Presentation on theme: "How do forensic investigators analyze ballistic evidence?"— Presentation transcript:
1 How do forensic investigators analyze ballistic evidence?
2 Major Differences in Rifles, Shotguns & Handguns shotgun barrel long made of fairly thin steel very smooth on the inside to allow the shot and wad to glide down barrel without friction thinner than rifle, since it does not have to withstand as much pressure.handgun barrel shorter designed to be shot while beingheld with one or two hands, rather thanbeing placed against the shooter’sshoulder. bores of most handgun barrels have agrooved patternrifle barrel: long thick walls with spiraling grooves cutinto the bore grooved pattern is called riflingWhy is this important to aforensic investigator?
4 Understanding rifling (of the barrel) How do forensic firearm examiners match bullet to gun type or a particular gun?Understanding rifling (of the barrel)• Rifling = spiral grooves etched in the barrel to promote spinning• A spinning bullet is a more accurate bullet.• Cutting the grooves leaves LANDS, or high parts, between the grooves• The GROOVES grab the bullet as it travels down the barrel,causing it to spin.• The grooves are TWISTED, clockwise or counterclockwise, to enhance spinning.Old smoothbore riflesweren’t accurate beyond100 X. Today’s rifles canbe accurate forthousands of yards.
6 Determining the rifling pattern in the barrel: counting the number of groove or land impressionsaround the circumference of the bullet holding the nose of the bullet pointing away from you, the direction the impressions run away from you (either to your left or right) determines the direction of twist.Deringer pistol used by John Wilkes Boothin the assassination of Abraham Lincoln.FBI examiners determined that the barrel of the pistol was rifled with seven grooves in a counterclockwise direcJon (left twist). Land & groove impression measured of .100 and .085 inches.
7 The lands and grooves on a bullet are measured in thousandths of an inch or in millimeters. The image below shows the micrometer positioned next to a land impression on a bullet.One way to measure individual rifling impressions is to use a micrometer.Were thesebullets shot fromthe same rifle?BA
8 INDIVIDUALIZED Evidence: FBI database: General Rifling Characteristics File - Lists the land, groove and twist characteristics of known weapons.CLASS Evidence:FBI DRUGFIRE program: the development and deployment of an automated firearms identification system to support serial, gang, and drug-related shooting investigationsIBIS: Integrated BulletIdentification System of the ATF1997, the National Integrated Ballistics Information Network (NIBIN) board was created to develop a national imaging system* bullet & shell casings can be compared with the known database.INDIVIDUALIZED Evidence: Rifling tools cut through each barrel differently" Barrel has minute, distinct characteristics" Cutting & etching equipment becomes worn anddamaged" Repeated firing wears down and damages grooves &lands" Bullets are nicked & scratched as they “ pass throughthe barrel = STRIATIONS
9 Handling & Analyzing Gunshot Residue What is gunshot residue = GSR?“Chemical and particulate components of gases released when gun is shotResidue clings to shooter’s hand, arm, clothing, face, and hairResidue settles on nearby furniture and wallsResidue may reach victim’s skin and clothing in close-range gunshotsResidue from exploding primer = Primer ResidueGun Powder Residue = from when gun is shot Burning gunpowder Smoke Unburned particulates
10 Determining the distance between firearm and target What characteristics will affect the observable residuepatterns around a bullet hole?✩ Without the weapon:★relaJve distances determined★analyze characterisJcs around bullet hole✩ With suspected weapon:★more exact determination★comparison of test shot with actual shotComparison -Use similar materialUse suspect weapon & ammuniJonFire shot at varying intervalsCompare residue patterns Barrel length & diameter Angle of shot Caliber Type of ammunition Condition of the weapon Weather conditions The target*
11 Typical Pattern of Residue o 1 inchheavy concentration of smoke‐like vaporous lead surroundsbullet hole. Clothing/skin will show scorch marks from flamedischarge of weapon.o 12‐18 inchesa halo of vaporous lead (smoke) deposited around bullet holeo 25‐36 inchesscattered specks of unburned and partially burned powdergrains can be foundo More than 3 feetwill not deposit any residue on target’s surface. Only visualindicator is a dark ring around the bullet hole = BULLET WIPE!
12 How do we detect gunshot residue?" Analyzing residue! Griess test is used to reveal GSR: sheet of treated (photographic) paper is pressed over area then, immersed in a reagent which reacts with inorganic nitrate particles(gunpowder) Chemically treat paper pattern is revealedLead Residue test is used to reveal GSR: spray area with a solution sodium rhodizonate, followed by a series ofoverspray causes lead particles to exhibit a pink color followed by a blue violetcolor
13 Examples of comparison residue patterns at different distances 3 inches9 inches15 inches
14 How do we detect gunshot residue? Griess test is used to reveal GSR: sheet of treated (photographic) paper is pressed over area then, immersed in a reagent which reacts with inorganic nitrate particles (gunpowder) Chemically treat paper pattern is revealedLead Residue test is used to reveal GSR: spray area with a solution sodium rhodizonate, followed by a series of overspray causes lead particles to exhibit a pink color followed by a blue violet color
15 Be careful forensic team…GSR Could land on someone near gun when it discharges Transfer by handling gun after it was shot Fades away rapidly Dissipates after two hours Can be wiped or washed awayPrimer residue containsblends of metalsLevels can determinewhether a person hasfired a gun recentlySoot on hand of suicide victim
16 Diphenylamine TestMoist swab used on suspect’s hands, arms, clothing Treated with diphenylamine, indicates metals*false positive for tobacco, cosmetics, urineIndicator testChemists have developed a reliable newtest for detecting the presence of gun shot residue.a residue particle --roughly 1/20 the size of a period—That has been magnified 200 timeswith a digital microscope.
17 Crime Scene Reconstruction Trajectory• Shooting distance• Position and location of the victim• Position and location of the offender• Sequence of shots• Direction of shots• Possibility that the wound(s) could have beenself inflicted• Identification of weapon used may link serial cases
19 Contact Range Gunshot Wounds The abrasion ring, and a veryclear muzzle imprint, areseen in this contact rangegunshot wound.An abrasion ring, forms when the force of the gases enteringbelow the skin blow the skin surface back against the muzzle of the gun,Since the barrel contacts theskin, the gases released by thefired round go into thesubcutaneous tissue and causethe star‐shaped laceration.Note also the grey‐blackdiscoloration from the soot, aswell as the faint abrasion ring.
20 Intermediate range gunshot wounds Powder tattooing is seen inthis intermediate rangegunshot wound. The actualentrance site is somewhatirregular, because the bulletcan tumble in flight.
21 Entrance and Exit Wounds Displayed here is an entrance at theleX and an exit at the right. ThisparJcular bullet struck at an angleto produce the ovoid entrance. Exitwounds vary considerably in sizeand shape because the bullet canbe deformed in its transit throughthe body. There may be no exitwound at all if the bullet's energy isabsorbed by the Jssues. Somebullets (such a a "hollowpoint") aredesigned to deform so that all theirenergy will be converted to Jssuedamage and not exit.Here is a slit‐like exitwound. Note that there isno powder or soot visible.