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Firearms Forensic analysis is vital to solve a crime that uses a gun.
In 2004, there were 12,00 homicides in the US The vast majority of U.S. homicides involve guns. And they are more powerful than ever. Lansing State Journal, July 2007
Firearms: A Quick History
Almost every gun is based on the same simple concept: You apply explosive pressure behind a projectile to launch it down a barrel. The earliest & simplest application of this idea is the cannon. The 1st handheld guns were essentially mini-cannons; you loaded some gunpowder & a steel ball & lit a fuse
War typically resulted in the need for improved weapons technology.
In the late 1800’s, the revolver quickly became popular due to it’s size & quick loading. It only had to be reloaded every 5-6 shots instead of after each shot.
Handguns reigned supreme for the past 200 years & to this day, remain the most popular & readily available firearm.
Types of Firearms Handguns (pistols) Revolver Semiautomatic Rifles
Shotguns Air or BB guns
Ammunition Components Cartridge case Primer Propellant Projectile
Bullets Made of lead, sometimes jacketed with brass, copper, or steel
Bullet size—diameter (caliber or gauge) Shapes
Often confused with the term ballistics Ballistics is the study of a projectile in motion. Inside the firearm After it leaves the firearm When it impacts the target Identification of Firearms is based upon this basic idea: A harder object marks a softer one & imparts/transfers its microscopic irregularities to that object.
Forensic Firearms Expert
Did a suspect use this gun to kill that person? Did these bullets come from that gun? Was it really self-defense? Is this a case of suicide, or is foul play involved? Bullet Comparison Weapons Function Is it safe? Has it been modified? Serial Number Restoration Gunpowder Residue Detection on clothes, hands, & wounds Muzzle-to-Target Distances
Pulling the Trigger Pulling the trigger releases the firing pin…
The firing pin strikes the primer… The primer ignites the gun powder… The powder generates gas that propels the bullet forward through the barrel & ejects the spent cartridge case.
Bullet Caliber Caliber: the diameter of the gun barrel.
Caliber is recorded in hundredths of an inch (.22 & .38) millimeters (9mm)
Cartridge Parts & How it works
Anatomy of a Bullet
Bullet Comparisons Each gun leaves distinct markings on a bullet passing through it. A gun barrel is made from a solid bar of steel that has been drilled/hallowed out. The drill leaves microscopic marks on the barrel’s inner surface. Gun manufacturers also add spiral grooves to the barrel. This is known as rifling. Lands: the space between the grooves. As a spinning bullet passes through the barrel, it is marked by these grooves.
Rifling The grooved spirals inside the barrel of a gun that produce lands and grooves on a bullet Lands & grooves are class characteristics
Striae Scratches on a fired bullet, like a barcode
Can serve as individual evidence Matching bullets or bullet to a firearm
Class Characteristics: Once a manufacturer chooses a rifling process, for a particular class of weapon, they keep it consistent. Lands & Grooves are the same for a model. .32 caliber Smith & Wesson has 5 lands & grooves twisting to the right. .32 caliber Colt has 6 lands & grooves twisting to the left. Class characteristics can eliminate certain makes but are not enough to ID a particular gun.
Imperfections in the manufacturing process make each barrel unique. Rifled barrels, even if made in succession will NOT have identical striation (scratch-like marks).
Bullet Comparisons To match bullets to a gun, test bullets must be fired through a suspect barrel for comparison. Goddard & Comparison Microscopes Examined bullets side-by-side (to match striated markings).
Cartridge Case Head Stamps Rimfire & Centerfire Cartridges
Usually brass or nickel-clad brass Class evidence Manufacturer Shape Caliber Composition Head Stamps Rimfire & Centerfire Cartridges
Cartridge Markings All moving components contact the cartridge rather than the bullet can leave useful impressions on shell cartridges. Cartridge Case Individual Characteristics: Breech face marks Firing pin impressions Chamber marks Extractor marks Ejector marks
Breech Marks When a cartridge is fired, the explosion forces the bullet down the barrel and the shell casing is forced back against the breech. This leaves impressions unique to the individual gun’s breech on the shell casing.
Breech face Marks
Actual Breech Marks
Firing Pin Marks In order to fire the cartridge, the primer must first be ignited. To accomplish this a firing pin strikes the center ring of the cartridge. This will in turn leave a distinct impression that is unique to the firing pin of that particular gun.
