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Forensic
Examination Guidelines for Silencers
(Adopted: 09/27/2005)
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
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.
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 A
reduction in sound is noted when shots are
fired with and without the device attached.
6.2.3 Quantitative
6.2.3.1 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.
7.0 Documentation
7.0.1 Sketching
7.0.2 Narrative description
7.0.3 Photographic
7.0.4 Audio or video recording
7.0.5 Measurements
7.0.6 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
-
Crum, Richard A. and Owen,
Edward M., "Silencer Testing," AFTE Journal, Volume 19,
Number 4, October 1987, pp. 438-439
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Association of Firearms
and Toolmark Examiners Procedures Manual, Sound
Suppressor Examination (FA-II-2), Version July 9, 2001
-
AFTE Glossary, Fourth
Edition (Silencer Reference)
-
Principles of Acoustic
Measurement and Analysis, Bruel & Kjaer factory brochure
-
Measuring Sound, Bruel &
Kjaer factory brochure
-
Department of Defense,
Design Criteria Standard, Noise Limits, MIL-STD-1474D,
12 February 1997
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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
-
Silverwater, Howard and
Koffman, Avi, “Noise Reduction Measurements of Home Made
Silencer,” AFTE Journal, Volume 30, Number 3, Summer
1998
-
De Kinder, Han, “Note on
the Application of Sound meters in firearms
identification” AFTE Journal, Volume 30, Number 3,
Summer 1998, pp. 510-511
-
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
-
Paulson, Alan C.,
“Silencer History and Performance Sporting and Tactical
Silencers”, Volume One, Paladin Press, Boulder Colorado
-
Haag, Lucien, “Firearms
Sound Levels and measurements”, Draft-Work in Progress
-
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
-
Interview with Ed Owen,
former Branch Chief, ATF Firearms Technology and
co-author of the article entitled: “Silencer Testing”.
Additional Reference
Material
-
Dater, Philip H., MD,
“Firearm Sound Level Measurements: Techniques and
Equipment, ATI Star Ore's, Division of Antares
Technologies, Inc., Boise, Idaho
-
Dater, Philip H., MD,
“Firearm Sound Levels and Hearing Damage, Antares
Technologies, Inc., P.O. Box 140618, Boise, Idaho
-
Parker, Neil R., “Tactical
Use of Suppressed Weapons for Law Enforcement, Antares
Technologies, Inc., P.O. Box 140618, Boise, Idaho
-
Gemtech Technologies
product brochure, P.O. Box 140618, Boise, Idaho
-
John’s Guns Sound
Suppressor Catalog, 761 FM2134, Voss, TX
-
Smith, Stephanie C.,
“Silencers”, AFTE Journal, Volume 37, Number 2, Spring
2005, pp. 136-138
Adopted:
09/27/2005
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