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NIJ, Firearm's Examiner Course, Module 8, retrieved Feb 26, 2020 …
NIJ, Firearm's Examiner Course, Module 8, retrieved Feb 26, 2020 ,
https://firearms-examiner.training.nij.gov/
© Sara Hancock, Feb 2020
Cycle Of Fire Steps
Feeding
Chambering
Locking
Firing
Obturation
Unlocking
Extraction
Ejection
Cocking
Step 1: Feeding
Insertion of cartridges into the chamber with the breech bolt pushing the cartridge into final position. Can be manual or performed by magazines or clips.
Step 2: Chambering
The insertion of the cartridge into the chamber. Chambering can be obstructed if the cartridge is incorrect or there is a foreign matter. Oil and grease can cause overpressure.
Step 3: Locking
The breech bolt mechanism locks the cartridge into position before firing. Firearms are equipped with an interrupted mechanism that can disconnect the trigger from the firing pin. Relationship is critical and referred to as timing.
Step 4: Firing
When the breech is locked, a trigger pull translates to a firing pin release. The firing pin has a hammer behind it that forces it toward the primer, restrained by a sear that is engaged by the trigger. The hammer drives the pin against the primer which fires the cartridge.
Step 5: Obturation
This occurs when the gases under high pressure are sealed. Cartridge cases must be flexible to allow expansion against the chamber wall. When the pressure is zero, the cartridge case must release itself from the chamber wall.
Step 6: Unlocking:
Reverse of the locking process and frequently in conjunction with extraction.
Step 7: Extraction
Sometimes cartridges stick in the chamber after firing. Cases are designed with a rim/groove so a claw can grasp the edge to achieve extraction.
Step 8: Ejection
The case is contacted by an ejector which
flips
the case out of the mechanism.
Step 9: Cocking
Exposed hammers can be cocked by manual retraction.
Evidence Receipt
Each firearm should be packaged and documentation should accompany it. The sequence of examination should be determined
before
examination. Any evidence must be marked according to lab protocol.
Examination Documentation
needs to be complete and accurate. Work habits need to be thorough and consistent. Lab forms assist in the process but checklists should not just
be
completed.
Firearms Identification Information
Includes:
Brand name/Trademark
Serial Number
Model Designation
Caliber Designation
Importer
Military Firearms
Difficult to identify because there is no set caliber. Symbols and codes are used to identify the manufacturer. Bore diameter and chamber provides a link to the caliber.
Proof Marks
Some older
foreign
firearms list a location. Examiner must then rely on proof marks.
Handguns
all firearms designed to be fired with one hand.
Revolver Types:
single or double action
Types of Pistols:
semiautomatic, full automatic, single and multi barreled.
Single-Action Revolver
requires a manual cocking. Pulling the trigger releases a cocked hammer which drives the firing pin to detonate the cartridge. Older models can be "safe moded" by pulling hammer back slightly but ensuring the firing pin cannot rest on the unfired cartridge. Cylinder is held in place by a removable pin. Opening the loading gate and unlocking the hammer can let the cylinder rotate.
Modern Single-Action Revlovers
include a safety feature that prevents accidental discharge. Some can use a hammer block safety.
Double-Action Revolver
allows the hammer to be cocked and released with a single pull of the trigger. Most revolvers can act as a double or single action mode. The cylinder locks into place by the cylinder stop. The cylinder release device is located on the left rear frame. The front of the cylinder has an ejection road which is used to eject fired cartridges.
Semiautomatic Pistols
are magazine fed repeating firearms that require a pull of the trigger to fire each shot. Energy of discharge performs cycle functions like extraction, ejection, cocking, and loading.
Three major actions:
blowback, recoil, gas.
Blowback
comprised of an unlocked breech that relies on weight of breechblock.
Recoil Action
a locked breech system that relies on recoil to unlock the breech.
Gas Operated Action
locked breech system in which gas is bled from the barrel to move a piston rearward that unlocks the breech and cycling the firearm.
Autoloading - Full Automatic Pistols
a single trigger pull does the following: feeds cartridges, fire shot, extract/eject cartridges, continues the cycle. Fall automatic pistols can be full automatic by design or a semiautomatic modified.
Single and Multibarreled Pistols
manual unload and reload. Most common is break open.
Trigger Systems:
single (most common), double, double-only.
Single
the trigger is only to discharge the firearm. The external hammer may be manually cocked. Slide must be pulled.
Double
hammer can be at rest, pulling trigger cocks hammer and fires weapon. Firing cycle cocks firearm after each shot until hammer is manually at rest.
Double-Only
cock and fires firearm after each shot, Hammer returns to rest after each shot.
Long or Shoulder Firearms
can be rifles or shotguns. Designed to be fired from shoulder using both hands.
Rifles
can include action types of: bolt, pump, autoload, lever, falling block, and rolling block. Four types of magazines are removable box, internal box, rotary, and tubular.
Visual Examination
determines:
Type of magazine
Type of action
Caliber designation
Telescopic sights
Shotguns
can have single or double barrel. Single barrel actions include break open, pump, bolt, and semiautomatic.Types of choke include full, modified, and cylinder.
Preliminary Examination
Ensure firearm is unloaded
Determine the brand and model
Determine caliber designation
Measure bore diameter
Measure chamber
Research type of design
Determine country of origin
Obtain the cartridge dimensions by casting the chamber
Determine action type
Determine safety type
Determine is any modifications or alterations have been made
Observe anything unusual
Note type of sighting system and accessories present
Examine the bore and determine class characteristics.
