Module 2: Ballistics - Firearm and Projectile Identification

Definition of Bullet Identification: A bullet is identified to a specific firearm through individual microscopic characteristics.
Microscopic Characteristics:
- Inside the barrel of a firearm, there exist microscopic imperfections and irregularities that are unique to each barrel.
- These imperfections distinguish one barrel from another.

Manufacturing Process:
- Most imperfections are introduced during manufacturing via tools like drills or cutters.
- Marks on these tools can change due to wear, leading to individual imperfections.
Other Causes:
- Abuse, corrosion, or damage can lead to additional imperfections.

Bullet Passage Through Barrel: As the bullet travels through the barrel:
- Imperfections cause microscopic striations or scratches on the bullet’s bearing surface.
- These striations can be repetitive from shot to shot, creating a linkage potential.
Test Shots:
- If a bullet is suspected of coming from a particular firearm, test shots are made by firing bullets from that firearm’s barrel.
- The evidence bullet is then compared microscopically to the test bullets.

Mutilation of Evidence Bullets:
- Damage can occur to the bullet shape, especially with hollow points (e.g., mushrooming).
Complexity of Markings:
- The variety and complexity of microscopic markings might affect the examination process.

Barrel Manufacturing:
- A gun barrel is created from a solid steel bar hollowed out through drilling.
- Rifling:
- The barrel's inner surface is impressed with spiral grooves, aiding in bullet stabilization.
- The lands (surfaces between grooves) play a role in bullet guiding.
Function of Rifling:
- Engaging rifling grooves gives the bullet a rapid spin, preventing tumbling and ensuring accuracy.
Caliber:
- The caliber of a weapon is measured between opposite lands and recorded in hundredths of an inch or millimeters (e.g., .22 caliber, 9mm).
- Example: A .38 caliber weapon may have a bore diameter from 0.345 to 0.365 inches.

Definition: Characteristics that are common among a certain group of firearms; cannot impart individuality.
Key Features: Number and width of lands and grooves; direction of twist (right or left).
- Example: A bullet with five lands and grooves and a right twist cannot come from a firearm with six lands and a left twist.
Quick Inclusion or Exclusion:
- Firearm examiners can quickly include/exclude firearms based on class characteristics.

Definition: Tiny imperfections produced accidentally during manufacturing or through wear and tear.
Interaction with Bullets:
- As a bullet moves through the barrel, it acquires striated markings from the barrel's unique imperfections.
Firing Pin Impressions:
- The shape of a firing pin produces unique impressions on the primer of a cartridge case, contributing to individual characteristics.

Importance: The comparison microscope is the essential tool for modern firearms identification.
Operation:
- Evidence bullets and test bullets are placed beneath a microscope, aligned to view simultaneously.
- Examination involves rotating bullets to find matching lands and grooves.
- Both longitudinal striations and the widths of the grooves are matched.

Challenges with Comparison:
- Bullets often become distorted upon firing, leading to loss of markings.
- Variability in striations due to wear on the barrel may complicate matches.
- Determining a "match" relies on the judgment of experienced firearm examiners.

Initial Assessment: Firearm examiners may be asked to determine the caliber and possible make without a suspect weapon.
Use of Class Characteristics:
- Assessing rifling marks, including lands, grooves, and twist, aids in narrowing possible manufacturing brands.
- Example: A bullet with specific class characteristics may distinguish between different manufacturers (e.g., Marlin rifles with micro-grooving).

Smooth Barrels: Unlike rifled firearms, shotguns have smooth barrels, producing no rifling marks on projectiles.
Projectiles Fired: Typically small lead balls or pellets pushed through by a wad during ignition.
Determining Shot Size: Weight and diameter measurements can identify the shot size and, occasionally, the ammunition manufacturer.
Gauge Measurement:
- The diameter of shotgun barrels is expressed in terms of gauge; higher gauge numbers reflect smaller diameters (e.g., 12 gauge = 0.73 in, 16 gauge = 0.67 in).
- Exception: .410 gauge refers to a diameter of 0.41 in.

Trigger Mechanism: Pulling triggers activates the firing pin, igniting the primer and expelling the bullet forward.
Breech Block: The rear part of the firearm where spent cartridges are impressed with markings from the firing mechanisms.
Extractor and Ejector:
- Extractor removes cartridges from the chamber.
- Ejector expels fired cases from the firearm.

Identification Through Markings: Markings on spent shells provide points for individualizing them back to a specific weapon.
Firing Pin Markings: The impression left by the firing pin can individualize it to a weapon due to its unique characteristics.
Breech Block Markings: Also contribute to individualization through distinctive surface markings.
Collective Analysis of Characteristics:
- In the absence of a suspect weapon, firing pin impressions and other markings hint at the weapon type or eliminate possible weapons from consideration.

Understanding firearm identification relies on the detailed examination of microscopic characteristics, both class and individual, crucial for linking bullets and cartridge cases to specific firearms, thereby playing a critical role in forensic investigations.