Types of Firearms May be classified by their state of automation –single-shot (manual) –semiautomatic –automatic May also be classified by aspects of their.

Types of Firearms

• May be classified by their state of automation– single-shot (manual)– semiautomatic– automatic

• May also be classified by aspects of their physical design– Rifled – Smoothbore

Handguns

• Designed to be fired with one hand– originally designed for cavalrymen who could

only spare one hand from the reins of the horse

• There are some single-shot handguns used for specialized target shooting

• Handguns with revolving cylinders are called revolvers

A Colt .45

Rifles• Designed to be fired with two hands, usually from

the shoulder• There are some small bore sporting rifles that are

single shot• Most are semiautomatic or automatic

– redirect the energy from either the bullet recoil or hot propellant gases to extract & eject the cartridge casing, then cock & load a new bullet

– Assault rifles may be used either in semi-automatic or automatic mode

Bullet Morphology

Cartridge Cases

Cutaway photos of three kinds of cartridge cases

Firing A Shot

Chambered bulletFiring pin hits primer

& sending a flash to ignite the powder

Bullet is propelled forward though the gun barrel as the spent cartridge case is slammed back against breechblock

Smoothbore Firearms

• Shotguns are the only smoothbore small arm currently in use

• Fire multiple small projectiles (pellets or shot) so accurate aiming not an issue – barrels smooth rather than rifled– barrels shaped or indented to control the spread

of the shot as it exits the muzzle• choke

Compression Formed Shotgun Shell

• Pellets are separated from the propellant by one or more overpowder wads– seal the propellant

gases behind the shot

The Rifled Barrel

• Has a series of spiral cut-outs that run the length of the barrel– depressions

• grooves

– raised portions• lands

Interior View of a Rifled Gun Barrel

• Horizontal striations on the grooves (G) come from the rifling process

• Vertical striations on the lands (L) come from the initial reaming of the barrel

• These markings impress the bullet as it travels through the barrel

Barrel Manufacture

• Before 1940, grooves cut one at a time by drawing a scraper repeatedly down the inside of the barrel as it was rotated

• Three modern methods of rifling– broaching– button rifling– hammer forging

Broaching

• The diameter of the hole reamed in the steel will be the final diameter (caliber) of the barrel

• Grooves cut with a tool called a gang broach– consists of a series of circular cutting tools with

projecting teeth positioned a regular intervals on a rod

Broaching

• The teeth cut away the metal in the grooves as the broach is forced through the barrel by hydraulic pressure

• Successive cutting disks increase in diameter until the desired groove depth is reached

Bore Diameter of Rifled Barrels

• Measured from the tops of opposing lands when there is an even number of lands & grooves

• measured from a circle tangent to the tops of the lands if there is an odd number of lands & grooves

Bore Diameter• All firearms may be characterized by their bore

diameter or caliber– the measure of the diameter in hundredths of an inch or

in millimeters• American & British weapons are normally given in hundredths

of an inch (.22; .45)

• other countries are normally in mm (9 mm)

• The larger the number, the larger the diameter– .22 is smaller than .45

Rifled Barrel

• The # of lands & grooves; their direction & rate of twist are characteristic of a particular product from a specific manufacturer– class characteristics

• Examples– .32 caliber Smith & Wesson revolvers

• 5 lands & grooves twisting to the right

– .32 caliber Colt• 6 lands & grooves twisting to the left

Firing Pin (Primer Pin)• Strikes the cartridge

primer to initiate the firing process

• Finished on a lathe or filing by hand

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

Primers

• Since gunpowder is relatively stable, it I ignited via a more flammable or explosive primer

• Types of systems– rimfire cartridge

• primer placed in the rolled rim of the cartridge casing

– centerfire cartridge

Some Rimfire Impressions

Firing Pin Match

How A Bullet Acquires Rifling Impressions

Bullet Comparisons• One bullet is rotated until a

well-defined land or groove comes into view

• The other bullet is then rotated in search of a matching region

• Analyst must allow for distortion between bullets due to mutilation on impact

Moving Components

• Some components of a firearm’s firing & cartridge ejection mechanism are of importance to the forensic examiner– the breechblock– the firing pin – the extractor– the ejector

Ammunition

Construction

Extractor & Ejector

• Extractor– extracts a spent cartridge from the chamber

• Ejector– ejects the extracted cartridge from the side of

the weapon body

• Metal parts of these mechanisms can leave individualizing markings on cartridge cases

Moving Components

• All of the moving components contact the cartridge rather than the bullet– can leave useful impressions on shotgun shell

cartridges

Automatic

• Fully automatic weapons will continue to fire & expend casings as long as the trigger is depressed and ammunition available

Machine & Submachine Guns

• Machine guns have a heavy recoil– designed to be fired from some type of sturdy

mounting

• Submachine guns are designed to be fired while being held in the hands

Bullets

• When black powder was used as a propellant– projectiles had relatively low velocities– bullets were usually almost pure lead

• With smokeless powders– burn rate & heat production much greater– pure lead bullets foul rifling

Common Types of Bullets For Rifled Firearms

• Lead Alloy– lead hardened by addition of antimony

• Semijacketed– have a copper-alloy or aluminum jacket that

covers part of the bullet surface• usually nose is left exposed

– nose configurations encourage penetration & expansion on impact

» soft-point, hollow-point, bronze point, etc

Common Types of Bullets For Rifled Firearms• Full jacketed

– consist of a lead core surrounded by a jacket of copper-nickel alloy or mild steel

