Firearms, Tool Marks, and Other Impressions Chapter 16 Firearms, Tool Marks, and Other Impressions

THE SCIENCE OF FIREARMS IDENTIFICATION INCLUDES: Comparison of bullets As well as: Knowledge of the operation of all types of weapons Restoration of obliterated serial numbers on weapons Detection and characterizations of gunpowder residue on garments and around wounds Estimation of muzzle-to-target distances Detection of gunpowder residues on hands Can not tell which bullet came from which shell prior to discharge

Forensics Firearms & Ballistics Firearms – Most common weapon used for deadly assault & are involved in an overwhelming number of robberies & other types of violent & assault crimes. History starts with the invention of gun powder and then develops devices that use the explosive power (gases produced) to force a projectile out of a tube/container.

What is a ‘firearm’? An assembly consisting of a barrel & a mechanism action that allows a projectile(s) to be propelled forward through the action of a fast combustion reaction. The propulsive force behind a projectile comes from the rapid combustion of gunpowder.

All firearms contain explosive material that is detonated with an enclosed chamber that provides only one direction of escape for pressure build-up.

Ammunition Bullets Shotgun Shells Rifle Ammunition Cartidge: The casing that holds the components of ammunition. Round: Each impact unit of firearm ammunition.

Fixed Ammunition Combines primer, main charge & bullet in a casing.

1 gram of solid carbon produces 85 liters of CO2(g) (If converted completely)

TYPES OF FIREARMS Revolver Rifle Shotgun Pistol

HANDGUNS: Derringer, Pistols, Revolvers Advantages: Small, lighter in weight, quick & easy to bring into action. Disadvantages: Less accurate over long distances, considered a better defensive Weapon.

Single & Multi-shot Handguns *Revolver – ammunition fed into barrel by a rotating cylinder containing individual rounds. *Semi-Automatic: Energy released from one firing will mechanically eject the spent case & reload the next round into the firing chamber. *Fully Automatic: Continuous firing when trigger pulled

Long Guns: Rifles *Refer to the length of the barrel *The rifle is the most well-known The longer barrels allows a weapon to have better accuracy

THE GUN BARREL The gun barrel is produced from a solid bar of steel that has been hollowed out by drilling. The drilling process leaves microscopic marks that give a uniqueness to the barrel. An additional step is required – rifling. Spiral grooves are impressed into the original bore. The raised portions, lands, are what comes in contact with the bullet as it travels through the barrel. *Give the bullet a spin (longer, accurate course. Caliber: Measurement from land  land across the barrel. *Recorded in hundredths of an inch or in millimeters. Example - .22 caliber, 9 mm.

Groove Land

RIFLING PATTERNS Handedness: Refers to the twist of the rifling patterns; Either to the right or left. Twist Rate: Length of barrel necessary for a grove to make one full rotation of the twist. Ex. 10 inches for 1 rotation  1:10 twist 25 inches for 1 rotation  1:25 twist

POLYGONAL RIFLING & FIRING MECHANISM Polygonal Rifling: Does not have lands & grooves, instead hills & valleys. Often hexagonal or octagonal pattern. Barrels are smooth with less striations. Advantages: Fast & inexpensive to manufacture Firing Mechanism: Single-shot, semi-automatic or fully automatic.

SHOTGUNS Type of ammunition: *Shot-many small pellets *Slug- Solid, single projectile Shotguns have smooth barrels, they are not rifled.

GAUGE Refers to the diameter of a shotgun barrel but it IS NOT the actual diameter, as caliber is. *Gauge = number of shot needed to equal 1 lb. As gauge increases, the diameter of the barrel decreases.

Lower gauge  Less shot (Pellets are bigger) Higher gauge  More shot (Pellets are smaller)

SPRAY PATTERN As the distance to the target increases, the pattern increases in size.

AIR GUNS Do not use combustion reactions to propel ammunition – The release of stored gas pressure provides the force. Compressed air, CO2, nitrogen. *Lightweight, easy to manufacture, rapidly fires, quiet, works in wet condition. *Less power, less accurate over long distances.

Caliber The diameter of the bore measured from land to land, usually expressed in hundredths of an inch (.22 cal) or in millimeters (9mm).

BALLISTICS Ballistics - the science of the travel of a projectile in flight. The flight path of a bullet includes: travel down the barrel, path through the air, and path through a target.

INTERNAL, EXTERNAL, TERMINAL BALLISTICS Internal: Path of bullet within the gun itself. *Spin, speed, pressure when fired *Pressure: Rifle  70,000psi, handgun  40,000psi *Barrel: Longer = higher acceleration External: Flight of bullet as it leaves the barrel towards target. *Flight properties determined by energy, bullet shape, mass environmental conditions, gravity *Bullet travels against air; causes friction

INTERNAL, EXTERNAL, TERMINAL BALLISTICS Terminal: When the bullet hits a target *May yaw, tumble, fragment, melt when it hits a target. *Land markings may still be seen.

BULLET TRAVEL Bullets do not typically follow a straight line to the target. Rotational forces are in effect that keep the bullet off a straight axis of flight.

16.2 – FORENSIC IDENTIFICATION OF FIREARMS Forensic Firearms Identification: Determining if a bullet, cartridge case or other ammunition component was fired by a particular firearm. Key pieces of information investigators want to know… *Type of gun that fired the recovered bullet *Likelihood that multiple bullets were fired from one specific weapon

What can be determined from a bullet recovered from a crime scene? -Caliber -Type of ammunition -Chemical composition of bullet -Composition of gunpowder residue -Residue of the barrel Rifling Techniques – how the barrel of a gun in molded/etched/etc.

FORMING RIFLING One modern method uses broach cutters to cut all the grooves simultaneously by forcing cutter down barrel and rotating the cutter. Another modern method uses a “button’ with the desired number of grooves that’s forces down barrel at high pressure. Uses no cutting but compresses metal. Alternative is to use a mandrel, The mandrel with correct (negative) rifling is inserted into an oversized bore and the barrel compressed around the mandrel by rolling or hammering the barrel.

Land Impression Striations Softer metal of the bullet is distorted & shaped to match the lands & grooves of a barrel 380 AUTO caliber FMJ bullets fired from a EAA Corp. pistol. 9mm LUGER caliber FMJ bullets fired from a BROWNING pistol.

The number of lands and grooves and direction of twist are points of comparison (class characteristics) The stria (random imperfections on a bullet) are individual characteristics. The widths of the lands and grooves on a bullet help determine if the bullet could have been fired from a firearm.

Comparison microphotograph What’s compared on bullets/shells? *Striations /stria from lands (on bullet) *Firing pin mark (on casing) *Breechblock mark (on casing)

CLASS & INDIVIDUAL CHARACTERISTICS Class Characteristics of a Barrel (same make & model): *Same number of lands and grooves *Same width and direction of twist Individual Characteristics: *Striation marks that result from the rifling process are negatively imparted onto the bullet. No two rifled barrels, even those manufactured in succession, have identical striation markings. In order to identify a bullet to the gun it was shot from, test bullets must be collected from the firearm. *Recovery box of cotton or Water Tank

Obtaining Bullets From a Suspect Gun If a gun is believed to be connected to a crime, a bullet must be fired from that gun to compare crime scene bullets to. WATER TANK: Bullets are not damaged & can be compared to.

Firing Pin Markings Firing pin impressions – Match; both markings are centered and towards the top. Results from the firing pin striking the rim of the casing. Firing pin marks don’t match – Impressions are in slightly different locations on the casing.

Extractor & Ejector Markings Breech Block Marks Matching breech marks – horizontal striations around firing pin impression are identical. Results from the cartridge case being pushed back against the breechblock in a firearm. Extractor & Ejector Markings Results from metal-to-metal contact as casings are ejected from the firearm.

Bullet and cartridge case identification through comparison microscope Comparing Bullets & Cartridge Cases                                                                                                                          Bullet and cartridge case identification through comparison microscope

GUNSHOT & PRIMER RESIDUES ON THE HANDS Detection of primer residue on hands of shooter *Contains lead, barium, copper, chromium antimony *Collected using adhesive tape *Swabbing both hands with cotton moistened with 5% nitric acid. Swabs are tested for the presence of above elements/compounds. (Very sensitive techniques must be used) Scanning Electron Microscopes (SEM) – Can visualize residue collected from adhesive tape. *Linked to an X-ray analyzer and an elemental analysis of the particles can be conducted. *SEM can detect particle size & shape also.

As residue moves away from the muzzle, it tends to spread out. This could help determine the muzzle-to-target distance.

DISTANCE DETERMINATION HANDGUNS & RIFLES: Spread & density of the residue pattern vary widely between weapons and ammunition. *Test patterns must be made *Without the weapon, the examiner is restricted to looking for recognizable characteristics around bullet hole. Experience with other residue patterns needed. In contact with or less than 1 inch from target: High concentration of smoke-like vaporous lead surrounds entrance hole. *Loose fibers may show scorch marks (melted) *The blowback of muzzle gases may produce a stellate (star-shaped) tear pattern. 12-18 inches or less from target: Halo of vaporous lead Up to 36 inches from target: Scattered gunpowder particles *Ball powder ammunition – Up to 6 to 8 feet

9-inch test standard showing visible particulate residue (lead, burned and unburned gunpowder) and vaporous lead (pink) residues.

Spot tests of chemical products to determine firing distance Primer residue can not determine firing distance Griess Test from the 3-inch test standard.  Orange color is a positive reaction to a presence of a pattern of nitrite residues.   3-inch test standard showing a very concentrated deposit of soot and vaporous lead residues.

Serial number restoration. Obliterated Serial Number Restoration Many items will have identifying serial numbers or markings stamped on the metal surfaces. *Offender tries to remove identifying data so that ownership cannot be traced. *Through use of chemical and physical restoration techniques, the numbers can sometimes be made readable again. (Hydrochloric acid, copper (II) chloride, water)   Serial number restoration.

AUTOMATED FIREARMS SEARCH SYSTEMS Two major federal law enforcement agencies have computerized databases for firearms evidence. *IBIS (Integrated Ballistic Identification System) – Developed for the ATF. Processes digital microscopic images of features found on bullets & casings. *NIBIN (National Integrated Ballistics Information Network) – FBI & ATF joined in 1999 to unify DRUGFIRE and IBIS.

16.3 – Forensic Impression Evidence Arises from imprinting or pressing a patterned or shaped object upon another object to leave behind some sort of image of the original patterned object. Positive Image: When an object leaves behind something that can be seen/visualized. Negative Image: When an object removes material from the surface it contacts.

Impressions provide key types of forensic information… 1) Identification of the objects from class/individual characteristics to establish a link between items 2) How many people/objects were involved 3) Description of the movements of the participants/objects 4) Establish a timeline & sequence of actions 5)Support/refute eyewitness, suspect & victim accounts

Striations Impression More use of a tool results in an increase of individual characteristics. Striations Impression

Collecting Tool Mark Evidence 1)Localize – locate the evidence *When practical, the entire object or the part bearing tool mark should be submitted to the crime lab. 2) Photograph – with scale, use different lighting / angles 3) Documentation – Describe the impression/marking 4) Liquid silicone casting material reproduces the fine details – 3-dimentional impressions 5) Lifting- 2-dimentional images *Do not try to re-fit the tool back into the tool mark. *Suspect tool must be packaged in separate containers (protect the tool from coming in contact with hard surfaces). *Look for evidence adhering to the tool (paint, blood, etc)

Shoe prints & Tire marks SHOE PRINTS: Found in many areas besides the floor  walls, doors,etc. DATABASES: Have information on thousands of patterns, treads, etc. *SICAR *TreadMark Individual characteristics: Wear & damage marks in sole, molding imperfections

Individualize

Set-up boarders around the shoeprint Pour plaster gently over impression Allow to dry

Photograph shoe print Cast shoe print

Tire Marks *Similar to footwear pattern in comparison (class & individual characteristics) *Tread design MAY tell you manufacturer, model, sometimes year *Individual characteristics – wear & damage marks *Pattern may change over time with use

Human bite marks

Ted Bundy Bite Mark