Forensic Examination Guidelines

for Silencers Page 1 of 5 Adopted 09/27/2005

FORENSIC EXAMINATION GUIDELINES FOR SILENCERS

1.0 Objective/Introduction

The objective of the following guidelines is to identify the essential elements

suggested for use in the forensic examination of silencers.

1.1 Establish procedures to reliably determine if a device is constructed or

fabricated to reduce, suppress, attenuate or diminish the report of a firearm.

1.2 Review and/or validate established silencer examination protocols.

2.0 Definitions/Terminology

Standard terminology from sources such as the Bureau of Alcohol, Tobacco, Firearms

and Explosives (ATF) Federal Firearms Regulations Reference Guide, Association of

Firearm and Tool Mark Examiners (AFTE) Procedures Manual and the AFTE Glossary

should be used in the documentation of silencer examinations.

2.1 Commonly used terms may include:

2.1.1 Suppressor

2.1.2 Firearm muffler

2.1.3 Decibel

2.1.4 Sound meter

2.1.5 Sound pressure level

2.1.6 Report

2.1.7 Muzzle blast

2.1.8 Internal components, e.g., baffle, ported tube, wipes, end caps,

bleed holes

3.0 Equipment/Supplies

Proper equipment should be used and checked for acceptable accuracy when

appropriate.

3.1 Equipment and supplies may include:

3.1.1 Sound meter

3.1.2 Microphone

3.1.3 X-ray apparatus

3.1.4 Optical aids – borescope, stereoscope, magnifier

3.1.5 Safety equipment – ear muffs, eye protection
Forensic Examination Guidelines

for Silencers Page 2 of 5 Adopted 09/27/2005

3.1.6 Chemicals for gunshot residue examinations (GSR)

3.1.7 Various tools for disassembly

3.1.8 Remote firing devices

3.1.9 Range or shooting facility

3.1.10 Distances measuring devices

4.0 Concepts

4.1 Muzzle blast is the most significant portion of the report of a firearm.

4.2 Muzzle blast is caused by high pressure of hot gases suddenly escaping from

the front end of the barrel into the atmosphere as the bullet exits.

4.3 Reducing the pressure results in less sound generated.

4.4 Reduce gas temperature (cooling).

4.5 Delaying gas exit through trapping and turbulence or by a combination of

above.

4.6 The functioning of the device or a portion of the device may not be the sole

determining factor in the legal classification.

4.7 The evaluation of the design and configuration of a device may be a

determining factor in the legal classification.

4.8 Quantitative and/or qualitative testing can be conducted.

5.0 Safety Considerations

General laboratory safety protocols should be followed accordingly.

5.1 Safe firearm handling

5.2 Range test firing safety procedures

5.3 Safe firing of suspect device

5.3.1 Structural integrity

5.3.2 Proper attachment

5.3.3 Proper alignment

5.3.4 Proper bore clearance

5.3.4.1 Clear of any extraneous obstructions

6.0 Procedures/Methods

6.1 Inspection – components and material used to fabricate.

6.1.1 Documentation of visible characteristics, which include external and

internal examination.

6.1.1.1 An internal exam may include the use of specialized

equipment.
Forensic Examination Guidelines

for Silencers Page 3 of 5 Adopted 09/27/2005

6.1.1.2 Disassembly of the device may be required, but should be

conducted only after a function test has been conducted.

6.1.2 Chemical and/or trace analysis can be utilized to detect gunshot

residue.

6.1.2.1 Copper residue testing, e.g., dithiooxamide

6.1.2.2 Lead residue testing, e.g., sodium rhodizonate

6.1.2.3 Nitrite testing

6.1.2.4 Gunpowder

6.2 Function testing

6.2.1 Refer to Section 5.0 for safety considerations

6.2.2 Qualitative

6.2.2.1.1 A reduction in sound is noted when shots are fired

with and without the device attached.

6.2.3 Quantitative

6.2.3.1.1 A firearm report is recorded and calculated with a

sound measuring instrument(s) when multiple

shots are fired with and without the device

attached.

6.2.4 If a silencer is permanently attached to a firearm, every effort should

be taken into consideration to closely replicate the evidence firearm

without the silencer i.e. make, model, caliber and barrel length.

6.3 Operations and Controls

6.3.1 Controls

6.3.1.1 When conducting function testing, every effort should be

made to standardize ammunition, position of equipment,

firearm firing position and environment.

6.3.2 Quantitative Operations

6.3.2.1 A decibel meter or other sound level measuring equipment

using the appropriate microphone should be utilized

following the manufacturer’s suggested protocols.

