firearms, took marks & other impressions

Firearms, Took Marks & Other Impressions

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.

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

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 (9mm)

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 (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).

Bullet Comparisons

Cartridge MarkingsAll 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.

DANGER

GRAPHIC IMAGES!

Contact Gunshot wound

This is a contact gunshot entrance wound.

Since the barrel contacts the skin, the gases released by the fired round go into the subcutaneous tissue & cause the star-shaped laceration.

Abrasion Ring

An abrasion ring is formed when the force of the gases entering below the skin blow the skin surface back against the muzzle of the gun.

The abrasion ring, and a very clear muzzle imprint, are seen in this contact range gunshot wound.

This is a contact range gunshot entrance wound with grey-black discoloration from the burned powder.

Displayed here is an entrance at the left and an exit at the right.

Exit wounds vary considerably in size and shape because the bullet can be deformed in its transit through the body.

There may be no exit wound at all if the bullet's energy is absorbed by the tissues. Some bullets (such a a "hollowpoint") are designed to deform so that all their energy will be converted to tissue damage and not exit.

• Powder tattooing is seen in this intermediate range gunshot wound. The actual entrance site is somewhat irregular, because the bullet can tumble in flight.

• The surface of the skull demonstrates the heavy soot in this contact range entrance wound, as well as radiating fracture lines. The direction of fire was thus toward the back of this picture.

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.

END GRAPHIC IMAGES!

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.

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

FIREARMS EVIDENCE COLLECTION

AMMUNITION Write on base or nose Package in pill box or envelope Wrap in tissue to protect

FIREARMS EVIDENCE COLLECTION

CLOTHING Protect & preserve

any residue Air dry if wet Package

separately in paper bags

Establish CHAIN OF CUSTODY

FIREARMS EVIDENCE COLLECTION

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