Firearms, Tool Marks, and Impressions

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 • http://www.youtube.com/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 • http://www.youtube.com/watch?v=AVVEz8hpmos • http://www.youtube.com/watch?v=2kEh3Kgwhk0

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

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 • 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

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