Examine interior of bore to ensure there is no projectiles
Check bore for biological material or GSR.
Check for bulges/damage
Examine chamber for damage or cracking.
Revolver Exam
Remove cylinder
Inspect cylinder for damage or smoke halos
Replace cylinder and check functionality
Cock hammer until locked and attempt to rotate cylinder
Manually hold hammer to rear
Then test trigger pull, safety functions, and finally the firearm.
Autoloading Pistol
Determine magazine capacity
Load magazine to capacity
Place magazine into firearm
If functions properly, count cartridges and record capacity.
Cycle action by pulling slide to rear and determine if a slide stop is present.
If present, lock slide to rear
If not present, hold slide to rear
Examine extractor and ejector
Determine if firearm is capable of automatic fire.
Determine is selector switch is present.
-Cock firing mechanism
Pull trigger
Hold trigger to rear
Release trigger
Pull trigger to rear to release firing mechanism.
Then, test trigger pull, safety functions, and firearm.
*Additional Step if capable by design
Perform full automatic functionality test
Determine automatic fire functionality by inserting three cartridges then pulling and holding trigger to rear.
Confirm functionality.
Load number of cartridges into magazine
Fire short burst then release trigger
If stops, functions properly
If does not, not functioning properly.*
Additional Step if modified
Perform full functionality test
Determine functionality by inserting three cartridges then firing.
Disassemble, examine, and document modifications.
Single and Multibarreled Pistols
Open action and determine capacity
Test trigger pull
Test safety functions
Test fire
Long Guns
Determine capacity
Load magazine to capacity
Place magazine in firearm for function
Remove cartridges, count, and record.
Cycle action to confirm proper function.
Determine if capable of automatic fire.
Determine if selector switch present.
If is: can be capable of automatic fire
If not: proceed with 5 on.
Cock firearm
Pull trigger
Holding trigger to rear, cycle.
If released, firearm capable of automatic fire.
If not released, firearm functioning properly.
Determine choke
Check markings
Measure interior diameter
Index choke relative to barrel using scribe
Test trigger pull
Test safety functions
Test fire.
Trigger Pulls
Trigger Weights
Spring scale
Digital Scale
Trigger Weights
most common method. System consists of shaped rod and different sizes of weights.
For single action, cock firearm. For double action, do not cock.
Place trigger under top portion of rod.
Slowly lift firearm upward with road to bring force on trigger
If weights come off without discharge, add more weight
If firearm discharges before weights off, remove weight.
Continue until least weight required to cause firearm to discharge.
Scales
Cock firearm
Attach end of scale to fixed object and position other end against trigger.
Slowly pull away from fixed object until discharge
Record pounds of pressure.
Operate digital scales according to manufacturer
Safety Features
Always keep firearm pointed in safe direction, always keep your finger off trigger, always keep firearm unloaded until ready to use.
In Lab
Be aware of all safety policies and procedures, be responsible for own safety and others, handle every firearm like it is loaded, follow procedures if firearm comes loaded.
Safety of Revolvers
Confirm emptiness
Place cartridge into cylinder
Rotate chamber so aligned and cock
Strike trigger to release hammer.
Hammer should catch half cock, transfer bar drops away from firing pin, or hammer block engages
All of these ensure firing pin does not make contact with primer.
If there are marks on the primer, the safety features do not work. If safety does not function, retest firearm. If no safety feature, examine interior.
Autoloading Pistols
Check for safety device
Confirm emptiness
Pull slide to rear or release forward to cock or manually cock firearm.
Place safety in safe position
Pull trigger
If safety functions improperly:
Confirm empty
Place primed cartridge case in chamber
Pull slide to rear and release it forward to cock firearm
Place safety in safe position
Pull trigger
Passive Features
Disconnector ensure firearm in battery before fired and semiautomatic is not in full automatic mode. Device disconnects trigger from firing mechanism.
To test
Confirm empty
Pull slide to rear and release forward
Ensure external safety disengaged
Pull trigger and hold it to rear
Single and Multibarreled Pistols
Check for external safety device
Confirm empty
Cock firearm
Place safety in safe position
Pull trigger.
If safety proper, trigger blocked from operating. If improperly functioning:
Confirm empty.
Place primed cartridge case in chamber
Cock firearm
Place safety in safe position
Pull trigger
Long Guns
Check for external safety device
Confirm that the firearm is empty
Function the action and cock
Place safety in safe position
Pull trigger
If fires:
Confirm empty
Place primed cartridge case in chamber
Function the action and cock firearm
Place safety in position
Pull trigger
To test disconnector
Confirm empty, function action and cock, ensure safety disengaged, pull trigger and hold to rear, cock firearm again, open action far enough to take out battery, pull trigger.
Testing
Function
Load with one cartridge
Announce intention to fire
Discharge
Bullet Recovery
Properly mark cartridge case and bullets
Determine recovery medium
Load firearm with one marked cartridge
Place firearm into recovery system opening
Announce intentions
Discharge
Repeat until test fires complete
Recover
Drop Testing
Enables examiner to determine if accidental discharge happens during a drop.
Place one-inch rubber pad onto solid floor
Place primed cartridge in chamber
Cock
Disengage safety
Drop firearm from height of three feet
Test all positions
If fired
examine internal components to determine any broken or missing parts
Shock Test
Performed like drop test except the firearm is struck with a dead blow hammer.