types of firearms may be classified by their state of automation –single-shot (manual)...
TRANSCRIPT
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