Forensic Science: Ballistics

Ballistic evidence provides critical insights into:
  - The type of firearm used.
  - The caliber of the bullet.
  - The total number of bullets fired at a crime scene.
  - The possible location and position of the shooter.
  - Whether a weapon had been fired recently.
  - Connections between firearms and potential previous criminal actions.

Development Timeline:
  - Gunpowder invented by the Chinese over a thousand years ago.
  - Early firearms were muzzle-loading matchlocks that used wicks to ignite gunpowder.
  - Advancements included the development of cartridges and breech-loading firearms.
  - Addition of rifling techniques increased accuracy.
  - Subsequent development of revolvers, semi-automatic, and automatic handguns.

Components of a Firearm Barrel:
  - Barrel
    - Land: The raised portion of rifling inside the barrel.
    - Groove: The recessed portion of rifling inside the barrel.
  - Bullet
    - Land Impression: The impression left by the lands of the barrel on the bullet.
    - Groove Impression: The impression left by the grooves of the barrel on the bullet.
  - Rifling: The spiral grooves formed in the bore of a firearm barrel that impart spin to the projectile when fired.

Definition of Caliber:
- Caliber is a measure of the diameter of the cartridge, typically expressed in hundredths of an inch.
Common Calibers Include:
- Inch calibers: .22, .25, .357, .38, .44, .45.
- Metric calibers: 9 mm.
Importance of Caliber Matching:
- The caliber of ammunition must match the firearm to ensure proper functioning. Misalignment can cause malfunctions or misfires.

Bullets are usually made of metal and packaged with the cartridge, which contains the primer and propellant powders.

**Operating Mechanism: **
  1. The firing pin strikes the base of the cartridge, igniting the primer powder.
  2. The primer ignites, creating a spark that passes through the flash hole to the main propellant supply.
  3. The pressure from this explosion forces the bullet from its casing into the barrel.
  4. The bullet spirals out of the barrel guided by the lands and grooves.

Class Characteristics of Bullets:
  - Vary by gun manufacturer and include:
    - Number of lands and grooves.
    - Width of lands and grooves.
    - Depth of lands and grooves.
    - Twist of rifling.
    - Pitch of rifling.
Individual Characteristics of Bullets:
- Each gun barrel has unique striations (machine markings) along the lands and grooves, as no two barrels have identical markings.

Analysis includes:
  - Manufacturer: E.g., Winchester.
  - Shape: Types include rimless and rimmed cartridges.
  - Caliber: Designations such as .45 ACP, 9 mm, 12 gauge.
  - Composition: Common materials include brass, steel, and plastic.

Unique identifiers on cartridge cases:
  - Firing Pin Impressions: Distinct marks left by the firing pin when striking the cartridge.
  - Breech Face Marks: Unique impressions from the rear of the firearm's chamber.
  - Ejector Marks: Marks left by the ejector mechanism.
  - Extractor Marks: Impressions left by the extract mechanism.

Firing pin must hit the center ring of the cartridge to ignite the primer, leaving a unique impression associated with the specific firearm.

The extracting pin and ejector leave unique marks on spent shell casings as they are ejected from the firearm, further helping in firearm identification.

Description:
  - GSR consists of particles of unburned powder and traces of smoke from gunshots.
  - Residues can be found on the shooter’s hand, arm, face, hair, or clothing, as well as potentially on victims.
  - Chemical testing can detect residues even if attempts to remove them were made.
  - Examination of residue patterns can help determine the shooter's distance from the victim.

Reference Points:
  - Two reference points are needed to define bullet trajectory.
  - Bullet holes in objects or victims provide entry and exit points, serving as reference locations.
  - Gunshot residues or spent cartridge casings can also act as reference points.
Laser Guidance:
- Investigators may utilize lasers to trace the straight-line path, thereby deducing the shooter's location.

Investigators use right triangle geometry to calculate trajectory:
  1. Necessary information includes the distance to the shooter and the angle of elevation.
  2. Applying the **Law of Tangents: **
     - an( ext{angle of elevation}) = rac{ ext{opposite (height of shooter)}}{ ext{adjacent (distance to shooter)}}
  3. The height of the shooter may need to incorporate the feet from ground level depending on placement.

Characteristics of Wounds:
  - Entrance wounds are generally smaller than exit wounds.
  - Fiber evidence and gunshot residue can assist in determining entry versus exit wounds.
  - The behavior of bullets within the body (whether they pass through or become lodged) depends on the bullet's speed and caliber.

Fired bullets carry patterns of lands and grooves that correspond with the rifling in their respective barrels.
Cartridge caliber measures its diameter and is crucial for firearm compatibility.
Forensic investigators also analyze firing pin, breechblock, extractor, and ejector marks.
Gunshot residue plays a significant role in crime scene recreation and interpretation of shooter's location.