Firearms, Tool Marks, and Impressions

Chapter 15

Firearms Identification

Determining whether a bullet or cartridge was fired by a particular weapon

Includes Bullet comparisons Restoring damaged

serial numbers Detection and analysis of

gunpowder residue Estimation of muzzle to

target distance Detection of gun powder

residues on hands

Gun Barrel Anatomy Gun barrel inner surface leaves

specific markings of bullet shell; very important to gun identification

During production, gun barrels are hollowed by drilling which leaves striations on the inner barrel

Bore—inner barrel

Rifling—spiral grooves formed in the bore

Lands—raised portion between the grooves in the bore

Caliber—diameter of bore of a rifled firearm; i.e. .22 inch caliber or 9 mm caliber

Bullet Markings

As bullets are fired through barrel, the markings on the barrel leave specific markings on the bullet

Test firings are done with suspect weapons to see if markings on new test bullets match crime scene markings; test firings are done box of cotton or water to make recovery easier

Comparison microscope is used to match bullet markings

Bullet Analysis Considerations

Often much of bullet markings are damaged at impact; only piece of rifle bore patterns are intact on damaged bullets

Barrel striations can be worn down with many firings

Recovered bullet markings can be used to determine brand and caliber of unknown weapon type

General Rifling Characteristics File

Some weapons have microgrooving—8 to 24 grooves in rifle barrel

FBI maintains a record of Rifling characteristics such as land and groove width dimensions

Shot Guns Shot guns have smooth

barrels; not rifled barrels

Not impressed with any specific markings

Fire small lead balls or pellets

By measuring diameter and weight of shot recovered at crime scene, size of shot used in shell and gauge of shot gun can be determined

Gauge—size designation of a shot gun

Cartridge Cases

At firing, shell or cartridge case pushed hard against breechblock—rear part of the barrel

Shell is marked by contact with metal surface of breechblock

Shape of firing pin may leave markings; can be individual character

http://www.youtube.com/watch?v=sBKJZTkbypo

Automated Firearms Search Systems

With the great increase in automatic weapon use, weapons are often used in multiple crimes

Bullet and cartridge surface characteristics of weapons from one crime can be stored and accessed to compare to shells or bullets recovered at another crime scene

National Integrated Ballistics Information Network

Ballistic Fingerprinting

Information is being stored from not only crime scene weapons, but from all weapons fired at test samples

Samples can be taken from handguns before they are sold so that if they are used in a crime, data would be available for ballistic matching

Gunpowder Residues

When firing, gunpowder is converted to gas.

However, powder is never fully consumed

Partially burned powders are propelled toward the target

The distribution of gunpowder residue around the bullet hole permits distance determination

Distance Determination

Very important in possible suicides to have gun fired very close to body

Also, always measured in self-defense claims to see if distance is viable for self-defense

Is especially helpful when weapon isn’t recovered to have distance information from fired weapon

Compare victim’s patterns to sample patterns made during test firing at white fabric or fabric like victim’s clothing

If no gun is recovered for test firing, patterns are analyzed

Close range firing leaves burnt fibers around bullet hole

Close-range shots may have a star-shaped tear pattern called a stellate

http://www.youtube.com/watch?v=AVVEz8hpmos

http://www.youtube.com/watch?v=2kEh3Kgwhk0

Powder Residues on Garments

All clothing must be analyzed for gunpowder residue

Can be detected visually by characteristic color, size, and shape

Sometimes gun powders are same color as clothing and hard to detect

Sometimes blood covers up gun powder; so is not as visible

Infrared photograph will enhance the contrast between the powders and garments

Greiss test-tests for nitrites which are often present in residue; transfer residue to chemically-treated gelatin coated photographic paper; hot iron is used to press paper onto target; chemical treatments make nitrites visible

Sodium rhodizonate is sprayed over surface to test for presence of lead; lead particles turn pink and then blue violet

Primer Residues on Hands

During firing, gunpowder and primer residues are blown back toward the shooter as well

Traces of powders show up on the shooting hand; So, can be figured out if the hand recently fired a firearm

Dermal nitrate test—hot parafilm to suspect’s hands with paintbrush; Dry wax was removed and tested with diphenylamine; bue color indicates positive reaction for nitrates

Urine, fertilizers, tobacco, and make up can all give test positive for nitrate presence

More current tests measure for the presence of primer residue on the hands that fired a gun

Testing for Primer Residues

Measure the presence of barium and antimony on the hands of gun suspects

One test applies adhesive tape to the hands to remove any residue; SEM is used to view particles; best test for primer residue

Another test swabs both hands with 5% nitric acid to collect any primer residues

Presence of barium or antimony shows recent firing or handling of gun

Primer residues can be removed by washing hands; has to be done less than two hours after firing

Neutron activation analysis can be done to test for barium and antimony

Flameless atomic absorption spectrophotometry can be done as well

http://www.youtube.com/watch?v=WkcioQGfAgU

Serial Number Restoration

Criminalist must try to restore number when it has been removed or obliterated

Stamped on metal body or frame with hard steel dies; hard force allows dies to sink into metal at prescribed depth

Serial numbers can be restored because imprint puts strain into metal deeper than the superficial letter imprints

Surface is cleaned and treated with etching agent; Agent dissolves faster in altered area than unaltered area leaving imprint of number

Collection and Preservation of

Firearms Do not pick up in a way

that disrupts gunpowder residue

Unload bullets from weapon

When weapons are found, positions and angles of barrels and chambers must be noted

Bullets that are collected must be handled gentler to avoid damaging scratches on surface that are vital for analysis

Tool Marks An impression, cut, gouge, or

abrasion caused by a tool coming in contact with another object

Most often found at burglary scenes where there is some forced entry

Often found on frame of door of window in wood from screwdriver or crowbar

Examination of marks can tell size and shape of tool; yield class characteristics

Only get individual characteristics if tool has some wear mark or nick that might help discern identity

Reference Samples

Duplicating tool mark impressions is difficult

Reference marks are made in soft metal surface like lead

Collection of Tool Mark Samples

Entire object with marks is collected as evidence (window jam, door jam, car door, etc.)

If too large to be brought back to lab, marks are photographed

Cast marks are made with silicone

http://www.youtube.com/watch?v=ZD-KSGDFX_g

Cannot try to fit suspect tools into crime scene marks because it may damage crime scene tool impressions

Other Impressions

Shoe, Fabric, tire impressions are often left at crime scenes

Impressions must be photographed

Recoverable items are taken back to crime lab

http://www.youtube.com/watch?v=KEyRE6KYbIs

Lifting Impressions

When impressioned items can’t be taken back to crime lab

Made in light deposits of dust or dirt

Lifting material is placed on top of entire impression and lifted

Electrostatic lifting—mylar film is placed on surface and high voltage electrode is used to transfer pattern to film

Casting Impressions

Photography first

Then, casting of shoe and tire marks in soil at crime scene

Class I dental stone, gypsum, is used for impression castes

Snow impression wax can be used for lifting impressions in the snow

Latent blood tests can be used to highlight foot impressions in blood

Comparing Impressions

Style, manufacture of shoes or tires, size of shoe can be determined from castings

Can be matched to reference samples

Shoeprint Image Capture and Retrieval—database in England of shoe print information

Bite mark impressions are very important pieces of evidence at crime scene—individual characters

http://www.youtube.com/watch?v=thfHB5e51q8&feature=fvw