Firearms 5 and 6

Firearm

An assembly of a barrel and action from which a projectile is propelled by products of combustion

Revolver

Has a cylinder with several chambers which rotate around an axis to align the chamber with the barrel for discharge

Pistol

Chamber is an integral part of the barrel

Single Action

Pull of trigger performs one action

Double Action

Pull of trigger performs 2 actions

Revolver Single Action

-Hammer must be cocked manually to rotate the cylinder for each shot

-Firearm is ten discharged by a pull of the trigger

-Process is repeated for each shot

-Pull of the trigger performs only one action: releases the hammer

Revolver Double Action

-The rotation of the cylinder, cocking and firing are performed by a pull of the trigger

-Pull of the trigger performs two actions: cocks hammer, releases hammer

Pistol Single Action

-Striker/hammer is cocked by pulling the slide fully to the rear and releasing slide to fly forward

-Pull of the trigger performs only one action: releases the striker/hammer

Pistol Double Action

Pull of trigger performs two actions: Cocks hammer/striker and releases hammer/striker

Purpose of Safety

Device on a firearm that is intended to prevent unintentional discharge when engaged in “SAFE” position

Manual Safety

Firearm safety feature that requires manual activation/manipulation

-Ex.) Thumb safety/trigger safety

Passive Safety

-Firearm safety feature that is inherent to the design of the firearm

-Engaged in “safe” position until operation of the firearm (i.e. pull of trigger, insertion of magazine) disengages it

-Ex.) Transfer bar, magazine safety, internal firing pin block

Types of Revolver Safety Features

Hammer Block

Transfer Bar

Hammer Block

-Device that blocks/prevents contact between the firing pin and the hammer

-Pull of trigger drops hammer block down and allows hammer to fall onto the firing pin and therefore strike the cartridge

Transfer Bar

-Is connected to the trigger

-Pull of trigger raises the transfer bar, hammer strikes the transfer bar, which transfers energy from hammer to firing pin to discharge the cartridge

Sear

Part in a firearm which holds the hammer/striker in a cocked position until the trigger is pulled

Trigger

Part of a firearm’s firing mechanism that is moved manually to cause the firearm to discharge

Recoil Operation

-The action is locked closed via cutouts in the slide that “lock into” lugs on the barrel

-After discharge, the barrel and slide travel rearward together a short distance, as a result of the energy of discharge

-The barrel then cams downward and unlocks/separates from the slide

-Ex.) 9 mm Glock model 17 pistol

Blowback Operation

-Action is not locked, it is held closed by the weight/mass of the slide

-Gas pressure forces empty cartridge case rearward at the same time forcing bullet out of cartridge case and down barrel

-Energy of motion created by rearward movement of the empty cartridge case after discharge is the source for the cycle of fire

-Ex.) 9 mm Hi-Point pistol model C9

Gas Operation

-Gas pressure from discharge of cartridge is siphoned off from the barrel into a tube

-Gas travels down the tube and directly impinges the bolt itself or pushes a piston inside the tube which impinges the bolt

-Impingement on bolt unlocks the action and cycles the firearm

-Ex.) 223 Rem Bushmaster rifle model M4

Action

Working mechanism of firearm is loaded, fired and unloaded

Four rules of firearm safety

1. Always assume the firearm is loaded until you check it for yourself

2. Always keep the firearm pointed in a safe direction

3. Always keep your finger off the trigger unless you intend to fire the firearm

4. Know what is behind your target

Cycle of Fire Steps

1. Feeding

2. Chambering

3. Locking

4. Firing

5. Obturation

6. Unlocking

7. Extracting

8. Ejecting

9. Re-cocking

Feeding

Cartridge is fed towards the chamber by the slide

Chambering

Cartridge is pushed all the way into the chamber

Locking

Action is closed and locked

Firing

Pull of trigger causes firing pin to impact primer creating initial spark to ignite the gun powder

Obturation

-Gas pressure builds up inside cartridge case from gunpowder burning and forces bullet out of cartridge and down barrel

-Cartridge case swells from gas pressure and presses against chamber walls, preventing gas from escaping out the chamber

Unlocking

As pressure forces slide rearward and opens the action; the slide and barrel move rearward together for a short distance, then the barrel cams down and the slide continues rearward movement

Extracting

Rearward movement of slide pulls empty cartridge case out of chamber by the extractor

Ejecting

Empty cartridge case hits the ejector and is ejected from the firearm

Re-cocking

Rearward movement of the slide re-cocks the striker; recoil spring decompresses driving the slide forward; the slide strips the top cartridge from the magazine beginning the cycle of fire again

Semi-auto

A firearm in which each pull and release of the trigger results in one complete cycle of fire from discharge to reload

Full-auto

A firearm which, when the trigger is pulled, will continuously discharge cartridges until the trigger is released, or until the firearm’s magazine is empty

Disconnector

-A device intended to disengage the sear from the trigger. In a semiautomatic firearm it is intended to prevent full automatic firing.

-In a manually operated firearm, it is intended to prevent firing without pulling the trigger.

Test for full auto discharge capacity

-Cock the firing mechanism

-Pull the trigger and keep it pulled to the rear – sear will release striker

-While continuing to hold the trigger to the rear, pull slide/cocking handle to the rear and release to fly forward – striker should be recocked and retained by sear

-Release trigger – listen for trigger to “reset” – click noise as disconnector disengages trigger from sear

-Pull trigger again

-If trigger was “reset”, then pull of trigger will cause sear to release striker again

-If trigger mechanism did not reset, the disconnector did not disengage the sear from the trigger and the firearm is (potentially) capable of full auto discharge

-In a manually operated firearm, the disconnector is intended to prevent firing without pulling the trigger.

NIBIN

National Integrated Ballistics Information Network

Goal of NIBIN

Find links between cases to provide investigative leads to law enforcement

Cartridge cases from these firearms are entered into NIBIN

-Semi and Full auto Pistols

-Semi and Full auto Revolvers

-No CCs from revolvers or shotguns

Serial Number

A unique identifier composed of numbers, letters, symbols, or a combination thereof, applied to an item by a manufacturer

Firearms with Serial Numbers

-Gun Control Act of 1968

-All firearms manufactured in/imported into the US after 1968 must have SN

-For tracking purposes, recall purposes

Firearms without Serial Numbers

-Guns before 1968

-Ghost guns

-3D printed guns

-Improvised firearms for personal use

Legal Serial Number Location

-Located on frame/receiver

-Possible duplicate/partial duplicate SNs may be found on cylinder, slide, barrel

Serial Number Structure

-Firearm Serial Number Structure Guide – DOJ publication

-Royal Canadian Mounted Police database

-Look at same model from Firearm Reference Collection

Plastic Deformation

-Permanent change in the size/shape of an item when a stress applied to the item exceeds the elastic limit of the item

-Serial number restoration techniques rely on the permanent change that occurs to the crystalline structure of the metal underneath a stamped number of a firearm.

What determines which chemicals are used in serial number restoration

Metal composition of firearm

-Ferrous (Magnetic properties)

-Non-Ferrous (Non-magnetic properties)

How do chemicals work

The chemicals react faster with the permanently altered metal under the SN area than with the non-altered metal surrounding the SN.