Forensic Firearms Analysis FA1

Ballistics 

Study of an object in flight and under the domain of physics 

What are the two types of firearms?

Handguns and shoulder firearms

Types of Handguns

Revolvers and pistols

Types of Shoulder Firearms

Rifles, shotguns, machine/military guns

How is ammo fed into a revolver? 

Fed into firing chamber through revolving cylinder 

How to load and unload revolver?

Cylinder swings out or can be released

How does the cylinder on a revolver revolve?

Cocking the hammer or pulling the trigger advances the cylinder and aligns the next round with the barrel

Semi-automatic 

Ammo fed by spring-loaded magazine firing one bullet for each trigger pull 

Squib Rounds

Firearm malfunction whereby fired projectile doesn’t have enough force behind it to exit the barrel causing a blockage

Two main causes of Squib Rounds

Inadequate powder put into the load/use of reloaded ammunition to save money No gunpowder in load (primer sends bullet far enough to block the barrel)

Why are Squib Rounds dangerous?

Gases and debris could fly back towards shooter 

Gun could explode 

What are the three types of rifles? 

Repeating, semi-automatic, automatic 

Repeating Rifle

One bullet fired for each trigger pull

Must expel cartridge, cock, and reload magazine

Bolt action or lever

Semi-automatic Rifle

Use energy of fired ammo to expel empty cartridge, cock, and reload

Automatic Rifle 

Machine gun 

Shotguns

Like rifle but has smooth bore

Fires pellets “shot”

Fires singe projectiles “slugs”

Shotgun barrel

May be constricted to “choke” pellets

Shotgun Diameter 

Gauge measures the diameter of the shotgun's bore, indicating the size of the ammunition it can fire

What would a 28 gauge mean?

28 lead balls the diameter of the bore equal to 1 pound

What is one exception to the gauge measurements of shotguns?

.410 Bore

Broach Rifling 

Long, hardened steel rod w/ cutting rings spaced down the rod 

Each successive cutting ring is larger in diameter 

Cutting rings have evenly spaced gaps around them for lands 

How to broach rifle?

Rod is twisted as it is pulled through the barrel forming the spiral rifle pattern

Button Rifling

Most common

Very hard steel plug

How to button rifle?

Steel plug forced down the barrel 

Grooves are formed under very high pressure 

Pressure hardens and polishes the inside of barrel 

Polygonal Rifling Impressions

By-product of Hammer Forging

Hammer Forging 

More rounded edges 

Solid steel mandrel produced w/ desired rifling shape and inserted into blank barrel 

Outside of barrel is then hammered 

Bore Diameter 

Circle which touches stops of lands 

Caliber

Used to mean bore diameter

More of an approximation

Lands on ammunition

Raised areas inside bore of a firearm that engage projectile as it travels down the barrel, enhancing spin and accuracy

Grooves of firearms 

Indented areas inside bore that reduce friction on the projectile and assist with stabilization during flight

Ammunition of Handguns/Rifles

Bullet of cartridge

Ammunition for shotguns 

Shell

Bullets or cartridges contain one or more of?

Single solid projectile

Propellant (fuel)

Primer (igniter of fuel)

Cartridge case

Shotgun ammunition needs?

Pellets/shot to be held together via wadding between pellets and propellant

3 main types of bullets

Lead/lead alloy

Fully jacketed

Semi-jacketed

Lead/lead alloy bullets

Hardened lead w/ minute amounts of other metals 

Fully jacketed bullets

Lead core wrapped in copper/nickel or steel

Semi-jacketed bullets 

Metal jacket only partially covering i.e., nose exposed 

Hollow point cartridge 

Propellant

Smokeless powder ignited by primer

Nitrocellulose + stabilizers

Primer

Small metal cup containing percussion-sensitive material

Struck by firing pin

Includes lead, antimony, and barium

What are the steps when ammunition is discharged? 

1) Hammer strikes primer cap (Centre fire or rim fire) 

2) Primer explodes 

3) Propellant ignites generating hot gases 

4) Bullet pushed out of cartridge case

5) Bullets travels down barrel scratched by L/G 

Bolt Action

Manually operated sliding bolt to load, fire, and eject cartridge casings

User must rotate pull handle back

Most common patterns of polygonal rifling

6/right and 8/right

Guns with polygonal rifling

Glock, Steyr, IMI, SIG Sauer

Hammer

Strikes the firing pin to initiate the ignition of the cartridge

Firing Pin

Strikes the primer at the base of cartridge, igniting the gunpowder and firing the bullet