunition to legal purchases via logged firearm certificates.
Examples:
Imperial British 303: Firing a bullet 0.303 inches wide.
WW 45 auto: Fires a 0.45-inch diameter bullet.
Winchester 270: Fires a 0.270-inch diameter bullet.
Primer Appearance
Indicates whether the cartridge has been fired.
Central firing pin impact on centerfire cartridges; off-center impacts suggest firearm issues.
Cartridge Structure Recap
Flash hole: Where hot material jets through from the primer to the propellant.
What to look for
Identify manufacturers, users, and calibers.
Determine if it's self-loading or single-shot.
Self-Loading vs. Single-Shot Mechanisms
Self-loading firearms: Tend to use ammunition with non-protruding base rims for efficient stacking in magazines.
Single-shot firearms: May use ammunition with protruding rims (e.g., bolt action rifles and revolvers).
Weapons are designed for a specific type of ammunition (rimmed or non-rimmed).
Ammunition Documentation
Accurately document ammunition with technical diagrams, measurements, and notes.
Use rulers, Vernier calipers, or micrometers for measurements.
Compare findings to factory ammunition specifications and databases.
Class vs. Individual Characteristics
Class characteristics:
General characteristics possessed by multiple items.
Useful for ruling out possibilities (e.g., caliber).
Individual characteristics:
Unique markings specific to a firearm.
Examples: ejector marks, extractor marks, firing pin marks, chamber markings.
Ballistic Fingerprint
Markings on cartridges that are unique to a specific firearm, similar to fingerprints.
Firearms develop individual markings over time due to wear and usage.
Cartridge cases and bullets can be related to a suspect weapon through these markings.
Shotgun Ammunition
Brass heads and head stamps; plastic containers hold projectiles.
Brass contains pressure from burning propellant; plastic holds ball bearings.
Head stamp provides manufacturer and calibre information; information on plastic can be misleading.
Summary
Cartridge case analysis: Head stamp information, firing pin marks, chamber markings.
Bullet analysis: Markings from interactions within the gun barrel.
Barrels can be interchanged, which can change the markings a firearm leaves.
Bullet Composition and Jacketing
Bullets are typically lead-containing, not pure lead (lead-antimony alloy, approximately 97% lead, 3% antimony).
Antimony increases structural integrity and melting point.
A jacket is put around bullets at high velocity, high energy, and high temperatures for stability and to engage with rifling.
Jacketing Benefits
Prevents lead from skipping down the barrel.
Ensures engagement with rifling, stabilizing the bullet.
Bullet Nomenclature
Learn bullet design terminology; be able to draw and label components.
Components
Heel: The bottom corner of the projectile.
Boat tail: A slanting area at the base for aerodynamic advantages.
Outer jacket: Surrounds the lead alloy core.
Bearing surface: The flat, parallel surface that interacts with rifling.
Head: The curved top of the bullet.
Ogive: The curvature of the head; affects the size of the bearing surface.
Metplatz: The very tip of the projectile.
"Make sure you're talking about bullet terms, so know these. If you're asked to draw a cartridge, make sure you know cartridge terms. If you're asked to draw a round of ammunition, has both, put the right terms in the right place."
Bullet Designs and Applications
Unjacketed bullets are used for slower bullets.
Jury Duty Reminder
Complete jury duty availability on the ballistics page before Wednesday.
Attend official assigned session; swapping is required if unable to attend. Coordinate with relevant authorities for necessary documentation and approvals for any changes to scheduled appearances.
Check timetables for clashes with lab sessions; plan ahead to manage any time conflicts.
Lecture Overview
Recap of key terminologies and processes from last year, emphasizing areas where students showed less proficiency.
Forensic application of ballistics in police intelligence and wider operations, including case studies of successful applications.
Aims
Build on understanding of cartridges, bullets, and their structures, incorporating the latest advancements in ammunition technology.
Expand knowledge of class and individual characteristics, enhancing pattern recognition skills to differentiate various ammunition types.
Review markings on bullets and cartridges, their interpretation, and evidential value, discussing recent court cases where ballistic evidence played a crucial role.
Live Ammunition
Defined as functional units referred to as rounds of ammunition. Each round consists of a cartridge case, primer, propellant, and projectile.
Key defining values: calibre and chambering, essential for firearm compatibility and safety.
Calibre
Diameter of the bullet fired from a firearm; affects ballistic performance and compatibility.
Also the internal diameter of the gun barrel; measured between two grooves diametrically opposite inside the barrel. This measurement is crucial for determining the correct ammunition.
Example: 7.62mm calibre means the bullet diameter is approximately 7.62mm.
It's a ballpark figure, not always precise, which can be frustrating. Manufacturing tolerances and variations in bullet design can cause slight discrepancies.
Bullets of the same calibre can have different profiles, shapes, and lengths, leading to varied performance and compatibility. These variations affect trajectory, velocity, and impact energy.
Chambering
Length of the cartridge case, which sits in the chamber of the firearm; critical for ensuring proper fit and function.
Example: 7.62 x 39mm ammunition; 7.62mm is the calibre, and 39mm is the chambering length.
Chambering values tend to be accurate. Deviations can lead to unsafe conditions.
Caliber Examples
7.62 x 39mm: Classic AK-47 ammunition, known for its reliability and widespread use.
7.62 x 51mm: Standard NATO military ammunition, offering greater range and penetration.
7.62 x 54R: Older style Russian military ammunition with a protruding rim, mainly used in vintage rifles.
7.62 x 25mm Tokarev: Much smaller ammunition with the same bullet diameter, noted for its high velocity and flat trajectory.
Caliber Unit Systems and Conversion
Metric (millimeters) and Imperial (inches) systems are used, complicating comparisons and requiring precise conversions.
5.56mm (military) is approximately equivalent to 0.223 inches (civilian). This conversion is crucial for ammunition compatibility in different firearms.
Caliber system: Decimal component removed from imperial value (e.g., .223 becomes 223 cal). This convention is common in firearm nomenclature.
Conversions aren't always perfect, requiring awareness of general conventions. Variations may lead to confusion, especially in international contexts.
Ammunition Variety
A wide range of ammunition exists for given calibers, each designed for specific purposes (e.g., hunting, target shooting, law enforcement).
Chambers are specific to the firearm and cartridge length; incorrect ammunition can cause damage or malfunction. This specificity ensures safe operation and prevents catastrophic failures.
Safety: Always use the correct ammunition. Using improper ammunition can lead to severe injury or death.
Consequences of Using Incorrect Ammunition
Too Big:
May not fit; weapon won't close. Attempting to force it can damage the firearm.
If it's a close fit, it may jam in battery after firing. This can lead to dangerous malfunctions and require professional gunsmith intervention.
Cartridge case expands and gets stuck in the chamber, requiring specialized tools to remove.
Too Small:
The firing pin may miss the primer, resulting in a failure to fire.
If it activates, the cartridge expands excessively, leading to rupture and potential explosion due to unusual pressure conditions on the propellant. This can destroy the firearm and cause severe injury.
Propellant may detonate instead of burning, causing catastrophic failure of the firearm and potential harm to the shooter. This is due to the uncontrolled release of energy.
Head Stamps
Located on the base of the cartridge case; provides useful information such as manufacturer, calibre, and date of production.
Can identify manufacturer, calibre, and other details, aiding in tracing ammunition to its source.
Can assist in tracing ammunition to legal purchases via logged firearm certificates, helping law enforcement in investigations.
Examples:
Imperial British 303: Firing a bullet 0.303 inches wide. Used extensively in historical British rifles.
WW 45 auto: Fires a 0.45-inch diameter bullet. A popular round in many semi-automatic pistols.
Winchester 270: Fires a 0.270-inch diameter bullet. Commonly used for hunting.
Primer Appearance
Indicates whether the cartridge has been fired or if there are any firearm issues.
Central firing pin impact on centerfire cartridges; off-center impacts suggest firearm issues such as a worn firing pin or misaligned chamber.
Cartridge Structure Recap
Flash hole: Where hot material jets through from the primer to the propellant, igniting the main charge.
What to look for
Identify manufacturers, users, and calibers to gather intelligence and trace ammunition.
Determine if it's self-loading or single-shot ammunition helps understand the type of firearm involved.
Self-Loading vs. Single-Shot Mechanisms
Self-loading firearms: Tend to use ammunition with non-protruding base rims for efficient stacking in magazines and reliable feeding.
Single-shot firearms: May use ammunition with protruding rims (e.g., bolt action rifles and revolvers), which simplifies extraction.
Weapons are designed for a specific type of ammunition (rimmed or non-rimmed). Using the wrong type can cause malfunctions or damage.
Ammunition Documentation
Accurately document ammunition with technical diagrams, measurements, and notes to ensure accurate analysis and comparison.
Use rulers, Vernier calipers, or micrometers for measurements to obtain precise data.
Compare findings to factory ammunition specifications and databases to identify discrepancies or anomalies.
Class vs. Individual Characteristics
Class characteristics:
General characteristics possessed by multiple items, such as caliber, shape, and weight.
Useful for ruling out possibilities (e.g., caliber) and narrowing down potential firearms.
Individual characteristics:
Unique markings specific to a firearm, caused by imperfections or wear.
Examples: ejector marks, extractor marks, firing pin marks, chamber markings. These marks can link a cartridge to a specific weapon.
Ballistic Fingerprint
Markings on cartridges that are unique to a specific firearm, similar to fingerprints, providing a unique identifier.
Firearms develop individual markings over time due to wear and usage, making each weapon's markings distinct.
Cartridge cases and bullets can be related to a suspect weapon through these markings, providing critical evidence in criminal investigations.
Shotgun Ammunition
Brass heads and head stamps provide structural integrity and identification; plastic containers hold projectiles.
Brass contains pressure from burning propellant, ensuring safe combustion; plastic holds ball bearings or slugs.
Head stamp provides manufacturer and calibre information; information on plastic can be misleading due to variations in manufacturing.
Summary
Cartridge case analysis: Head stamp information, firing pin marks, chamber markings offer key insights.
Bullet analysis: Markings from interactions within the gun barrel can be matched to a specific firearm.
Barrels can be interchanged, which can change the markings a firearm leaves, complicating forensic analysis.
Bullet Composition and Jacketing
Bullets are typically lead-containing, not pure lead (lead-antimony alloy, approximately 97% lead, 3% antimony). The lead-antimony alloy enhances bullet hardness and performance.
Antimony increases structural integrity and melting point, preventing deformation during firing.
A jacket is put around bullets at high velocity, high energy, and high temperatures for stability and to engage with rifling, improving accuracy and range.
Jacketing Benefits
Prevents lead from skipping down the barrel, ensuring a smooth, consistent flight path.
Ensures engagement with rifling, stabilizing the bullet and increasing accuracy.
Bullet Nomenclature
Learn bullet design terminology; be able to draw and label components to effectively communicate findings.
Components
Heel: The bottom corner of the projectile, often tapered or rounded.
Boat tail: A slanting area at the base for aerodynamic advantages, reducing drag.
Outer jacket: Surrounds the lead alloy core, providing structural support and preventing deformation.
Bearing surface: The flat, parallel surface that interacts with rifling, imparting spin to the bullet.
Head: The curved top of the bullet, designed for optimal aerodynamic performance.
Ogive: The curvature of the head; affects the size of the bearing surface and influences ballistic coefficient.
Metplatz: The very tip of the projectile, impacting target first.
"Make sure you're talking about bullet terms, so know these. If you're asked to draw a cartridge, make sure you know cartridge terms. If you're asked to draw a round of ammunition, has both, put the right terms in the right place."
Bullet Designs and Applications
Unjacketed bullets are used for slower bullets, such as those in low-velocity cartridges, where jacket separation is not a concern.