Firearms and Toolmarks- Final Exam

What can function testing include?

• Determining if the firearm is in mechanical operating condition

• Testing firing

• Determining if safety features are functioning properly

• Testing scenarios for accidental discharge

Types of revolvers

• Double-Action

• Single-Action

• Top-Break

Single-Action

Action requiring the manual cocking of the firing mechanism

Double-Action

Action in which a single pull of the trigger cocks and releases the firing mechanism

Top-Break

Two safeties that may be on a revolver

1. Hammer Block

2. Transfer Bar

Conventional

Rifling with sharp, defined edges between that land and groove

Polygonal

Rifling in which lands and grooves have a rounded profile

General Rifling Characteristics (GRC)

• Number of lands and grooves

• Width of lands and grooves

• Direction of twist

• Caliber

Hybrid

design that incorporates aspects of both single action and double action

Two types of firing mechanisms for pistols

1. Hammer Fired

2. Striker Fired

Blowback

mechanism has the barrel of the firearm fixed to the frame and the weight of the slide and tension on the recoil spring keeps the breech closed during firing

Recoil

mechanism is the barrel of the firearm no fixed to the frame and the slide and barrel travel/lock together for a portion of the recoil

Gas-operated

mechanism uses the propellant gases to unlock the breech and complete the cycle of extracting and ejecting

Three safeties that may be on a pistol

1. Key Lock

2. Disconnect

3. Manual grip safety

Accidental

discharge typically involves the discharge of a firearm without the trigger being pulled

Unintentional

discharge involved the trigger being pulled without the intent to fire

Semiautomatic

action in which each pull of the trigger results in a complete firing cycle

Automatic

firearm design that feeds cartridges, fires, extracts, and ejects cartridge cases as long as the trigger is fully depressed and there are cartridges in the feed system

Two types of rifles

1. Gas-operated

2. Lever action

Direct impingement

Gas is bled rearward to cycle the action in a rifle

Indirect impingement

Gas moves a piston, piston cycles the action in a rifle

Two common safety features on a rifle

1. cross bolt safety

2. disconnector

Military versions of the AR-type rifle may have a selector switch to choose between….

• Semiautomatic

• Automatic fire

• Burst mode

• Safe mode

AR-type rifles have what types of cartridges?

223 Remington/5.56 × 45 mm

AK-type and SKS-type rifles have what types of cartridges?

7.62 × 39 mm

Two types of shotguns

1. Slide-Action

2. Break open

Two common safety features on a shotgun

1. Trigger Safety

2. Lever Safety

True or False: 10 gauge has a smaller barrel diameter than 28 gauge

False

Choke

interior constriction at or near the muzzle end of a shotgun barrel bore to control shot dispersion

Rifles and Shotguns are….

Single Action

Breechface

area of a firearm that supports the head during firing

Firing pin aperture

opening in the firearm slide where the firing pin comes through

Shear

Types of marks produced when the firearm unlocks and scrapes the cartridge case across the surface of the breech face

Class Characteristics of Breechface Mark

• Parallel

• Circular

• Arched

• Granular

• Cross-Hatched

Parallel Breechface Mark

Circular Breechface Mark

Arched Breechface Mark

Granular Breechface Mark

Hemispherical Firing Pin Mark

Rectangular Firing Pin Mark

Elliptical Firing Pin Mark

Circular Firing Pin Mark

D-shaped Firing Pin Mark

Two marks that can be created on a cartridge of a firearm without it being fired

1. Extractor

2. Ejector

Reproducibility

the appearance of the same marks in repeated test samples describes what concept?

What are some reasons that some features might not reproduce over all samples?

• Different cartridge materials

• Different projectile weights

True or False: NIBIN digitally captures images of cartridge cases, searches them against other images in the database, and automatically makes a match of the identified cartridge cases

False

True or False: The way a gun is held when fired can sometimes affect how marks are left behind on cartridge components

True

Variations in mark reproducibility from force or angle in the firearm can be attributed to this:

large firearm tolerances (loose fit of parts)

Distance Determination

the process of determining the distance from the firearm, usually the muzzle, to the target based on patterns of gunpowder or gunshot residues deposited upon that target

Gunshot residues can include these two things

1. Lead

2. Nitrate

Gunshot residues are expelled from what areas of the firearm

1. Muzzle

2. Ejection Port

3. Cylinder Gap

4. Breech

Modified Griess Test

Chemical Testing for nitrate compounds

Sodium Rhodizonate Test

Chemical Testing for lead residues

How firearms should be packaged

• Unload → Ziptie → Proper seal w/ initials → immobilize item

True or False: For guns in water, submit the firearms fully submerged in the water it was found in

True

Describe how to package tools for examination in the firearms fully submerged in the water it was found in

protect the working surface → under proper seal → and immobilize

Tool

an object used to gain mechanical advantage

factors that affect the production of toolmarks

1. Direction

2. Force

3. Rate of Speed

What general rifling characteristic is this?

Caliber

What general rifling characteristics is this?

Land

What general rifling characteristics is this?

Grooves

What general rifling characteristic is this?

Twist

two common causes of malfunctions

1. failure to load

2. failure to extract/eject

toolmark identification

the discipline of forensic science where the primary concern is to determine if a particular tools produced a tool mark

Tool

an object used to gain mechanical advantage, thought of as the harder of two objects

Subclass Characteristics

consistent among different items same tool in the same approximate state of wear

Striated Toolmark

tool is moved across the object (force and motion parallel to surface)

Impressed Toolmark

leaves an impression (force and motion perpendicular)

Microscopic Comparisons: Identification

Identified as produced by same tool

Microscopic Comparisons: Elimination

• Eliminated by a particular tool

• Eliminated by the same tool

Microscopic Comparisons: Inconclusive

not be identified or eliminated as having been produced by a particular tool

Microscopic Comparisons: Not suitable (unsuitable) for comparison

• No microscopic markings present

  • Relative hardness between tool and surface too similar – marks not transferred from tool

• Markings present are of poor quality and/or quantity

  • Tool may not produce suitable markings

  • Damaged after the mark was imparted (overlapping marks)

  • Blood and other body fluids can erode markings on metallic materials

  • Poor quality of a cast toolmark

Toolmark

Mark is produced when two objects come into forcible contact with one another- the softer surface receiving the mark

Tool Classification

• Compression

  • Hammer

  • Punch

  • Stamp

• Flat Action (Scraping)

  • Screwdriver

  • Chisel

  • Pry Bar

• Slicing

  • Knife

  • Box Cutter

  • Axe

• Gripping

  • Pipe Wrench

  • Groove and Joint Pliers

• Pinching

  • Bolt Cutter

  • Diagonal Cutter

• Shearing

  • Scissors

  • Tin Snips

  • Shears

Groove Joint Pliers

• Gripping Tool

  • Class Characteristics include:

    • Jaw width

    • Jaw length

    • Teeth spacing

    • Jaw lip length

    • Number of teeth

    • Maximum opening

Improper Packaging

• Plastic or paper bags

• Unprotected edges

Preferred Packaging

• Rigid boxes or container

• Protect the edges and working surfaces of the tool

• Consider trace evidence

AR-type rifle

AK-type rifle

Bolt-Action or Slide-Action Rifle

Lever-Action Rifle