Firearms NotesFirearms Notes

Forensic ScienceForensic Science

Categories of FirearmsCategories of Firearms

Shotguns- gaugeShotguns- gauge Rifles-caliberRifles-caliber Handguns-caliberHandguns-caliber

Rifles and handguns are rifled Rifles and handguns are rifled (have (have spiraled grooves built into barrel)spiraled grooves built into barrel)

GaugeGauge

GaugeGauge - the system used to determine - the system used to determine the interior diameter of the barrel of a the interior diameter of the barrel of a shotgun. shotgun.

If you take one pound of lead and If you take one pound of lead and divide that pound into 16 balls of equal divide that pound into 16 balls of equal size, the diameter of those balls will fit size, the diameter of those balls will fit a 16 gauge shotgun. (it is the same a 16 gauge shotgun. (it is the same inner diameter as a 16 gauge shotgun)inner diameter as a 16 gauge shotgun)

CaliberCaliber CaliberCaliber - the system used to determine the - the system used to determine the

bore diameter of a handgun or rifle – bore diameter of a handgun or rifle – generally a measurement in either mm or generally a measurement in either mm or inches. inches.

BoreBore diameterdiameter - the distance from land to - the distance from land to land in the inside of a rifled barrel. land in the inside of a rifled barrel.

Lands Lands – the higher areas on the inside – the higher areas on the inside surface of the barrelsurface of the barrel

Lands and GroovesLands and Grooves Cut or pressed, the Cut or pressed, the

entire length of a entire length of a barrel, into the inner barrel, into the inner surface of a barrel surface of a barrel in a twisting motion.in a twisting motion.

Lower areas are Lower areas are groovesgrooves

Higher areas are Higher areas are landslands

RiflingRifling RiflingRifling - a “cut” - a “cut”

made down the made down the center of the center of the barrel of a rifle. barrel of a rifle.

Purpose - Purpose - The The “cut” is made in a “cut” is made in a twisted pattern to twisted pattern to cause the bullet to cause the bullet to spiral as it moves spiral as it moves through the air.through the air.

Rifling Cont’dRifling Cont’d

A rifle’s “Twist” is the barrel distance for A rifle’s “Twist” is the barrel distance for one spiral to be completed. Can vary from one spiral to be completed. Can vary from 8 to 20 inches. 8 to 20 inches.

How much twist a gun has depends on the How much twist a gun has depends on the type of ammunition to be used (mass of type of ammunition to be used (mass of bullet and amount and type of powder)bullet and amount and type of powder)

The number of grooves inside the barrel The number of grooves inside the barrel can also vary. From 3 to 8 grooves can also vary. From 3 to 8 grooves

For example, rifle can have a 4 groove 14 For example, rifle can have a 4 groove 14 twist. twist.

Anatomy of a FirearmAnatomy of a Firearm

BreechblockBreechblock Supports base of the Supports base of the

cartridge in the chambercartridge in the chamber

Often finished by hand filingOften finished by hand filing characteristic striations on characteristic striations on

this part contact the back this part contact the back of the cartridge leaving of the cartridge leaving individualizing markings on individualizing markings on the cartridge basethe cartridge base

Anatomy of a FirearmAnatomy of a Firearm

Firing PinFiring Pin Strikes the cartridge Strikes the cartridge

primer to initiate the primer to initiate the firing processfiring process

Finished on a lathe or Finished on a lathe or filing by handfiling by hand

Can transfer striations Can transfer striations to the soft metal of to the soft metal of the primer capthe primer cap

Firearm Anatomy, Cont.Firearm Anatomy, Cont. Extractor and EjectorExtractor and Ejector

extracts and ejectors a spent extracts and ejectors a spent cartridge from the chambercartridge from the chamber

Metal parts of extractor and Metal parts of extractor and ejector can leave ejector can leave individualizing markings on individualizing markings on cartridge casescartridge cases

All of the moving components All of the moving components contact the cartridge rather contact the cartridge rather than the bullet and can leave than the bullet and can leave useful impressions on shotgun useful impressions on shotgun shell cartridgesshell cartridges

AmmunitionAmmunition

A cartridge A cartridge (ammunition) is (ammunition) is made up of gun made up of gun powder, a primary powder, a primary explosive and a explosive and a bullet all held bullet all held within a cartridge within a cartridge case case

bullet

powder

Primer cup

Cartridge case

What Happens When the What Happens When the Trigger is Pulled?Trigger is Pulled?

