Firearms, Tool Marks, and Impressions Chapter 15
Mar 31, 2015
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