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Firearms & Ballistics Notes - lect 15
Key Reminders
Write up labs, analytical and ballistic labs.
Work on group presentations.
Third-year students: Court starts on Thursday, which means jury duty. Check your jury duty slot on the Moodle page (SI5560).
Arrive ahead of your time slot.
The session this week overlaps with a jury duty slot. Wait outside the courtroom until told to enter during a mid-session break.
The courtroom is in the Wagoda Building, over the bridge between Elliott and Rutherford Colleges (opposite the library).
Internal Ballistics
Internal ballistics is broken down into three key phases:
Lock time
Ignition time
Barrel time
Lock Time
Lock time is from the point where the weapon is activated (pulling the trigger) to the point that the primer is activated.
The process is largely mechanical, converting a linear motion of pulling a trigger into releasing a firing pin to impact the primer cup.
Sears: Special levers that translate motion between the firing pin assembly and the trigger.
Accidental shootings often occur when internal mechanisms and safety mechanisms fail.
Safety Mechanisms
Trigger Safety: Physically blocks the trigger.
Sear Safeties: Block the motion of the sears.
Grip Safety: A special variant of the sear safety.
Fire Selection Lever
Common on automatic and semi-automatic weapons (assault rifles, military-type weapons).
Allows selection from different modes:
Safe Mode: Blocks the motion of the sears (a sear safety).
Semi-Automatic/Single Fire: Pull the trigger once per shot.
Automatic Fire: Hold the trigger for continuous firing.
Burst Fire: Fires a short burst (e.g., three rounds) with each trigger pull.
Useful in a military context for controlled ammunition usage and grouping of bullets.
Magazine Interlock
Employs multiple mechanisms.
The holding mechanism to stop the magazine from falling out. Usually, it's just like a simple little clasp or a little kind of series of buttons that kind of interact with each other.
Linked to the sears, preventing firing if the magazine is not fully inserted.
Rationale: Even with an empty magazine, a round may still be in the chamber.
Safety Protocol: Always check that a firearm is fully unloaded, even if told it is.
Remove the magazine, cock the weapon, and visually inspect the chamber.
Take control of firearm safety; do not rely on others' assurances.
Firing Pin Safeties
Physically blocks the firing pin from hitting the ammunition.
The safest type of safety mechanism.
If the firing pin is under spring pressure it is ready to be launched and impacts the primer cup. If we physically block it, if the spring breaks, if the firing pin accidentally comes forward, it physically can't hit the primer.
May use spring-loaded mechanisms or rotating formulations.
Blocks firing pin movement until deactivated.
Accidental Discharge
If an accidental discharge is claimed:
Responsibility: The gun owner is responsible for maintenance and care.
Both the owner and the user share responsibility.
Certification: The firearm must be certified safe and unloaded by a competent person before use.
Treatment: Always treat a firearm as if it's loaded.
Never point a firearm at someone, even if unloaded.
General Tests
Replication of Accidental Discharge:
The idea is to copy how the gun was dropped and see if it goes off to find out if there is a fault inside the gun that makes this happen consistently.
Ensure the unit is unloaded, then copy the action and dry fire repeatedly.
Subject the firearm to low-velocity impacts, replicating claimed scenarios (e.g., dropping onto a desk or floor).
Problem: Additional drop tests increase the risk of creating damage never there in the first place.
Repeat the process up to 10 times, noting the results. Any single failure supports the claimed accidental discharge.
Trigger Tests:
Measures the force required to pull the trigger and activate the gun.
Can indicate if the weapon has been modified or damaged.
Procedure: Secure the gun in a weapon stand, cock it, and use a force gauge to measure the trigger pull.
Compare the measurement against factory specifications to identify deviations.
Repeat at least 10 times. A significant drop in force supports a claim of accidental discharge or tampering.
Example values (Newtons):
Two-two rimfire: 15-25 N
Military assault rifle: 20-35 N
Single-action revolvers: Lower force requirement
Double-action revolvers: Higher force requirement (rotates cylinder and fires)
Self-loading pistols: Relatively low.
Shotguns: generally 20-40 range.
