Forensic Ballistics Notes - lect 2

Jury Duty Reminder

• Log availability for jury duty on the FSI 5560 Moodle page.

• Indicate availability for all dates if applicable.

• Complete this task within the next week for allocation to specific slots.

Proof Markings on Firearms

• Legal firearms must undergo a proofing process to ensure safety.

• Proofing involves firing special ammunition under controlled conditions at specialized proof houses (e.g., Birmingham and London in the UK).

• Proofing ammunition has approximately 30% higher pressure than standard ammunition.

• If the firearm withstands the proofing ammunition, it receives markings indicating it has been proofed, which can be traced back to the proof house.

• A proof certificate accompanies the firearm, including the weapon's serial numbers.

• It is a criminal offense to sell an unproofed or out-of-proof firearm.

• If pressure-bearing components are replaced, the firearm must be reproofed.

• Multiple serial numbers may indicate legally changed components, but paperwork is required.

General Firing Procedure

• A live round is loaded into the breach (area adjacent to the chamber).

• The round is fed into the chamber (area holding the ammunition).

• Pulling the trigger initiates internal mechanisms, causing a firing pin or hammer to impact the primer cup.

• The primer contains an impact-sensitive explosive, leading to propellant ignition.

• Burning propellant produces gas, increasing gas pressure, and propelling the bullet down the barrel.

• Accidental shootings can be investigated by understanding these processes and verifying functions.

Classes of Firearms

• Pistols: Handheld firearms, including revolvers and semi-automatic weapons.

• Semi-automatic: The weapon cycles and readies the next round after each trigger pull, but requires a separate trigger pull for each shot.

• Fully automatic: The weapon continues to fire as long as the trigger is held down.

• Converting a semi-automatic weapon to fully automatic can be illegal, often achieved using "auto sears".

• In the UK, automatic firearms are generally prohibited, requiring special permissions from the Home Office for ownership or use.

• Rifles: Typically bolt action, but can be semi-automatic or automatic.

• Bolt action rifles are classic sniper rifles that cycle the weapon manually.

• Submachine guns: Smaller firearms that fire handgun ammunition, making them more concealable and easier to use in close quarters.

• Military assault rifles

• Shotguns: Characterized by a smoothbore barrel (no rifling), designed for firing multiple projectiles (shot).

• Rifling is not typical in shotguns because the shot would bounce down the barrel and not engage with rifle, also damaging the rifling, so smooth bore is more suitable.

• Air weapons: Least regulated in the UK, but legal classifications and certificates may be required depending on energy output.

• Modified or Homemade Firearms (Zip Guns): These are constructed from disparate parts including modified or 3D-printed parts.

Modified and Homemade Firearms

• Often referred to as "zip guns", these can be made from old firearm parts, modified firearms, or 3D-printed components.

• 3D-printed weapons pose a growing challenge due to advancements in printing methods and material resilience.

• Early 3D-printed firearms were dangerous to the user due to polymer limitations.

• Improved materials now allow 3D-printed gun barrels to withstand multiple shots.
  These firearms made of plastics may evade metal detectors.

• Plans for 3D-printed firearms are often accessible online.

• Monitoring digital intelligence helps track individuals downloading or uploading these plans.

• These weapons are dangerous and come in various forms, presenting challenges for forensic ballistics investigations.

Barrels, Rifling, and Twists

• Key concepts include barrel rifling, bullet twists, and related calculations.

Barrel and Chamber Terminology

• Chamber: The area that holds the actual round of ammunition.

• Rifling marks: Lines inside the barrel, angled for the bullet's spin.

• Firing pin impacts the primer, igniting the propellant; pressure builds, and the bullet accelerates down the barrel.

Firing Process Stages

• Primer activation by firing pin impact.

• Propellant burning, initiated by heat from the primer through the flash hole, thermally propagates through the propellant.

• Pressure builds until it overcomes the bullet's resistance, accelerating it down the barrel.

• The tight fit of the bullet ensures optimum pressure and velocity.

• Propellant continues burning as the bullet moves down the barrel.

• The bullet engages with the rifling, causing it to spin.

• The system is carefully calibrated for bullet acceleration and rotation (gyroscopic stability).

Rifling Details

• Lands: Raised areas inside the barrel.

• Grooves: Sunken areas between the lands.

• The lands imprint landmarks (sunken areas) on the bullet, while grooves create raised areas.

• Caliber is measured as the internal diameter between two opposite grooves.

Twist Direction and Rate

• Twist direction (left or right) is a class characteristic; right-hand twist is more common.

• Twist rate indicates how many rotations the rifling makes per inch (e.g., 1 in 10 means one rotation every 10 inches).

