Chapter Eight

Recovered Firearm

1. Important information for reconstruction can come from recovered firearms, recovered bullets or cases. - as well as location and markings from al three. Trigger Pull: - the amount of force needed to pull (depress) the trigger sufficiently for the firearm to fire. Safety checks to assure that the weapon would not discharge if dropped. Bullets may have trace evidence.

Firearms Examination

A firearm is a device for accelerating a projectile to a high speed, sending it towards a selected target. When the copper-coated piece of lead hits its target, it has some sort of destructive effect. The energy that provides the driving force for the projectile is contained in the cartridge. A cartridge consists of four basic components: the case, powder, projectile, and primer.

Tool mark

A pattern resulting from a harder marking device (tool) being forced against a softer object. Are usually either striations (scratches) or impressions.

Collection of Tool Marks

A tool mark on a movable object should be protected and sent to the lab. A tool mark on a fixed item, should be thoroughly photographed and the cast using silicone rubber, A suspect tool should never be placed into the tool mark, there could be damage to the mark

Collection and Preservation of Firearms Evidence

Careful Handling of Firearm, Bullets, and Cartridge Cases: - A firearm must be rendered before collecting and packaging. Mark Revolver at Cylinder Position and Mark Recovered Bullets and Cases: - With revolvers, the cylinder position that is lined up with the barrel must be recorded. Submit Evidence to the Lab for Eval: - Bullets and cases recovered from the scene must be carefully marked for identification or sealed in containers that are marked. Firearms Evidence Examination: - Firearm evidence should always be sent to the lab for analysis so that connections can be made to other cases using the firearm database.

Cartridge Components

Cartridge: the assembly of a bullet, gunpowder, and primer in a casing that is placed in the chamber of a firearm. The case: the container for the other three components. the powder: small disks, cylinders, or balls of smokeless powder (cellulose nitrate), or propellant that produces little smoke. the projectile (bullet): usually made from lead of copper-coated lead. An object that can be fired or launched. The primer: consists of a shock sensitive material. Provides the initial spark or flame that causes the cartridge to fire. firing pin: the pin or rod in the firing part of a firearm. It strikes the cartridge primer to fire the cartridge.

Cartridge

Cartridges are placed in a magazine, which is located in the grip of the firearm. - magazine: the container for the cartridges in most semiautomatics and automatics. Empty cartridge cases are automatically ejected.

Functionality and Obtaining Control

Firearms are test fired to determine functionality, proper working of the safety features, and to obtain known specimens. Functionality is the ability of a firearm to fire a projectile when the trigger is pulled. Two or three test firings are done into water-filled bullet recovery tanks. - the bullet recovery tanks are usually large horizontal or vertical stainless steel tank filled with water and used to stop the flight and allow recovery of a projectile when a firearm is fired into it.

Individualizing Features

Imperfections in the lands during manufacture are caused by wear, nicks, scratches etc., will be mirrored on the surface of fired bullets. These rifling imperfections are unique to each barrel and form the basis for projectile-to-barrel identification.

Types of Tool Marks

Indented: marks occur when the tool is pushed into the surface. Striated: marks occur when the tool slides across the surface. Combination: marks occur when the tool is pushed in and then slides across the surface

Rifling

Rifling: refers to the spiral grooves cut or impressed into the barrel of a handgun or rifle to cause the exciting projectile to spin. Lands: the spiralling raised areas between the grooves inside the barrel of a handgun or rifle. Grooves: the depressed areas inside the barrel of the handgun or rifle. (The raised areas are known as "lands" and the depressed areas, "grooves") A spinning projectile has a flatter trajectory and flies a truer path to the target. Twist: the direction in which the lands and grooves spiral down the inside of a handgun or rifle barrel. Land impressions: the depressed helical grooves in a bullet left by the lands inside the barrel from which it was fired.

Types of Firearms

Semiautomatics: fire one bullet then reload with a fresh cartridge each time the trigger is pulled and released. Automatics: continue to fire projectiles until the trigger is released. Rifle: a firearm that is designed to be fired from the shoulder and normally has a relatively long barrel. Handguns: a firearm with a barrel less than a particular length (designed to be fired in or or both hands) Revolvers: have rotating cylindrical cartridge holders that usually hold five to nine cartridges and allow the weapon to fire semiautomatically.

Computer Databases

The FBI maintains a central computerized firearm file: - General Rifling Characteristic file (GRC) Firearm examiner can search the file with the caliber - number of lands and grooves, - their widths, and direction of twist for the make and - model of the firearm that could have fired the crime scene bullet.

Examination and Comparison of Tool Marks

The examiner then attempts to reproduce the tool mark by making test impressions in similar material. The tool mark and test impressions are then examined using a comparison microscope. Visual examination.

Firing Train

The firing train process: - the firing pin strikes the primer "cap" located on the base of the case, compressing the cap and causing the shock sensitive primer to ignite. - the primer then transfer a spark flame to the powder charge, located in the body of the case. - the burning powder rapidly gives off heat and gases and the pressure forces (the projectile from the case and down the barrel of the weapon). - the expanding gases also push the spent cartridge case or shell back with equal force against the breech face.

Class Characteristics

The number of lands and grooves in a firearm barrel. The widths of the lands and grooves. The direction in which the lands and grooves spiral (the "twist") can be either right or left. The caliber, the distance between two opposing lands in hundreds of an inch (US) or in millimetres.

Residue from Softer Object on Tool

The tool mark should be first examined using a stereomicroscope for residue. If the residue on the evidence tool is shown to be consistent with the surface that was scratched, the association of the tool with the surface is strengthened.

Examiner may conclude

The tool mark was made by that tool and no other tool. The marks are consistent in class characteristics and show a number of other similarities (the tool could have made the mark) The marks are clearly different (disassociation) The marks are not sufficiently clear (inconclusive)

Class and Individual Characteristics

Tool marks can have either class or individual characteristics. The basic size and shape of the tool mark is a class characteristics and can tell something about the kind of tool that made the mark.

Physical Examination

Two key parts: - ensuring that no cartridges are left in the firearm. - sighting down the barrel to verify that it is not obstructed. The weapon is then "dry fired" (no cartridge) to ensure that all the different parts are working. The basic class characteristics are noted along with make, model, and serial number.