Firearms Examiner Training. Dillon JH, Jr. 2013. Firearms Examiner…
Firearms Examiner Training. Dillon JH, Jr. 2013. Firearms Examiner Training. [Accessed on Feb 18, 2020]
Module 7: Equipment and Instrumentation
I. Measurement, Standards, and Accuracy
A. Questioned items are compared to known standards to find a common source.
The instruments used in this examination are used to measure
i. The weight of a bullet
ii. The range of weights required to squeeze the trigger and fire a particular firearm (trigger pull)
• Can assist in determining the level of difficulty related to discharging a particular firearm.
– 0.22 caliber rimfire rifles
◊ 3lb to 5lb
– Military rifles
◊ 4lb to 7lb
– Single-action revolvers
◊ 4lb to 6lb
– Double-action revolvers
◊ 10lb to 15lb
– Semiautomatic pistols
◊ 4lb to 5lb
• Unusual trigger pull reasons
– Inherent design characteristics
– Poor quality factory assembly work
– Customization of a firearm
– Flawed lock mechanism
– Flawed safety feature
• Measuring trigger pull approaches
– Standard trigger weights
◊ Effective when weight is carefully added in small increments.
◊ Excellent data is produced if the weights are suspended from a point on the trigger where the finger would rest and exert pressure to the rear.
– A spring gauge
◊ Lightweight and easily calibrated.
– Mechanical trigger pull testers
◊ Nondigital, portable
◊ Can be mounted or handheld
◊ Can be used for both handguns and long guns
◊ Peak values are recorded, but no digital information in the form of charts or graphs is provided for case documentation.
– Digital trigger pull testers
– The TriggerScan system
◊ Engineered specifically for gunsmiths and firearm examiners.
◊ The data can be stored, printed, and retrieved for case documentation.
◊ The data can be customized for the needed information for each case.
◊ The system includes a calibration tool for making adjustments against a known standard.
• Common conceptual threads
– A range of trigger pull values must be established
– Multiple tests should be performed to establish reproducibility within the range of values
– Contemporaneous documentation must be kept for each test result
– Results can be expressed as a range of weights:
◊ From a low end at which the trigger never releases the sear
◊ To an upper end at which the trigger always releases the sear
iii. A way of expressing the force of gravity on an object, assuming certain standard conditions.
iv. Digital Scales
– Multiple strain gauges
◊ A paper-thin device that normally covers an area less than that of a dime.
– A load cell
– A signal amplifier
– A specialized microprocessor with built-in proprietary software
– Determining the difference (if any) between the output reading produced and a known standard, and remedying that difference.
– Ensuring that the microprocessor correctly relates the amplified digital output signal to a standard weight.
i. The width of land and groove impressions in a fired bullet
ii. The width of a toolmark
iii. Essential in documenting many class characteristics of firearm/toolmark evidence.
iv. Instruments used for dimensional data
• Glass measuring reticule
– Compatible with microscope eyepieces
– Filar Micrometer Eyepieces
◊ Optical-mechanical system
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• Electronic reticule
– Combine a stereomicroscope, a digital camera, and a computer.
– The software saves and exports the image data for use in case documentation
• Digital measuring equipment and software
– Two major options for taking and recording measurements
◊ Electronic reticules similar to those available for stereo microscopes
◊ Mouse-driven point-to-point techniques displayed on-screen (proprietary in nature and software dependent)
– The accompanying software provides options for saving, storage, and retrieval.
– Measuring class characteristics like thickness, width, or diameter
– The device consists of an outer thimble and inner spindle joined by a fine pitch screw mechanism.
– Electronic models
◊ Are easily zeroed at any point
◊ Are easy to use
◊ Offer a choice of units, usually English and metric
◊ Are designed for analog use in the event of battery failure
◊ Are accurate
◊ Provide easy to read liquid crystal displays (LCD)
– Used to measure distance between two opposing sides of an object
– Digital calipers also have a scale for manual use in the event of battery failure.
• Machinists scale
– Useful for measuring barrel length
• MP6 optical projector
– Used for measuring bullet diameter, land and groove impressions, and other small items
c. Velocity of a fired bullet (in rare cases)
• Sense the passage of a bullet past two sensors separated by a known distance.
• Data may include
– Highest velocity recorded
– Lowest velocity recorded
– Spread of the high and low velocities
– Average velocity
– Standard deviation
Accurate measurements taken has multiple effects
a. Accurate results
b. Certification of the firearm examiner
i. Competency and proficiency tests
c. Laboratory accreditation of the firearms section
B. Standards, NIST Traceability, and Laboratory Accreditation
Standards used in measuring weight and dimensions in an accredited laboratory should be traceable to and fall within the tolerances of a standard maintained by NIST.
a. The standards may or may not be used in forensic firearms work in a given laboratory.
b. There are increasing challenges relating to NIST standards and traceability in the future including
i. Widespread recognition of a need for accreditation of forensic laboratories. Laboratory accreditation can be achieved through the following accreditation bodies and their respective programs.