Professional target rifle: 1.5-2 N
Case Example
1992, Wales: A male went on trial for shooting a friend in the head with a shotgun.
Defense: The gun discharged accidentally while placing it in the rear of the victim's car.
Testing: The firearm was tested about 100 times, but nothing went off accidentally, no trigger force changes happened.
Dismantling: A gunsmith found a broken spring on the safety engagement set.
Outcome: The broken spring introduced doubt, and the case was dismissed.
Scientific vs. Legal Perspectives: Scientific assessment focuses on consistency; legal focus is on the possibility of malfunction.
Ignition Time
From activation of the primer charge to initial movement of the projectile.
Based on:
Primer type.
Propellant design.
Chamber dimensions.
Tightness of the bullet inside the cartridge neck.
Want the bullet to be nice and tight inside the neck of the cartridge when it's hold. Pressure to build and build before the bullet starts to move.
Primer cup contains an explosive priming mixture detonated by the firing pin crushing it against the anvil.
Detonation/explosion of the primer leads to deflagration/burning of the propellant.
The rate of burning is affected by grain size, shape, primer type, propellant chemistry, and chamber dimensions.
Barrel Time
From the first movement of the projectile to when the projectile exits the muzzle of the gun.
The barrel must go or the bullet go all the way down the barrel.
Increased chamber and barrel pressure accelerate the projectile.
Factors:
Projectile size, shape, and mass.
Internal barrel friction.
Rifling type and rate of twist.
Chamber pressure and propellant burn rate.
Need to clean our barrels fairly regularly, otherwise, you get buildup of stuff, which can lead to barrel blockages, which again, are extremely dangerous
This is the acceleration phase.
Important Equations
Thermodynamics:
Ideal Gas Law - Not displayed but can be written as: PV = nRT
Boyle's Law - Not displayed but can be written as: P1V1 = P2V2
Force, Pressure, and Area:
P = \frac{F}{A}
F = PA
Force, Mass, and Acceleration:
F = ma
Pressure Curve
Lock time: No change in pressure.
Ignition time: Sharp increase in pressure due to primer and propellant activation.
The bullet starts to move once we hit the peak pressure for the system
Barrel time: Pressure decreases but remains high as propellant continues to burn while the bullet accelerates down the barrel.
Propellant continues burning after the bullet leaves the barrel until pressure returns to atmospheric levels.
Key Reminders
Write up labs, analytical and ballistic labs. Ensure all experimental data is accurately recorded and conclusions are well-supported by evidence.
Work on group presentations. Coordinate with team members to ensure a comprehensive and cohesive presentation. Practice presentation skills and allocate time for questions and answers.
Third-year students: Court starts on Thursday, which means jury duty. Check your jury duty slot on the Moodle page (SI5560). Understand the importance of jury duty and its role in the legal system.
Arrive ahead of your time slot. Plan your commute and ensure punctuality.
The session this week overlaps with a jury duty slot. Wait outside the courtroom until told to enter during a mid-session break. Be patient and respectful of the court proceedings.
The courtroom is in the Wagoda Building, over the bridge between Elliott and Rutherford Colleges (opposite the library). Familiarize yourself with the location to avoid delays.
Internal Ballistics
Internal ballistics is broken down into three key phases. Understanding each phase is crucial for forensic analysis.
Lock time: The delay between trigger pull and primer ignition.
Ignition time: The time it takes for the propellant to ignite and start building pressure.
Barrel time: The duration the projectile spends traveling down the barrel.
Lock Time
Lock time is from the point where the weapon is activated (pulling the trigger) to the point that the primer is activated. This involves mechanical movements within the firearm.
The process is largely mechanical, converting a linear motion of pulling a trigger into releasing a firing pin to impact the primer cup. Mechanical efficiency affects precision and reliability.
Sears: Special levers that translate motion between the firing pin assembly and the trigger. Sears are critical for controlled firing and preventing accidental discharges.
Accidental shootings often occur when internal mechanisms and safety mechanisms fail. Regular maintenance and inspection are necessary to prevent such failures.
