15-1 prentice hall ©2008 pearson education, inc. upper saddle river, nj 07458 forensic science an...

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PRENTICE HALL ©2008 Pearson Education, Inc. Upper Saddle River, NJ 07458

FORENSIC SCIENCE An IntroductionBy Richard Saferstein

BALLISTICSChapter 15

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Introduction• Striations, other physical variations and

irregularities can match a spent bullet to a specific gun.

• Individualizing any physical evidence is always the most desirable but usually possible in ballistics.

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Gun Barrel Markings• Barrel’s inner surface has ridges that leave markings

on the bullet passing through it.• Each barrel has INDIVIDUAL markings.• Barrels are produced from a solid bar of steel hollowed

out by drilling.• Microscopic drill marks left on the barrel’s inner

surface are random and irregular.• The markings are unique to each gun (same or

different manufacturers).

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Gun Barrel Markings• RIFLING: Spiral grooves placed on the inner surface

of a gun barrel.

• LANDS: The sections of the original barrel (bore) that are left between the grooves.

• RIFLING: Spiral grooves provide a more rapid spin on the fired bullet which increases shooting accuracy.

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Gun Barrel Markings• CALIBER: the diameter of the gun barrel, measured

between opposite lands.

• CLASS CHARACTERISTICS:

Once a manufacturer chooses the rifling process, that weapon barrel design will remain consistent, each will have the

same number of lands and grooves and with the same approximate width and direction of twist.

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Striations• STRIATIONS: fine lines impressed into the metal as

the “negatives” of minute imperfections on the rifling cutter’s surface.

• Also produced by minute chips of steel pushed against the barrel’s inner surface by a moving breech cutter.

• Striations form the INDIVIDUAL characteristics of the barrel.

• It is the inner surface of the barrel of a gun that leaves its striation markings on a bullet passing through it.

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Bullet Examination• No two gun barrels, even those manufactured in

succession, will have identical striation markings.• The number of lands and grooves and their direction of

twist are FIRST steps of an comparison between bullet evidence and a known test-fired bullet.

• Any differences in these characteristics immediately eliminate the possibility that both bullets traveled through the same barrel.

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The Comparison Microscope• The comparison microscope is the most important tool

to a firearms examiner. • Two bullets can be observed and compared

simultaneously within the same field of view. • Not only must the lands and grooves of the test and

evidence bullet have identical widths, but the longitudinal striations on each

must coincide.

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Shotguns• Shotguns have a smooth barrel - NO RIFLING.• Shotguns shells are usually made of plastic and contain small lead balls or pellets• Gauge: diameter of the shotgun barrel.• Gauge is inversely proportional to the diameter of the barrel.• Higher the gauge number - the smaller the barrel’s diameter. Choke: collar placed at end of shotgun to control shot dispersion.

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Firing a Weapon• Pulling the trigger releases the firing pin which strikes

the primer and ignites the gun powder.

• Expanding gases generated by the burning gunpowder do TWO things:

• 1) propel the bullet forward through the barrel

• 2) push the spent cartridge case/shell back with an equal force against the breechblock.

• Cartridge/shell will have impressions left by the firing pin and any loading mechanisms.

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Cartridge Case Comparison• The firing pin can leave marks on spent ammunition.• The shape of the firing pin will be impressed into the

primer on the cartridge case.• Ejecting/extracting the cartridge case leaves

impressions on the surface of the case.

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Cartridge Case Comparison• Additional distinctive markings on the spent

case can be caused by the:– Ejector: throws the spent cartridge or fired shell-

case from the firearm.– Extractor: fired case is mechanically removed from

the firing chamber.– Magazine or clip: holds multiple bullets and allows

for faster loading times.

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NIBIN/IBIS• National Integrated Ballistics Information

Network, NIBIN – database files from bullets and cartridge casings retrieved from crime scenes or test fires from retrieved firearms.

• Combines digital photographs from IBIS with similar data from NIBIN.

• NJ has 6 NIBIN stations: One each in Bergen, Camden, Somerset, and Union counties. In Essex County there are two in Newark PD and Essex County Sherriff’s Office.

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Gunpowder Residue• Distance fired is determined by careful

comparison of powder-residue pattern located on the victim’s clothing or skin versus patterns made when the suspected weapon is test-fired at varying distances from a target.

• Comparison between test and evidence patterns may show enough similarity in shape and density to form an opinion about the firing distance.

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Gunshot “LOOK FORS”• 1 inch or LESS: a star-shaped (stellate) tear pattern

around the bullet hole entrance and a rim deposit of lead vapor.

• 12-18 inches: Just the halo (smoke) deposited around a bullet hole .

• 25-36 inches: Scattered specks of unburned and partially burned powder grains without soot/smoke ring.

• 3 feet or greater: no powder residue and a dark ring around the hole, known as a “bullet wipe”.

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Evidence Collection & Testing• All evidence relevant to a shooting must be

collected from the crime scene, surfaces of all items are first examined microscopically for the presence of gunpowder residue.

• Chemical tests: Greiss test detect gunpowder residues that are not visible.

• The firing distances involving shotguns must again be related to test firing.

• The muzzle to target distances can be established by measuring the spread of the discharged shot.

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Primer Residue on Hands• The firing of a weapon not only propels

explosive residues toward the target are also “blown back” toward the shooter.

• Residues are deposited on the firing hand/clothing of the shooter. A positive gunshot residue test can indicate someone has recently fired a weapon.

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Primer Residue on Hands• Examiners measure the amount of

barium and antimony on the suspect’s hands, such as the web of the thumb, back of the hand, and the palm.

• Look at morphology of particles containing these elements to determine whether or not a person has fired, handled a weapon, or was near a discharged firearm.

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Serial Numbers• Increasingly, the criminalist is requested to

restore a serial number when it has been removed or obliterated by grinding, rifling, or punching.

• Restoration of serial numbers is possible through chemical etching because the metal crystals in the stamped zone are placed under a permanent strain that extends a short distance beneath the original numbers.

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Firearm Evidence Collection• Firearms are collected by holding the weapon by the

edge of the trigger guard or by the checkered portions of the grip.

• Before the weapon is sent to the laboratory, all precautions must be taken to prevent accidental discharge of a loaded weapon.

• In most cases, it will be necessary to unload the weapon.

• When a revolver is recovered, the chambers, their positions, and corresponding cartridges must be recorded.

• Firearm evidence must be marked for identification (usually a tag on the trigger guard) and a chain of custody must be established.

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Firearm Evidence Collection• Bullets recovered at the crime scene are scribed with

the investigator’s initials, either on the base or the nose of the bullet.

• The obliteration of striation markings that may be present on the bullet must be scrupulously avoided.

• The investigator must protect the bullet by wrapping it in tissue paper before placing it in a pillbox or an envelope for shipment to the crime laboratory.

• Fired casings must be identified by the investigator’s initials placed near the outside or inside mouth of the shell.

• Discharged shotgun shells are initialed on the paper or plastic tube remaining on the shell or on the metal nearest the mouth of the shell.

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