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 bullet Firing 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 Ejector 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 Come in a variety of different shapes 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

Bullet & Cartridge Comparisons 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