1. Pulling the trigger causes the firing pin 1. Pulling the trigger causes the firing pin to strike the back of the primer cup, to strike the back of the primer cup, leaving the impression of the firing pin on leaving the impression of the firing pin on the back of the primer cup. The primer the back of the primer cup. The primer cup contains a primary explosive. (A cup contains a primary explosive. (A primary explosive is one that is sensitive to primary explosive is one that is sensitive to friction, heat and or mechanical shock.) friction, heat and or mechanical shock.)

2. When the primary explosive receives 2. When the primary explosive receives this mechanical shock it explodes. this mechanical shock it explodes.

Pulling the Trigger Cont’dPulling the Trigger Cont’d

3. This explosion causes the gun powder in the 3. This explosion causes the gun powder in the cartridge case to ignite. The burning of gun cartridge case to ignite. The burning of gun powder (KNOpowder (KNO33 + sulfur + carbon) produces the + sulfur + carbon) produces the gases COgases CO22, nitrogen oxides and SO, nitrogen oxides and SO22. .

4. The gases, produced in this reaction, take 4. The gases, produced in this reaction, take up greater space causing the bullet that is in up greater space causing the bullet that is in front of them to be forced forward through the front of them to be forced forward through the rifled barrel of the gun. Rifling marks and rifled barrel of the gun. Rifling marks and striations from the inside surface of the barrel striations from the inside surface of the barrel are impressed upon the sides of the bullet. are impressed upon the sides of the bullet.

Action and ReactionAction and Reaction

For every action there is an equal and For every action there is an equal and opposite reaction. opposite reaction.

When the bullet is pushed forward, it When the bullet is pushed forward, it pushes back on the cartridge case and the pushes back on the cartridge case and the gun itself (recoil). gun itself (recoil).

The cartridge case is pushed back against The cartridge case is pushed back against the breechface (rear wall of the firing the breechface (rear wall of the firing chamber) of the gun. The striations found chamber) of the gun. The striations found on the breechface are impressed upon the on the breechface are impressed upon the back of the cartridge case. back of the cartridge case.

Removing the Cartridge Removing the Cartridge CaseCase

The cartridge case is then removed from The cartridge case is then removed from the firing chamber by an extractor. The the firing chamber by an extractor. The extractor, a metal device, leaves a mark extractor, a metal device, leaves a mark on the edge of the cartridge case rim. on the edge of the cartridge case rim.

The ejector then pushes the cartridge case The ejector then pushes the cartridge case out of the firearm, leaving its mark on the out of the firearm, leaving its mark on the side of the cartridge case. These marks, side of the cartridge case. These marks, made by the extractor and ejector, will made by the extractor and ejector, will match the marks made on other cartridge match the marks made on other cartridge cases fired by that same firearm.cases fired by that same firearm.

Class vs. IndividualClass vs. Individual

Class vs. IndividualClass vs. Individual

Breechface marks, firing pin marks and Breechface marks, firing pin marks and the striations found on the sides of the the striations found on the sides of the bullet have bullet have individualindividual characterisitics. characterisitics.

Rifling marks on the bullet only show the Rifling marks on the bullet only show the number of grooves and information that number of grooves and information that can give the twist.can give the twist. This is This is classclass information only. information only. It can It can sometimessometimes tell the manufacturer of a tell the manufacturer of a

firearm. firearm.