• American Society of Crime Laboratory Directors/Laboratory Accreditation Board (ASCLD/LAB)
– ASCLD/LAB Legacy: The original accreditation program offered by this agency. This program will be phased out in favor of ASCLD/LAB ISO.
– ASCLD/LAB ISO: Based on ISO/IEC 17025:2005 the international standard developed by the International Organization for Standardization (ISO) applicable to forensic laboratory accreditation.
• Forensic Quality Services (FQS)
– FQS ISO: Also based on ISO/IEC 17025:2005.
ii. Support of accreditation programs by the Association of Firearm and Tool Mark Examiners.
iii. Emphasis placed on formalized laboratory technical protocols by professional groups, such as the Scientific Working Group for Firearms and Toolmarks (SWGGUN).
II. Stereo and Comparison Microscopes
A. Allow the simultaneous comparison of the unique striations and impressed marks found on the surfaces on the surfaces of objects bearing toolmarks (including fired bullets and cartridge cases)
C. Compound Microscopy
Problems with single lens and compound microscopy
a. Dimensional distortion
b. Color inaccuracies (especially at the edges of the round convex lenses used in the system)
c. Inverted images
B. Primitive Microscopy
Uses a convex lens on both sides and have a short focal length to make objects appear larger.
The use of physically joined compound microscopes (one for each eye) in the examination of single objects.
Maintenance and Calibration
a. Standards for preparing a stereomicroscope for actual case work
• The stereomicroscope will be cleaned, serviced and certified (by a factory certified technician).
• These steps will be documented in the instruments maintenance/calibration logbook.
• The stereomicroscope will be standardized utilizing a 100-0.01 division 3x objective. This objective will either be obtained from the manufacturer or have a NIST traceable certificate.
• The microscope may be calibrated with a ruler with divisions of 0.01. This ruler should also have a NIST traceable certificate.
• This will also be documented in the instruments maintenance/calibration logbook.
iii. For each use
• The stereo microscope will be checked to ensure that it is functioning properly.
• This check will be performed by observing an item under the microscope and utilizing past experience to determine if the instrument appears to be giving a true and accurate representation of the evidence.
• This check does not need to be documented.
E. Comparison Microscopy
Before comparison microscopy forensic firearms examination involved
a. The sequential examination of fired components using a single compound microscope
b. Large format photography of microscopic details for each component through a compound microscope
c. Side-by-side comparison of the photographic results
d. Preparation of exhibits based on the photographs
Problems with this approach
a. The evidence items were examined in sequence, not simultaneously
b. the evidence items could only be simultaneously compared using photographic prints
c. The photographs taken were two-dimensional representations of three-dimensional objects.
Features of the monocular instrument
a. Binocular viewing
b. Rotating nosepieces with a variety of objective lenses
c. A choice of illumination systems, e.g., variable fiber optic, LED, and highly-intensity fluorescent light sources
d. Push-button focusing
e. Image capture systems with file export and printing capabilities
f. Specialized specimen mounts
g. Motorized control of the x, y, and z axis settings, which can be saved for reproducibility
h. Monitors to facilitate examinations, training, and image capture.
i. Optical and digital capability for image superimposition or conventional side-by-side comparisons
j. Digital reference marks to easily return to areas of interest.
Maintenance and Calibration
• The comparison microscope will be cleaned, serviced, and certified (by a factory certified technician)
• These steps will be documented in the instruments maintenance/calibration logbook
• The comparison microscope will be calibrated with a glass slide with 0.04 ruled to 0.001
• The slide should have a NIST traceable certificate
• This will be documented in the instruments maintenance/calibration logbook
iii. For each use
• The comparison microscope will be checked to ensure that it is functioning properly.
• This check will be performed by placing two similar items on each stage (test to test) and observing the agreement between these items. This would be documented in the examiners case notes.
III. Small Tools and Supplies
A. A number of safety items, hand tools, and expendable supplies should be available to each examiner.
B. Core items
Laboratory safety items
a. Personal protective equipment (PPE)
b. Disinfectant solution
c. Cotton swabs and detergent
Small tools for general use
a. Metal scribe for marking evidence and test standards
b. Jewelers screwdriver set
c. Small file set
d. Tweezers and/or forceps
f. Electronic micrometer
g. Steel machinists scale in English and metric units
h. Precision knives
i. Hammers, brass and polymer heads
j. Leather mallets, large and small sizes
k. Drift punch set
l. Bench block
m. Needle-nosed pliers
o. Jewelers loupe in various magnifications
p. Magnifying glass
t. Gunsmiths scewdriver set
u. Conventional pliers
v. Parallel jawed pliers
w. Roll pin punch set
x. Small vise
z. Partitioned specimen trays
ab. Inertia bullet puller
ac. Hemostat for use as a gripping tool
Items associated with firearms and toolmark examinations may include
a. Appropriate containers for test standards, such as small boxes
b. Appropriate containers for evidence items
c. Permanent markers in various colors
d. Casting material, such as Mikrosil™
Items usually associated only with toolmark comparisons may include
a. Clay or plasticene for mounting odd-shaped toolmarked items on microscope stages
b. Lead and copper wire in various diameters for the production of test standards
c. Sheet lead and brass for the production of test standards
Items generally associated with the comparison microscopy of fired ammunition components may include
a. Appropriate ammunition for use in firing test standards
b. StikkiWAX®, or the equivalent, for mounting items on specimen holders
IV. Imaging Equipment
A. Film Imaging
B. Digital Imaging
Current comparison microscopes typically have integrated systems for digital image capture
