Impression Evidence Firearms Examination Tom Anderson

Impression and Firearm Evidence In this week's Seminar, be prepared to discuss the use of impression evidence and firearm examination evidence. These two types of trace evidence can be the difference between the conviction and acquittal of a suspect. Use the readings and other assignments to prepare for the Seminar.

Impression Marks “Minute imperfections on a large variety of objects such as tools, footwear, tires and so on produce markings in their normal and sometimes unusual usage. These markings are often characteristics of the type of tool or object used. In many instances, very small and sometimes microscopically unique markings are left that can be traced directly to the object and instrument in questions (Fisher, 2004, p. 222)

Types of Marks Compression Marks Marks which are left when an instrument is either forced or pushed onto material able to pick up these marks. These marks can come from shoes, tires bites, fabric, and tools such as a hammer or screwdriver.

Types of Marks (cont.) Scrapings or Striated Marks These marks are made by the pressure applied to a tool or other object and the item is dragged or slid across an item where small microscopic striations are left be hind.

Types of Marks (cont.)

Retrieving Compression Marks Casting is one way of retrieving these marks. Photographing is another.

Bite mark Impressions

Casting

Impression Evidence

Casting (cont.)

Firearms Examination

Believe it or not, handguns are the weapon of choice, not the infamous ASSAULT RIFLE ! Two types of Barrels: 1. Smooth Bore “A tube” 2. Rifled Barrel Caliber is the approximate Bore diameter

Firearms Examination

Striated Marks

Striated Marks (cont.)

Firearms When a firearm is fired, several identifying marks are created from the weapon. The first is the firing pin striking the base of the bullet, creating this unique mark.

Firearms Examination Cotton waste bullet recovery system

Types of evidence The firing pin creates an impression on the primer in the shell casing. This impression is called a tool mark which can be matched to a single firing pin due to irregularities. The shell is blown backwards into the breechblock, which makes marks on the shell casing. The shell is ejected by some weapons, which creates metal- to-metal contact with the shell. These markings can be compared to test-fired shell casings. When the explosion occurs, gunshot residue (burned and unburned) powder is expelled forwards and backwards. This means the shooter will have residue on his/her hands and clothing.

Firearms (cont.)

Bullet striations

Lands and Grooves inside a cut-away Barrel

Firearms (cont.) Chamber marks: Roughness in the chamber of a firearm can scratch the outer walls of a cartridge case when loaded and removed from the chamber. Extractor Marks: Another action mark, usually found in a striated form, is created by the extractor of most auto-loading or repeating firearms.

Firearms (cont.) Chamber mark

Firearms (cont.) Extractor mark

Firearms (cont.) Gun Powder Residue: When the firing pin of a firearm strikes the primer of a cartridge the primer compound ignites sending a flame into the cartridge case. Gunpowder in the cartridge case starts to burn, causing it to change from a solid material to a gas. This change creates pressure within the cartridge, which in turn forces the bullet down the barrel and down range. Pressure building behind the bullet is released when the bullet exits the muzzle of the firearm.

Impression Evidence How do we match the suspected tool to the object of interest?? Comparative Examination

Firearms Examination Lands and Groove Examinations Firing Pin Strikes, extractor marks Fingerprinting of gun and bullets GSR testing of suspects Body fluid analysis Always use care in handling suspect weapons. If unsure of operation, STOP, get help.

Reference Fisher, B., A., J. (2004). Techniques of Crime Scene Investigation (7 th ed.). Washington: CRC Press.

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