Firearms Took Marks Other Impressions Chapter 15 Mr
- Slides: 55
Firearms, Took Marks & Other Impressions Chapter 15 Mr. Carl
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 1 st 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.
Firearms Identification • 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
Firearms 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?
Forensic Firearms Expert • 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.
Caliber: the diameter of the gun barrel. • Caliber is recorded in – hundredths of an inch (. 22 &. 38) – millimeters (9 mm)
Bullet Anatomy
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.
Bullet Comparisons • 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.
Class Characteristics • 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.
Individual Characteristics • Imperfections in the manufacturing process make each barrel unique. • Rifled barrels, even if made in succession will NOT have identical striation (scratchlike 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).
Bullet Comparisons
Cartridge Markings All moving components contact the cartridge rather than the bullet can leave useful impressions on shell cartridges – Breechface marks – Firing pin impressions – Chamber marks – Extractor & Ejector markings
Breechface Marks
Actual Breech Marks
Firing Pin Marks
Chamber Marks
Ejector Marks
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)
Gunshot Residue (GSR) • 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.
Trajectory
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.
Before & After – Historically, serial numbers are successfully restored 63% of the time on steel & 54% of the time for aluminum or zinc.
FIREARMS EVIDENCE COLLECTION n n n Make sure it is unloaded!!!!! DO NOT put a pencil into a barrel REVOLVERS n n AUTOMATICS n n n Indicate location of fired & unfired ammunition Check magazine for number of rounds Fingerprint magazine Place ID tag on trigger guard
FIREARMS EVIDENCE COLLECTION n AMMUNITION n Write on base or nose n Package in pill box or envelope n Wrap in tissue to protect
FIREARMS EVIDENCE COLLECTION n CLOTHING Protect & preserve any residue n Air dry if wet n Package separately in paper bags n n Establish CHAIN OF CUSTODY
TOOL MARK IMPRESSIONS • Impressions • Cuts • Gouges • Abrasions
TOOL MARK IMPRESSIONS • SUBMIT ENTIRE OBJECT • PACKAGE SEPARATELY (ALWAYS!) • DO NOT ATTEMPT TO FIT THE TOOL INTO THE TOOL MARK
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