a. Image storage
b. Image enhancement
c. Image overlay
d. Side-by-side comparisons
e. Image file export
V. Field Support Equipment
A. Firearms/toolmarks examiners may be required to provide on-site support during the investigation of a major case.
B. This type of field support usually involves two types of response
Major crime scene searches
a. The scope of the firearm examiners role in this type of response is defined by agency policy, and the examiner is one of the following
i. Forensic content expert immediately available to consult with the leader of the crime scene search or the senior investigator present
ii. Member of a formally organized evidence response team that has sole charge of the crime scene search
b. Items that may be collected include
i. Small items
• Small toolmarked items
• Fired bullets and cartridge cases
• Unfired ammunition
ii. Mid-sized items
• Clothing and other items bearing gunshot residues
• Rifles and shotguns
• Medium-sized toolmarked items
• Bulk ammunition
iii. Large items
• Vehicle parts
• Safe doors
• Furniture items
Shooting reconstruction/bullet path analyses/trajectory analyses (outside the scope of the training)
C. Computer-Based Technologies
Integrated Ballistics Identification System (IBIS®)
a. The use of this technology is coordinated by the NIBIN, a group governed by an executive board with
i. Bureau of Alcohol, Tobacco, Firearms and Explosives
ii. Federal Bureau of Investigation
iii. State or local law enforcement agency
i. Allows images to be altered
• Light intensity and direction
• Dimensional aspects
i. A confocal microscope designed to simultaneously gather two- and three- dimensional images of bullet surface topography
ii. A digital camera to capture the data
iii. An automated bullet holder to physically manipulate bullets, including damaged bullets
iv. Computer hardware
v. Proprietary software to manage image capture, transmission, and storage on a computer server
VI. Firing Facilities
An indoor range for firearms function testing, including remote test firing
Bullet recovery system for obtaining known standards for comparisons with evidence bullets
Access to a range for testing requiring longer distances
B. Indoor Ranges
An indoor range facility dedicated to laboratory use is ideal for the efficient operation of a forensic firearms unit.
Bullet Recovery Equipment
a. Cotton Box
• Easily assembled from readily available materials using in-house agency resources
• Requires a designated firing range
• May require considerable time to find fired bullets (all bullets must be found and removed to avoid confusion with other cases)
• Presents the potential for gunshot residues to ignite the cotton near the firing aperture
b. Water Recovery Tank
i. These should be considered when constructing a horizontal water tank
• The angle of the fired bullet should be approximately 30 to avoid ricochets off the surface of the water, minimize damage to the bullet
• The bullet should travel a minimum of ten to twelve feet on the diagonal in order to decelerate most fired bullets
• The tank should be positioned to facilitate firing at a 30-degree angle while allowing the shooter to maintain a comfortable shooting position
• Tanks, which can weigh as much as five tons, should be located on the building ground level
• Stainless steel construction
• Underwater lighting to avoid surface reflection
• Water filtration and purification
• Air-tight lid and exhaust system
• Assisted lifting system to open the lid
• Nonskid shooting platform
• Secure drain system to preclude bullet loss
• Light colored interior finish to improve platform
• Connection to a floor drain
iii. The space allocated for bullet recovery must address the same requirements as any other shooting facility, to include the following concerns
• Adequate ventilation
• Sound mitigation
• A means for lead removal from bullet traps
• Periodic removal of accumulated surface lead from walls, ceiling, and floors
• A posted emergency plan for range users
• First aid equipment
• Periodic training of all personnel in first aid with special emphasis on gunshot wounds
• Periodic training of all personnel in mouth-to-mouth resuscitation
• Landlines to emergency response personnel and managers
• Baseline and periodic testing for lead levels in the blood for all personnel
• Baseline and periodic hearing test for all personnel
• Availability of eye and hearing protection in the space
c. Other Bullet Recovery Systems
i. Wet telephone books
ii. Oil or water-soaked sawdust
iii. Ballistic gelatin
iv. Plastic milk jugs
v. Blocks of ice
C. Outdoor Ranges
Necessary under the following circumstances
a. Longer distance shot pattern testing
b. Indoor facility construction limits the use of high power ammunition
c. Additional space required to reproduce a shooting scenario
d. Indoor facility construction limits the use of fully automatic firearms
e. Accuracy testing at greater ranges
a. Availability and scheduling
b. Permission may be required to conduct testing
c. Adherence to local laws and agency regulations
d. Weather conditions