Author: John H. Dillon, Jr.

Jack Dillon earned a B. S. from the United States Naval Academy and an M. Ed. from the University of Virginia. Commissioned in the United States Marine Corps in 1964, he attained the rank of Captain of Marines. Appointed a Special Agent, FBI, in 1970, he investigated diverse criminal violations, including organized crime, bank robberies, extortions, and kidnappings. In 1976 he received orders to the Firearms/Toolmarks Unit of the FBI Laboratory for training as an examiner, where he evaluated evidence and provided on-site field support in domestic cases, as well as abroad. He also designed and taught basic and management-level forensic courses at the FBI Academy, Quantico, Virginia, 1982-1988. Jack retired from the FBI as Chief of the Firearms/Toolmarks Unit in 1994, and continues to consult in forensic firearm cases and in training design and delivery.

Introduction

The use of gunshot residues for determining the distance of the muzzle of a firearm from a target at the time of discharge is a subdiscipline of the forensic firearms specialty. This module covers the examination, testing, reproduction and comparison of residues, and distance determination.

Applying scientific methods, the subdiscipline of using gunshot residues for determining distance is performed in the following manner:

• Testing is conducted only when the suspect firearm is available or per laboratory protocol.
• Visual and microscopic examination is made of evidence items for residues, which may or may not be visible to the unaided eye.
• Testing of evidence items is performed using a sequence of standardized chromophoric (color-producing) chemical tests which are specific for certain metals or compounds typically produced by fired ammunition components, including burned gunpowder.
• Reproduction of these residues in the form of standards fired by the examiner at known distances uses the suspect firearm and ammunition that is as similar as possible to that used in the incident(s).
• Comparison of the residues previously developed on evidence items is made against known standards to determine the distance of a suspect firearm from the target at the time of discharge.

Distance determinations based on gunshot residues are not performed in isolation from the rest of the firearms-related evidence in a shooting case. In summary, this module is directed at acquiring an understanding of the chemical makeup of ammunition propellants, what types of combustion products they produce, and the mechanics of extracting the distance-related information they represent.

Objectives

At the conclusion of this module the student should be able to do the following:

• Describe in general terms the chemical reaction that takes place when propellant powder burns
• Describe the visual and microscopic examination of evidence for the presence of visible gunshot residues
• Describe the physical effects caused by the discharge of a firearm and the passage of a bullet
• Describe the techniques for preparation of test media and chemical reagents required for the Modified Griess Test
• Describe the techniques for preparation of test media and chemical reagents for the Dithiooxamide Test
• Describe the techniques for preparation of test media and chemical reagents for the Sodium Rhodizonate Test
• Explain the chemical reactions that take place in the Modified Griess Test
• Explain the chemical reactions that take place in the Sodium Rhodizonate Test
• Explain the chemical reactions that take place in the Dithiooxamide Test
• Describe the performance of the Modified Griess Test for the detection of nitrite residues on porous objects
• Describe the performance of the reverse Modified Griess Test for the detection of nitrite residues on nonporous objects
• Describe the performance of the Dithiooxamide Test for nickel or cuprous materials
• Describe the performance of the Sodium Rhodizonate Test for the detection of lead residues
• Describe the performance of the Bashinski Transfer Technique/Sodium Rhodizonate Test for the detection of lead residues on darker (blue, black, violet) surfaces
• Describe how to document gunshot residue test/technique procedures and results
• Describe the process of reproducing positive results in gunshot residue examinations
• Articulate conclusions based on gunshot residue test/technique results and the accumulated raw data represented in contemporaneous notes

AFTE Knowledge and Ability Factors

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