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Firearms, Tool Marks, and Impressions

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1 Firearms, Tool Marks, and Impressions
Chapter 15

2 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

3 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

4 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

5 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

6 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

7 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

8 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 watch?v=sBKJZTkbypo

9 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

10 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

11 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

12 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

13 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

14 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

15 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

16 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

17 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

18 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

19 Reference Samples Duplicating tool mark impressions is difficult
Reference marks are made in soft metal surface like lead

20 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 ?v=ZD-KSGDFX_g Cannot try to fit suspect tools into crime scene marks because it may damage crime scene tool impressions

21 Other Impressions Shoe, Fabric, tire impressions are often left at crime scenes Impressions must be photographed Recoverable items are taken back to crime lab watch?v=KEyRE6KYbIs

22 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

23 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

24 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 ?v=thfHB5e51q8&feature=fvw

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