– usually core is left exposed at the base of the bullet– Semiautomatic pistols use full jacketed bullets

• noses of bullets must slide up a ramp when rounds chambered

Bullet Shapes

• Influence aerodynamic properties– round 0r pointed noses reduce air resistance

• increases striking power

– some have boattailed bases• act as a rudder to reduce turbulance & decrease drag

Shotguns

• Typically characterized by their gauge• Gauge is a term which comes from the days

when all firearms fired spherical lead balls– gauges were expressed as the number of

appropriately-sized bullets that could be made from one pound of lead

• 10-gauge shotgun– 10 lead balls of about the same diameter as the barrel

could be made from a pound of lead

Cartridges Cases

• Made from brass– may be nickel-plated to prevent corrosion

• Come in a variety of different shapes– Revolvers fire straight rimmed cartridges

– Self-loading pistols fire straight rimless cartridges

• Can have cannelures rolled into them to prevent the bullet from being pushed back into the case

Cartridge Cases

• Bullets may be held in place by criming or staking the mouth of the cartridge case into a cannelure on the bullet

• May have nformation stamped on the head of the cartridge– identify manufacturer– identify caliber

Shotgun Shells

• Most contain pellets which come in a variety of sizes– the smaller the number, the larger the pellet

• 000 buckshot pellets are 0.36 inches in diameter

• No. 12 birdshot pellets are 0.05 inches in diameter

• The load in each shotshell depends on the gauge of the shell & the size of the pellets– 12-gauge No. 1 buckshot contain 16 pellets

– 16-gauge no. 1 buckshot contains 12 pellets

Shotgun Gauge

• The higher the gauge number, the smaller the diameter of the barrel– 12-gauge shotgun has a bore diameter of 0.730

inches– 16-gauge shotgun has a bore diameter of 0.670

inches

• Exception– .410-gauge shotgun has a barrel 0.410 inches in

diameter

Shot gun Shells

• Shotgun pellets may be made of pure lead (drop shot), lead alloy (chilled shot) or soft steel

• Smaller size pellets are manufactured by pouring molten metal through a perforated plate & allowing it to fall some distance– solidifies as spheres

• Larger size are made by pressing lengths of wire between cup-shaped dies

Impression Evidence

Bullet & Cartridge Comparisons

Rifling Impression Patterns

Bullet comparisons

• Test bullet generated by firing into a water chamber or cotton box

• Bullets are then examined using a comparison microscope

Cartridge Case Comparisions

Marks which might be found on a spent cartridge

Cartridge Case Comparisons

• The hole in the center is the imprint from the primer pin

• Striation within circle are breechblock impressions

Breechblock Comparison

Evidence Cartridge Reference Cartridge

Breechblock Comparison

Breechblock Face Microscopic Comparison

Bullet Matches

Extractor Matches

Automated Firearm Search Systems

• National computerized networks have been developed to allow labs to search each other’s data

• DRUGFIRE (FBI)– concentrates on cartridge markings although

bullet striae can be stored as well

• IBIS (Integrated Ballistic Identification System)

Automated Firearm Search Systems

• IBIS (Integrated Ballistic Identification System) – developed by the Bureau of Alcohol Tobacco &

Firearms (ATF)– incorporates two separate software programs

• Bulletproof– for bullet-specific markings

• Brasscatcher– for cartridge cases

• Restoring an obliterated serial number.

• Proper collection and preservation of firearm evidence.

• Comparing suspect’s tool to a tool mark.

• Significance of class and individual characteristics to the comparison of impressions.

• Common field reagents for enhancing bloody footprints.

• Firearms: a discipline mainly concerned with determining whether a bullet or cartridge was fired by a particular weapon. Not to be confused with ballistics, which is the study of a projectile in motion

• Grooves: the cut or low-lying portions between the lands in a rifled

• Rifling: the spiral grooves that are formed in the bore of a firearm barrel to impact a spin to the projectile when it is fired

• Bore: the interior of a firearm

• Lands: the raised portion between the grooves in a rifled bore

• Caliber: the diameter of the bore of a rifled firearm. The caliber is usually expressed in hundredths of an inch or millimeters, e.g. .22 caliber and 9 mm

• Gauge: size designation of a shotgun, originally the number of lead balls with the same diameter as the barrel that would make a pound. For example, a 12-gauge shotgun would have a bore diameter of a lead ball. 1/12 pound in weight. The only exception is the .410 shotgun, in which bore size is 0.41 inch

• Breechblock: the rear part of a firearm barrel

• Ejector: the mechanisms in a firearm that throws they cartridge or fired case from the firearm

• Extractor: the mechanism in a firearm by which a cartridge of a fixed case in withdrawn from the chamber

• Distance determination: the process of determining the distance between the firearm and a target usually based on the distribution of powder patte3rn or the spread of a shot pattern

• Greiss Test: a chemical test to develop patterns of gunpowder residues around bullet holes

• Choke: an interior constriction placed at or near the muzzle end of a shotgun’s barrel for the purpose of controlling shot dispersion

Tool Mark Comparison Examples

Matching evidence (L) and reference (R) prymarks