Safety Mechanisms
Trigger Safety: Physically blocks the trigger, preventing it from being pulled unintentionally. Simple yet effective in preventing many accidental discharges.
Sear Safeties: Block the motion of the sears, ensuring the firing pin cannot be released unless the trigger is intentionally pulled. Enhances safety by adding a layer of mechanical protection.
Grip Safety: A special variant of the sear safety, requiring the shooter to have a firm grip on the weapon for it to fire. Commonly found on pistols, adding another layer of safety.
Fire Selection Lever
Common on automatic and semi-automatic weapons (assault rifles, military-type weapons). Allows the user to select different firing modes based on the situation.
Allows selection from different modes:
Safe Mode: Blocks the motion of the sears (a sear safety). Ensures the weapon cannot fire, regardless of trigger manipulation.
Semi-Automatic/Single Fire: Pull the trigger once per shot. Provides controlled, single-shot firing.
Automatic Fire: Hold the trigger for continuous firing. Allows rapid, continuous firing until the trigger is released or ammunition is exhausted.
Burst Fire: Fires a short burst (e.g., three rounds) with each trigger pull. Balances firepower with ammunition conservation.
Useful in a military context for controlled ammunition usage and grouping of bullets. Enables soldiers to manage ammunition effectively during combat.
Magazine Interlock
Employs multiple mechanisms to ensure the magazine is correctly seated and interacts with the firing mechanism.
The holding mechanism to stop the magazine from falling out. Usually, it's just like a simple little clasp or a little kind of series of buttons that kind of interact with each other. Essential for reliable feeding of ammunition into the chamber.
Linked to the sears, preventing firing if the magazine is not fully inserted. Prevents firing when the magazine is detached, enhancing safety.
Rationale: Even with an empty magazine, a round may still be in the chamber. Emphasizes the danger of assuming a weapon is unloaded.
Safety Protocol: Always check that a firearm is fully unloaded, even if told it is. Never trust someone else's assurance; perform a manual check.
Remove the magazine, cock the weapon, and visually inspect the chamber. Verify there is no round in the chamber before handling further.
Take control of firearm safety; do not rely on others' assurances. Personal responsibility is paramount in firearm safety.
Firing Pin Safeties
- Physically blocks the firing pin from hitting the ammunition. Prevents accidental discharge by adding a mechanical barrier.
The safest type of safety mechanism. Provides an additional layer of security against unintentional firing.
- If the firing pin is under spring pressure it is ready to be launched and impacts the primer cup. If we physically block it, if the spring breaks, if the firing pin accidentally comes forward, it physically can't hit the primer. Crucial in preventing discharge due to mechanical failure.
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May use spring-loaded mechanisms or rotating formulations. Different designs achieve the same goal of blocking the firing pin.
Blocks firing pin movement until deactivated. Ensures the firing pin cannot move forward unless the safety is disengaged.
Accidental Discharge
If an accidental discharge is claimed: Investigate thoroughly to determine the cause and contributing factors.
Responsibility: The gun owner is responsible for maintenance and care. Owners must ensure their firearms are in safe working condition.
Both the owner and the user share responsibility. Anyone handling a firearm must understand its operation and safety mechanisms.
Certification: The firearm must be certified safe and unloaded by a competent person before use. A qualified gunsmith or armorer should perform the certification.
Treatment: Always treat a firearm as if it's loaded. Reinforces the importance of constant vigilance.
Never point a firearm at someone, even if unloaded. Avoids the possibility of accidental injury or death.
General Tests
Replication of Accidental Discharge: Attempts to recreate the conditions under which the accidental discharge occurred.
The idea is to copy how the gun was dropped and see if it goes off to find out if there is a fault inside the gun that makes this happen consistently. Identifies potential mechanical failures that could cause an accidental discharge.
Ensure the unit is unloaded, then copy the action and dry fire repeatedly. Safety is paramount during testing.
Subject the firearm to low-velocity impacts, replicating claimed scenarios (e.g., dropping onto a desk or floor). Simulates real-world conditions.
Problem: Additional drop tests increase the risk of creating damage never there in the first place. Weigh the risks and benefits of repeated testing.