• Twist rate does not indicate barrel length; it only indicates the rate of rotation.

Described Angle

• The described angle is the angle between the longitudinal axis of the bullet and the rifling markings on the bullet.

• It can be measured from bullets fired from a specific type of rifling and is a class characteristic.

Measuring Rifling Marks

• The width of landmarks can be measured using calipers or microscopes.

• Count the number of lands and grooves imparted on the bullet; the numbers should be equal.

• Measure the described angle using microscopy and imaging.

• Be consistent in measurements for relative comparisons.

• The shiny lines are the transitions between land and groove marks, not the marks themselves.

Twist Rate Calculation

• $L = \frac{C}{tan(\theta)}$ where:

  • $L$ = length of barrel for one twist.

  • $C$ = Bore circumference ($\pi$ x diameter).

  • $\theta$ = The described angle.

• Example: For a 9mm bullet with a 4.5-degree described rifling mark angle:

• $L = \frac{\pi \times (9 \times 10^{-3})}{tan(4.5)}$ (initial calculation in SI units).

• Conversion from meters to inches: multiply by 39.37.

• The calculated result is often rounded to the nearest whole number, e.g., 14.144 inches becomes a 1 in 14 twist rate.

Rifling Profile Types

• Standard Rifling: Rigid transitions and clear lands and grooves.

• Polygonal Rifling: Smooth transitions and less engagement with the bullet; makes forensic analysis challenging, but is cheaper for manufacturers.

• Trapezoidal Rifling: Between standard and polygonal in terms of definition.

Rifling Production Methods

• Four main processes:

  • Broach Cuts: a single point tool cuts one groove at a time. This is a long process and requires high precision.

  • Electrochemical Etching: acid is used to etch away the barrel between by using a negative of your desire rifling form.

  • Hammer Forged: hammers squeeze the barrel onto a former.

  • Button Rifling: tungsten carbide button is pushed and rotated through the barrel.

• Log availability for jury duty on the FSI 5560 Moodle page.

• Indicate availability for all dates if applicable.

• Complete this task within the next week for allocation to specific slots.

Proof Markings on Firearms

• Legal firearms must undergo a proofing process conducted at specialized proof houses to ensure they meet safety standards before being sold to the public. This process is critical for verifying the integrity of the firearm.

• Proofing involves firing special ammunition that generates approximately 30% higher pressure than standard ammunition under controlled conditions at specialized proof houses (e.g., Birmingham and London in the UK). The firearm's ability to withstand this high-pressure test is a key indicator of its safety.

• Proofing ammunition has approximately 30% higher pressure than standard ammunition to simulate extreme conditions and ensure the firearm can handle typical use safely.

• If the firearm successfully withstands the proofing ammunition, it receives specific markings indicating it has been proofed. These markings can be traced back to the proof house that conducted the test, providing accountability and verification.

• A proof certificate accompanies the firearm, including essential details such as the weapon's serial numbers and a record of the proofing process. This certificate serves as documentation of the firearm's safety compliance.

• It is a criminal offense to sell an unproofed or out-of-proof firearm, emphasizing the legal requirement for firearms to undergo proofing before sale. This helps prevent unsafe weapons from reaching the public.

• If pressure-bearing components are replaced, the firearm must be reproofed to ensure the modified weapon still meets safety standards. This is crucial because changes to these components can affect the firearm's ability to handle pressure safely.

• Multiple serial numbers may indicate legally changed components, but paperwork is required to verify the legality of these changes. Proper documentation is essential to avoid legal issues.

General Firing Procedure

• A live round is loaded into the breach (area adjacent to the chamber), readying the firearm for firing.

• The round is then fed into the chamber (area holding the ammunition), aligning it for ignition.

• Pulling the trigger initiates internal mechanisms, causing a firing pin or hammer to strike the primer cup.

• The primer contains an impact-sensitive explosive, which, when struck, ignites the propellant.

• Burning propellant produces gas, increasing gas pressure inside the cartridge, and this pressure propels the bullet down the barrel.

• Accidental shootings can be investigated by understanding these processes and verifying the correct function of each step. Identifying malfunctions can help in determining the cause of accidental discharges.

Classes of Firearms

• Pistols: Handheld firearms, including revolvers and semi-automatic weapons, designed for single-handed use.

• Semi-automatic: The weapon automatically cycles and readies the next round after each trigger pull, but requires a separate trigger pull for each subsequent shot. This mechanism allows for rapid firing while maintaining control.

• Fully automatic: The weapon continues to fire continuously as long as the trigger is held down, enabling a high rate of fire. These firearms are often restricted due to their potential use in violent crimes.

• Converting a semi-automatic weapon to fully automatic can be illegal in many jurisdictions, often achieved using