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Equipment and Instrumentation
retrieved Feb 18, 2020, https://firearms…
Equipment and Instrumentation
retrieved Feb 18, 2020, https://firearms-examiner.training.nij.gov/
Module 7
Measurement, Standards, and Accuracy
- comparison of questioned items to known standards
- measure weight, dimensions, velocity
Standards, NIST Traceability, and Laboratory Accreditation
- should be traceable to and fall within the tolerances of a standard maintained by NIST
Weight, Force, and Their Measurement
- Precision laboratory beam-style balances are being replaced by digital and electronic scales
Digital Scales
- to weigh small objects
- contain: multiple strain gauges, a load cell,
a signal amplifier, and a load cell, a signal amplifier
Calibration
- ensuring that the microprocessor correctly relates the amplified digital output signal to a standard weight
- manufacturers provide known weights
- heat, humidity, and unlevel surfaces can affect readings
- NIST traceable weights for calibration are a requirement
Trigger Pull Measurements
- an assist in determining the level of difficulty related to discharging a particular firearm
- Trigger pull values can point to other root causes of functional and operational problems
Standard Trigger Weights
- Spring Gauges: lightweight and easily calibrated
- Mechanical Testers: non digital device, portable and can typically be calibrated by turning the bezel-type ring around the needle indicator
Digital Trigger Pull Testers
- push and pull testing devices
- they capture and store information relating to the full range of trigger motion
- TriggerScan: advanced system engineered specifically for gunsmiths and firearm examiners
Dimensional Data and Measurement Tools
- most forms of measurements have been replaced with digital displays
- instruments include:
- Glass measuring reticule
- Electronic reticule
- Digital measuring equipment and software
- Micrometer
- Calipers
- Machinists scale
- MP6 optical projector
Glass Measuring Reticules
- Filar Micrometer Eyepieces
- Electronic Reticules
- Eyepiece Measuring Reticules
Digital Measuring Equipment
- Electronic reticules
- Mouse-driven point-to-point techniques on-screen
- Micrometer: measuring class characteristics such as thickness, width, or diameter
Other Equipment
- Calipers: measure the distance between two opposing sides of an object
- Steel Machinists Scale: barrel length
- MP 6 Optical Projector: measuring land and groove impressions, bullet diameter and other small items
Muzzle Velocity of Bullets
- Chronographs: designed to sense the passage of a bullet past two sensors separated by a known distance
- muzzle velocity for each shot is calculated based on the distance between the sensors
Stereo and Comparison Microscopes
- enhanced to allow simultaneous examination of two separate objects
Compound Microscopy
- Stereomicroscopy: based on joined compound microscopes. Single objective lens is shared by both eyepieces
Maintenance and Calibration
- Annually: cleaned, serviced, certified, documented
- Quarterly: standardized, calibrated, documented
- Each Use: checked for proper function
Comparison Microscopy
- an optical bridge joins together the stages of two compound microscopes
- Binocular viewing
- Rotating nosepieces
- Image capture systems with file export and printing capabilities
- Motorized control
- Optical and digital capability for image superimposition or conventional side-by- side comparisons
Maintenance and Calibration
- Annually: cleaned, serviced, certified, documented
- Quarterly: standardized, calibrated, documented
- Each Use: checked for proper function, test two similar items on each stage, documented
Small Tools and Supplies
- each case could need different supplies
- Core Items:
- PPE
- Disinfectant solution
- cotton swabs and detergent
- Other small tools for general use
Supplies
- Items associated with firearms and toolmark examinations
- Items associated only with toolmark comparisons
- Items associated with the comparison microscopy of fired ammunition
Imaging Equipment
- various approaches to documenting identifications
- Imaging is generally the preferred method, replacing sketching and narrative format
Film Imaging
- cameras with adapters came into use on comparison microscopes
- allowed mounting the camera without disturbing the view through the binocular eyepieces
Digital Imaging
- Digital cameras can be converted to microscope cameras
- High-quality pictures are obtained using this method
- Alternatively, an examiner may obtain quality images using a handheld digital camera held up to either eyepiece of a comparison microscope
Field Support Equipment
two types of response:
- Major crime scene searches
- Shooting reconstruction/bullet path analyses/trajectory analyses
Crime Scene Search Responses
- examiner should ensure that there are appropriate evidence containers present to properly collect, preserve, and document firearms/toolmarks evidence at the scene.
Mid-sized items:
- Clothing and other items bearing gunshot residues
- Rifles and shotguns
- Medium-sized toolmarked items
- Bulk ammunition
Large items:
- Vehicle parts
- Safe doors
- Furniture items
Small items:
- Small tool mark items
- Fired bullets and cartridge cases
- Unfired ammunition
- Handguns
Computer-Based Technologies
- used to screen firearms-related evidence items for potential linkages to local, regional, and international incidents
- specialized computer algorithms, pattern recognition technology, and digital imaging.
IBIS
- Integrated Ballistics Identification System
- coordinated by NIBIN
- nationally interconnected, computer-assisted system facilitates the transmission of digital images of fired bullets and cartridge cases
MatchPoint+™
allows images to be altered in:
- Magnification
- Light intensity and direction
- Contrast
- Depth
- Dimensional aspects
BulletTrax-3D™
- designed to address the comparison needs of the forensic firearms examiner.
- can conduct preliminary correlations prior to actual microscopic comparisons
Firing Facilities
three requirements:
- 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
Indoor Ranges
- deal for the efficient operation of a forensic firearms unit
- automated overhead target placement and retrieval system
- bullet trap arrangement
- ventilation
- Sound and vibration mitigation
Bullet Recovery Equipment
- fired bullet samples must be collected using techniques that decelerate and securely capture the fired bullets within a short distance
- performed without damage to the microscopic detail on the surface of the fired bullets
Cotton Box
- constructed of wood or one-eighth to one-quarter inch thick metal plates
- filled with cotton waste to decelerate bullets
- simple, inexpensive, easy to assemble
Water Recovery Tank
- Water fills the tank to just below the firing port
- angle of the fired bullet should be approximately 30 degree
- the bullet should travel a minimum of ten to twelve feet on the diagonal in order to decelerate most fired bullets.
- can weigh as much as five ton
Other Bullet Recovery Systems
- Wet telephone books
- Oil or water-soaked sawdust
- Ballistic gelatin
- Plastic milk jugs
- Blocks of ice
Outdoor Ranges
necessary if:
- Longer distance shot pattern testing
- high power ammunition
- more space needed to recreate shooting scene
- fully automatic firearms
- Accuracy testing at greater ranges