Firing Pin Marks
Extracting Pin & Ejector Marks
The extracting pin and ejector throw the spent shell casing from the chamber of the gun. These leave marks on the shell casing that are unique to those parts on that particular firearm.
Other Factors Perfect matches sometimes difficult b/c:
Presence of grit & rust in a barrel Recovered bullets too mutilated or distorted on impact A spent bullet’s weight can sometimes determine the gun make. Microgrooves: 8-24 grooves; it’s not as common General Rifling Characteristics File FBI database of known land/groove width for all weapons.
Shotguns Smooth barrel
Projectile NOT marked as it passes through Fire small lead balls or pellets contained within a shell. Characterized by: diameter of the shot size & shape of the wad Gauge: diameter of the barrel ( gauge diameter) Identification can still be made by comparison of extractor/ejector markings on shotgun shell.
Features of a Semiautomatic Handgun
Firearms Evidence Individual: Striae Firing pin marks Breech marks
Extractor marks Ejector marks Chamber marks Class: Bullet type Bullet caliber Bullet weight Lands and grooves Rifling Cartridge case Head stamp
Gunshot Residue (GSR) When a weapon is fired:
Primer and propellant particles blow back toward the shooter. Combustion products (mostly NO2-), unburned propellant, and particles of lead follow the bullet, spreading out with distance.
Gunpowder Chemistry GSR Sources: victim, clothing or target
shooter’s hands Gunpowder Chemistry Major detectable elements are: lead (Pb), barium (Ba) & antimony (Sb) Virtually all cartridge cases are made of brass (copper & zinc); also detectable.
Griess Test Tests for the presence of nitrates (partially burned or unburned gunpowder) Swab of shooter’s hand Must produce a pattern for a distance determination
Results of GSR Hand Test
Negative results may be caused by: Washing the hands Shooter may have been wearing gloves Lead free ammunition A rifle or shotgun may not deposit GSR on hands
GSR on the hand of a suicide victim, proving he was holding the weapon when it was fired.
With a contact or very close range gunshot wound, it is possible to have blood spatter as well as GSR on the hand of the person firing the weapon.
Trajectory DC Sniper
SERIAL NUMBER RESTORATION
When a serial number is stamped into a gun, the metal underneath the number is compressed & hardened. If the number is filed-off, the hardened area may still be present. By using an acid solution the metal can be slowly eaten away. In this process the softer metal will be eaten away first and the number may reappear.
FIREARMS EVIDENCE COLLECTION
Make sure it is unloaded!!!!! DO NOT put a pencil into a barrel REVOLVERS Indicate location of fired & unfired ammunition AUTOMATICS Check magazine for number of rounds Fingerprint magazine Place ID tag on trigger guard
AMMUNITION Write on base or nose Package in pill box or envelope
Wrap in tissue to protect
Protect & preserve any residue Air dry if wet
CLOTHING Protect & preserve any residue Air dry if wet Package separately in paper bags Establish CHAIN OF CUSTODY
Toolmarks Tools often used in burglaries may leave a mark.
Class characteristics: type, size, shape Individual characteristics: features from wear and damage
TOOL MARK IMPRESSIONS Impressions Cuts Gouges Abrasions
TOOL MARK IMPRESSIONS COLLECTION
SUBMIT ENTIRE OBJECT PACKAGE SEPARATELY (ALWAYS!) DO NOT ATTEMPT TO FIT THE TOOL INTO THE TOOL MARK
Matching Toolmarks Photography & casting are important to match tool with mark ? ?
Class characteristics— manufacturer, type, model, size Individual characteristics— wear patterns, nicks, marks, occlusions (like pebbles or sticks)
Captured by oblique-angle photography or chemical enhancement
casting in soil lifting.
Treated much the same as shoeprints
Tire marks Treated much the same as shoeprints Class characteristics involve design, size, type, and model. Wear and damage cause defects that can lead to individualization. TreadMate is a database containing data on more than 5,000 vehicle tires and tread patterns. Tire Treads
Bite marks Result from assault or sexual attack, common in domestic violence Individual evidence, if enough impressions Bite marks were the prime evidence in the conviction of serial killer Ted Bundy.
Serial Numbers/Restoration of serial numbers
Items of value may have ID numbers stamped into them. Grinding is usually used to obliterate identification numbers. To restore ID numbers on metal, an acid etching solution is employed. HCl-CuCl2
Forensics: Debate Both Sides of the Issue; Gun Control Laws Should gun sales and ownership be subject to federal regulation? Introduction Pro/con sides Assertion Evidence Personal opinion
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