6.3.2.2 When conducting testing, the position of the firearm, sound

level meter and microphone must remain constant.

6.3.2.2.1 The microphone should be positioned

approximately 90 degrees from the muzzle of the

firearm to optimize sound pressure level

recordings, while maintaining maximum safety.

6.3.2.3 Measurements should be recorded as to the spatial

relationship of the firearm to the sound level meter.

6.3.2.4 The environment must remain relatively constant

throughout the testing.

6.3.2.5 Ammunition utilized should be from the same lot and/or

box.
Forensic Examination Guidelines

for Silencers Page 4 of 5 Adopted 09/27/2005

7.0 Documentation

7.1 Sketching

7.2 Narrative description

7.3 Photographic

7.4 Audio or video recording

7.5 Measurements

7.6 Computer assisted programs

8.0 Training / Qualification of Personnel

Training covering a broad range of topics may be required to conduct silencer

examination.

8.1 Training may include the following:

8.1.1 Study of published materials and/or general procedure manuals

8.1.2 Equipment use, particularly the sound level manufacturer’s protocols

8.1.3 Study under the guidance of experienced examiner

8.1.4 Report writing and testimony preparation

8.1.5 Mock casework and testimony presentation.

ACKNOWLEDGEMENT

The Scientific Working Group on Firearms and Toolmarks would like to thank Bob

Burrows, senior firearms enforcement officer, Bureau of Alcohol, Tobacco, Firearms

and Explosive, and Doug Murphy, Federal Bureau of Investigation, Firearm and

Toolmark Laboratory for their technical input to this project.

References

1. Crum, Richard A. and Owen, Edward M., "Silencer Testing," AFTE Journal, Volume

19, Number 4, October 1987, pp. 438-439

2. Association of Firearms and Toolmark Examiners Procedures Manual, Sound

Suppressor Examination (FA-II-2), Version July 9, 2001

3. AFTE Glossary, Fourth Edition (Silencer Reference)

4. Principles of Acoustic Measurement and Analysis, Bruel & Kjaer factory brochure

5. Measuring Sound, Bruel & Kjaer factory brochure

6. Department of Defense, Design Criteria Standard, Noise Limits, MIL-STD-1474D, 12

February 1997

7. Hueske, Edward E., “Silencers - A Review And A Look At The State Of The Art," AFTE

Journal, Volume 23, Number 2, April 1991, pp. 668-678

8. Silverwater, Howard and Koffman, Avi, “Noise Reduction Measurements of Home

Made Silencer,” AFTE Journal, Volume 30, Number 3, Summer 1998
Forensic Examination Guidelines

for Silencers Page 5 of 5 Adopted 09/27/2005

9. De Kinder, Han, “Note on the Application of Sound meters in firearms identification”

AFTE Journal, Volume 30, Number 3, Summer 1998, pp. 510-511

10. Grandy, Jennifer, “The Construction and Relative Efficiency of a Homemade Sound

Suppressor VS. a Commercial Model”, AFTE Journal, Volume 33, Number 2, Spring

2001, pp. 103-111

11. Paulson, Alan C., “Silencer History and Performance Sporting and Tactical Silencers”,

Volume One, Paladin Press, Boulder Colorado

12. Haag, Lucien, “Firearms Sound Levels and measurements”, Draft-Work in Progress

13. Unpublished Empirical Silencer Testing conducted by the SWGGUN silencer sub

committee on Thursday, April 24,2003 at the Federal Bureau of Investigation,

Firearms and Toolmark Laboratory, Quantico, Virginia

14. Interview with Ed Owen, former Branch Chief, ATF Firearms Technology and coauthor of the article entitled: “Silencer Testing”.

Additional Reference Material

1. Dater, Philip H., MD, “Firearm Sound Level Measurements: Techniques and

Equipment, ATI Star Ore's, Division of Antares Technologies, Inc., Boise, Idaho

2. Dater, Philip H., MD, “Firearm Sound Levels and Hearing Damage, Antares

Technologies, Inc., P.O. Box 140618, Boise, Idaho

3. Parker, Neil R., “Tactical Use of Suppressed Weapons for Law Enforcement, Antares

Technologies, Inc., P.O. Box 140618, Boise, Idaho

4. Gemtech Technologies product brochure, P.O. Box 140618, Boise, Idaho

5. John’s Guns Sound Suppressor Catalog, 761 FM2134, Voss, TX

6. Smith, Stephanie C., “Silencers”, AFTE Journal, Volume 37, Number 2, Spring 2005,

pp. 136-138