Repeat the process up to 10 times, noting the results. Thorough documentation is necessary for accurate analysis. Any single failure supports the claimed accidental discharge.
Trigger Tests: Measures the force required to pull the trigger, providing insight into the weapon's condition.
Measures the force required to pull the trigger and activate the gun. Determines if the trigger mechanism is within factory specifications.
Can indicate if the weapon has been modified or damaged. Alterations to the trigger pull can indicate tampering or wear.
Procedure: Secure the gun in a weapon stand, cock it, and use a force gauge to measure the trigger pull. Ensures accurate and repeatable measurements.
Compare the measurement against factory specifications to identify deviations. Deviations from factory standards may indicate a problem.
Repeat at least 10 times. Multiple measurements ensure accuracy and consistency. A significant drop in force supports a claim of accidental discharge or tampering.
Example values (Newtons):
Two-two rimfire: 15-25 N
Military assault rifle: 20-35 N
Single-action revolvers: Lower force requirement
Double-action revolvers: Higher force requirement (rotates cylinder and fires)
Self-loading pistols: Relatively low.
Shotguns: generally 20-40 range.
Professional target rifle: 1.5-2 N
Case Example
1992, Wales: A male went on trial for shooting a friend in the head with a shotgun. Illustrates how forensic investigation is used in legal cases.
Defense: The gun discharged accidentally while placing it in the rear of the victim's car. The defense argued that the discharge was not intentional.
Testing: The firearm was tested about 100 times, but nothing went off accidentally, no trigger force changes happened. Extensive testing failed to replicate the claimed accidental discharge.
Dismantling: A gunsmith found a broken spring on the safety engagement set. Revealed a mechanical fault that could have contributed to the discharge.
Outcome: The broken spring introduced doubt, and the case was dismissed. Demonstrates how a single point of doubt can affect a legal outcome.
Scientific vs. Legal Perspectives: Scientific assessment focuses on consistency; legal focus is on the possibility of malfunction. The difference in perspectives can lead to varying conclusions.
Ignition Time
From activation of the primer charge to initial movement of the projectile. Characterized by rapid pressure buildup.
Based on:
Primer type. Different primers have varying explosive power.
Propellant design. Propellant characteristics affect the rate of burning.
Chamber dimensions. Chamber size influences pressure dynamics.
Tightness of the bullet inside the cartridge neck. Affects how quickly pressure builds.
Want the bullet to be nice and tight inside the neck of the cartridge when it's hold. Pressure to build and build before the bullet starts to move. Promotes efficient energy transfer to the bullet.
Primer cup contains an explosive priming mixture detonated by the firing pin crushing it against the anvil. The primer initiates the combustion process.
Detonation/explosion of the primer leads to deflagration/burning of the propellant. The rate of burning is affected by grain size, shape, primer type, propellant chemistry, and chamber dimensions. Affects the pressure curve and projectile velocity.
Barrel Time
From the first movement of the projectile to when the projectile exits the muzzle of the gun. The projectile accelerates along the barrel.
The barrel must go or the bullet go all the way down the barrel. Consistent barrel length is necessary for predictable ballistics.
Increased chamber and barrel pressure accelerate the projectile. Maximizing projectile velocity is essential for accuracy and range.
Factors:
Projectile size, shape, and mass. Influence acceleration and stability.
Internal barrel friction. Reduces projectile velocity.
Rifling type and rate of twist. Imparts spin for stability.
Chamber pressure and propellant burn rate. Crucial for achieving desired velocity.
Need to clean our barrels fairly regularly, otherwise, you get buildup of stuff, which can lead to barrel blockages, which again, are extremely dangerous. Regular maintenance is crucial for safety and performance.
This is the acceleration phase. The projectile gains kinetic energy.
Important Equations
Thermodynamics:
Ideal Gas Law - Not displayed but can be written as: PV = nRT
Boyle's Law - Not displayed but can be written as: P1V1 = P2V2
Force, Pressure, and Area:
P = \frac{F}{A}
F = PA
Force, Mass, and Acceleration:
F = ma
Pressure Curve
Lock time: No change in pressure. The system is idle during this phase.
Ignition time: