FORENSICSFirearms, Tool Marks, and Other

Impressions

• The firearm lab is responsible for many functions including:

–The comparison of bullets and their markings.

–Knowledge of the operations of all types of weapons.

–The restoration of obliterated serial numbers on weapons.

–The detection and characterization of gunpowder residues on clothing and around wounds.

–The estimation of muzzle-to-target distances.

–The detection of powder residues on hands.

• BULLET COMPARISONS

1. The inner surface of the barrel of a gun leaves its markings on every bullet passing through it. These markings are unique to each gun. This can be used to connect a gun to a crime.

• 2. Gun barrels are produced from a solid bar of steel that is hollowed out by drilling. The microscopic marks left by the drills are random and irregular making them unique.

• Added to these are other marks left by the manufacturer.

• The inside of the barrel has spiral grooves. This is called rifling. The surfaces of the original bore (the interior of a firearm barrel) remaining between the grooves is called lands.

• As a bullet travels through a barrel the rifling grooves cause the bullet to start spinning. This is because a spinning bullet will not tumble end over end after leaving the barrel but will stay on a straight, accurate path.

• The diameter of a gun barrel measured between opposite lands is known as the caliber of the weapon.

• Caliber is normally recorded in hundredths of inches or millimeters. ( .22, .38, 9mm)

• Ways of making grooves.–Broach cutters; concentric steel rings are passed through the barrel. It is rotated as it passes through the barrel cutting the spiral grooves in the desired direction and rate of twist.

–The button process; a steel plug with the desired number of grooves is forced by very high pressure through the barrel. The button is rotated to give the desired direction and rate of twist.

–The mandrel rifling process; a mandrel is a hardened steel rod with the reverse impression of the rifling desired.

The mandrel is inserted into a slightly oversized bore, and the barrel is compressed with hammering or heavy steel rollers into the mandrels form.

• Different manufacturers use different methods for their specific guns. This will give all the same model guns by the same manufacturer class characteristics. Ex. All Smith and Wesson .32 revolvers have five lands and grooves twisting to the right.

• The Colt .32 revolver has six lands and grooves twisting to the left.

• Remember the difference between class and individual characteristics!

• A lengthwise section of a gun barrel reveals many fine lines called striations running the length of the barrel. These are made by whatever method was used to make the lands and grooves.

• The random distribution and irregularities of these markings are different in every barrel.

• No two rifled barrels, even those manufactured in succession, will have exactly the same striation markings.

• These striations form the individual characteristics of the barrel.

• As the bullet passes through the barrel, its surface is impressed with the rifling markings of the barrel.

• The bullet emerges from the barrel with the impression of the bore’s interior surface. These impressions give the bullet and the barrel both class and individual characteristics.

• There is no practical way to make a direct comparison between markings on a fired bullet and those found within a barrel. A test bullet must be fired from the suspect gun into a recovery tank filled with water or cotton.

• In the early stages of comparison the number of lands and grooves and the direction of twist are observed. If these are different, a bullet or weapon can be eliminated from suspicion. (ex. Colt v. Smith & Wesson.

• Once it has been determined that two bullets carry the same class characteristics, the effort must be made to match the striations marks on both bullets. This can only be done with the use of a comparison microscope.

• The test and evidence bullets are mounted on cylindrical adjustable holders beneath the objective lenses of the microscope, both pointing in the same direction.

• Both bullets are observed simultaneously within the same field of view. The observer rotates one bullet until a well defined land or groove comes into view.

• The other bullet is then rotated until the two can be matched up. The two bullets are simultaneously rotated together to see if there are similar markings around the periphery.

• Unfortunately most of the time it is impossible to match striations around the entire bullets periphery. Bullets can be scarred, mutilated, or distorted upon impact.

• Also the striation marks in the barrel are not permanent and change over time due to wear. However these changes occur slowly. There are no rules as to how many points must be made to confirm a match.

• The final determination must be made by an expert.

• Occasionally a firearm expert will be presented a spent bullet without an accompanying weapon and asked to provide information with regard to caliber and make of the weapon it came from.

• If the bullet has not lost much metal the weight may be used to determine its caliber. Sometimes the number of lands and grooves, the direction of twist, widths of lands and grooves are useful class characteristics.

• These can be used to eliminate certain types and manufactures of guns. Sometimes a bullet will have rifling marks that set it apart from most manufactured weapons. Ex.

• Marlin rifles are rifled by microgrooving and may have 8 to 24 grooves impressed into their barrels.

• The FBI maintains a record known as the General Rifling Characteristics File.

• This file contains a listing of class characteristics, such as lands and groove number and widths for known weapons.

• Unlike rifles, shotguns have a smooth barrel. Projectiles passing through a shotgun barrel will not be impressed with any characteristic markings that can be related back to the weapon.

• Shotguns generally fire small lead balls or pellets contained within a shotgun shell.

• A paper or plastic wad pushes the pellets through the barrel upon ignition of the cartridge’s powder charge.

• By weighing and measuring the diameter of the shot recovered at the crime scene, the examiner can usually determine the size of shot used.

• The size and shape of the recovered wad may also reveal the gauge of the shotgun used and in some instances may indicate the manufacturer of the shell.

• The diameter of the shotgun barrel is expressed by the term gauge. The higher the gauge number, the smaller the barrel’s diameter.

• Example – a 12-gauge shotgun has a bore diameter of 0.730 inches whereas a 16-gauge shotgun has a bore diameter of 0.670 inches.

• The exception is the .410 shotgun which has a bore diameter of 0.410 inches.

• CARTRIDGE CASES

• 1. when the trigger is pulled the weapons firing pin is released. The firing pin strikes the primer which in turn ignites the powder.

• The expanding gases generated by the burning gunpowder propels the bullet forward through the barrel. At the same time it pushes the spent cartridge case or shell back with equal force against the breechlock.

• The breechlock is the rear part of a firearm barrel.

• 2. As the bullet is marked by its passage through the barrel the shell is also impressed with markings by its contact with the metal surfaces of the weapon’s firing and loading mechanisms.

• Just like bullets, the markings on the shells can be reproduced in test-fired cartridges for matching and comparison to suspect shells and weapons.

• 3. The shape of the firing pin will be impressed into the softer metal of the primer on the cartridge case revealing very small distortions of the firing pin.

• These very small imperfections may be distinctive enough to individualize the pin impressions to a single weapon.

• The cartridge may also have distinctive markings made from impacting the breechlock. The breechlock may have striations on its surface that will be specific to its weapon.

• 4. Other distinctive markings that could possibly be found on the shell may be made by contact with the ejector mechanism and the magazine or clip.

• The ejector is the mechanism that throws the cartridge or fired case from the firearm. The extractor is the mechanism by which a cartridge of a fired case is withdrawn from the chamber.

• 5. These markings may also be impressed onto the surface of the brass portions of shells fired by a shotgun. The markings left by the firing pin, ejector, and extractor may also be used to identify the type or make of the weapon that fired the shell.

• 9 mm

• 38 special

• 357

Kahr PM9

Glock 26

• Glock 26

•

Beretta PX4 Storm

Springfield Ultra Compact 1911

AR 15

• 500 magnum

• 6. Automated Firearm Search System – The use of firearms in the commission of a crime has risen significantly in recent years. The likelihood that a specific weapon will be used in multiple crimes has also risen.

• The use of computerized imaging technology has made possible the storage of bullet and cartridge surface characteristics in a manner similar to the storage of fingerprint files.

• Now crime libraries can be networked and share information on bullets and cartridges retrieved from several jurisdictions.

• The FBI and the ATF offered two different and incompatible systems. The FBI system was called DRUGFIRE. The ATF system was called IBIS (Integrated Ballistic Identification System).

• In 1999 the FBI and ATF joined forces to introduce the NIBIN (National Integrated Ballistic Information Network) program.

• The new unified system is called IBIS. The ATF has responsibility for the overall system while the FBI is responsible for communications.

• IBIS serves as a screening tool for firearms evidence. No computerized system can replace the skills of the firearms examiner.

• IBIS will be able to screen hundreds of unsolved firearm cases and may narrow the possibilities to several firearms. However, the ultimate comparison for making a final conclusion will be determined by a forensic examiner using traditional microscopic methods.

• GUNPOWDER RESIDUE

• 1. In incidents involving gunshot wounds it is often necessary to determine the distance from which the weapon was fired.

• Investigators are primarily interested in firing – distance determination. This is important because it can verify or refute a witnesses statement.

• Distance determination is the process of determining the distance between the firearm and the target, usually based on the distribution of powder patterns or the spread of the shot patterns.

• Ex. Suicide–A careful examination of the wounds usually reveals characteristics associated with very close range gunshot wounds. The absence of such characteristics is a strong indication that the wound was NOT self-inflicted.

• 2. Most ammunition is propelled by expanding gases created by burning gunpowder. This powder is never totally burned.

• When a firearm is discharged unburned and partially burned particles of powder in addition to smoke are propelled out of the barrel along with the bullet toward the target.

• If the muzzle of the weapon is close enough to the target these will be deposited onto the target. It is the distribution of gunpowder particles and other discharge residues around the bullet hole that permits an assessment of the distance from which the gun was fire.

• 3. The accuracy of the determination will vary with the scene. If the type weapon can be determined a more accurate estimation can be made.

• 4. The precise distance from which a gun has been fired must be determined by a careful comparison of the powder residue patterns located on the victims clothing or skin …

against test patterns made when the suspect weapon is fired at varying distances from the target. The same weapon and the same type ammunition must be used in the test.

• By comparing the test and evidence patterns the examiner may find enough similarities in the shape and density to base an opinion as to the distance from which the shot was fired.

• GUNSHOT WOUND CHARACTERISTICS

–1. a weapon fired in contact with or less than 1 inch from a target will have a heavy concentration of smoke-like vaporous lead markings around the bullet entrance wound.

Loose fibers surrounding the hole will show scorch marks or signs of melting. There may also be a star-shaped tear pattern around the hole caused by muzzle gases.

• There will be a halo of vaporous lead deposited around a hole in cases of discharge 12 – 18 inches from the target.

• At around 25 inches there will be scattered specks of unburned and partially burned powder without soot. A weapon fired from more than three feet from the target will usually not deposit any powder residue on the targets surface.

• Only a dark ring around the perimeter of the hole called a bullet wipe. The bullet wipe consists of a mixture of carbon, dirt, primer residue, and lead wiped off the bullets surface as it passes into the target.

• When determining distance involving shotguns, close range shots will produce a hole slightly larger than the bore of the gun. As the distance increases the spread of the shot will increase.

• Ex. A 12-gauge spread pattern will increase 1 inch for every yard in distance. There are several factors that can change this including barrel length, size and quantity of pellets, choke, etc.

• A choke is an interior constriction placed at or near the end of a shotgun barrel for the purpose of narrowing the pattern.

• PRIMER RESIDUE on HANDS

• 1. When a gun is fired some of the discharged powder and residue are blown back on the shooters hands and clothing. Usually on the thumb and back of the hand.

• Residue on the palm of the hand indicated a person has handled a gun.

• Determination of whether or not a person has fired a gun is normally made by measuring the amounts of barium and antimony on the hands.

• The most popular technique used is to apply adhesive tape to the surface of the hand to remove any residue. Another method involves using a 5% nitric acid solution to swab the hands and send the swab to the lab.

• SERIAL NUMBER RESTORATION

• Serial numbers are stamped on a metal body or frame or on a plate with steel dies. These dies strike the metal with a force that allows the numbers to be pressed into the metal to a certain depth.

Restoration of serial numbers can be accomplished because the metal crystals in the stamped zone are placed under a permanent strain that extends a short distance beneath the original number.

• When an etching agent is applied the strained area will dissolve at a faster rate than the unaltered metal. The etched pattern will appear in the form of the original number.

• The most common etching agent is a mixture of hydrochloric acid, copper chloride, and water.

• TOOL MARKS

• 1. A tool mark is considered to be any impression, cut, gouge, or abrasion caused by a tool coming in contact with another object.

• 2. Generally these marks occur in the form of indented impressions into a softer surface or abrasion marks caused by the tool cutting or sliding against another object.

• 3. Just as the machined surfaces of a firearm are impressed with random striations during its manufacture, other tools will have microscopic irregularities.

• The shape and pattern of these imperfections are further modified by damage and wear during the life of the tool. This will lead to the unlikelihood that any two tools will be identical. (individual characteristics)

• These markings left by a tool can be compared to marks made by a suspect tool in the lab to connect it to a particular crime.

• 4. Other impressions found at crime scenes include shoe prints, tire, and fabric impressions. The primary consideration in collecting impressions at the crime scene is the preservation of the impression or its reproduction.

• First, photos are taken from several angles with different lighting to get every detail.

• If the impression is on an object that can be transported it will be moved to the lab.

• Otherwise the impression must be lifted in a manner similar to fingerprints.

• PATHFINDER is a device that uses electrostatic charges to pick up dust imprints and deposit them onto a mylar sheet of film.

• When shoe and tire marks are left in soft earth they can be cast. Class 1 dental stone is the preferred medium for making a cast of shoe impressions. Snow Impression Wax is used to cast impressions made in snow.

• Human bite mark impressions on skin and food have proven to be important items of evidence for convicting defendants in a number of homicide and rape cases.

• If a number of points of similarities between test and suspect marks are present a forensic odontologist may conclude that a bite mark was made by one particular individual.

• Tire Information

example - P225/50R15 89H

P – signifies the tire is a P-metric size. Primarily used on passenger vehicles.

• If there isn't a letter preceding the three-digit numeric portion of a tire size, it signifies the tire is a "Metric" size (also called "Euro-metric" because these sizes originated in Europe). Used primarily on European cars, they are also used on vans and sport utility vehicles.

• If a tire size begins with "LT," it signifies the tire is a "Light Truck-metric" size used on vehicles capable of carrying heavy cargo or towing large trailers. Includes medium and heavy-duty (3/4- and 1-ton pickup trucks, sport utility vehicles and full-size vans).

225 indicates the tire is 225 mm across from the widest point of its outer sidewall to the widest point of its inner sidewall.

50 indicates that the tires sidewall height from rim to tread is 50% of its section width. The higher the number, the taller the tire.

R indicates radial construction

• Radial tires are by far the most popular type of tire today representing over 98% of all tires sold.

16 indicates the tire and wheel diameter designed to be matched together.

• Load rating- The higher the number the greater the amount of load it can carry.

Speed RatingsSpeed Rating Max MPH Typical UsageM 81 mph  N 87 mph Spare TiresP 93 mph  Q 99 mph Winter TiresR 106 mpg Truck Tires

S 112 mph Cars and Minivans

T 118 mph Cars and Minivans

U 124 mphSport-Performance Cars

H 130 mphSport-Performance Cars

V 149 mphSport-Performance Cars, High-Performance Cars

W 168 mphUltra High-Performance Cars

Y 186 mphUltra High-Performance Cars