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Firearms, Took Marks & Other Impressions

Learn about the history of firearms, the identification of weapons based on marks and impressions, and the role of forensic firearms experts in criminal investigations. Explore topics such as bullet comparisons, caliber, individual characteristics, and gunshot residue analysis.

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Firearms, Took Marks & Other Impressions

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  1. Firearms, Took Marks & Other Impressions Chapter 15 Mr. Carl

  2. Firearms: A Quick History • Almost every gun is based on the same simple concept: You apply explosive pressure behind a projectile to launch it down a barrel. • The earliest & simplest application of this idea is the cannon.

  3. The 1st handheld guns were essentially mini-cannons; you loaded some gunpowder & a steel ball & lit a fuse.

  4. War typically resulted in the need for improved weapons technology. • In the late 1800’s, the revolver quickly became popular due to it’s size & quick loading. • It only had to be reloaded every 5-6 shots instead of after each shot.

  5. Handguns reigned supreme for the past 200 years & to this day, remain the most popular & readily available firearm.

  6. Firearms Identification • Often confused with the term ballistics • Ballistics is the study of a projectile in motion. • Inside the firearm • After it leaves the firearm • When it impacts the target

  7. Firearms Identification Identification of Firearms is based upon this basic idea: A harder object marks a softer one & imparts/transfers its microscopic irregularities to that object.

  8. Forensic Firearms Expert • Did a suspect use this gun to kill that person? • Did these bullets come from that gun? • Was it really self-defense? • Is this a case of suicide, or is foul play involved?

  9. Forensic Firearms Expert • Bullet Comparison • Weapons Function • Is it safe? Has it been modified? • Serial Number Restoration • Gunpowder Residue Detection • on clothes, hands, & wounds • Muzzle-to-Target Distances

  10. Pulling the Trigger • Pulling the trigger releases the firing pin… • The firing pin strikes the primer… • The primer ignites the gun powder… • The powder generates gas that propels the bullet forward through the barrel & ejects the spent cartridge case.

  11. Caliber: the diameter of the gun barrel. Caliber is recorded in hundredths of an inch (.22 & .38) millimeters (9mm)

  12. Bullet Anatomy

  13. Anatomy of a Bullet

  14. Bullet Comparisons • Each gun leaves distinct markings on a bullet passing through it. • A gun barrel is made from a solid bar of steel that has been drilled/hallowed out. • The drill leaves microscopic marks on the barrel’s inner surface.

  15. Bullet Comparisons • Gun manufacturers also add spiral grooves to the barrel. This is known as rifling. • Lands: the space between the grooves. • As a spinning bullet passes through the barrel, it is marked by these grooves.

  16. Class Characteristics • Class Characteristics: Once a manufacturer chooses a rifling process, for a particular class of weapon, they keep it consistent. • Lands & Grooves are the same for a model. • .32 caliber Smith & Wesson has 5 lands & grooves twisting to the right. • .32 caliber Colt has 6 lands & grooves twisting to the left. • Class characteristics can eliminate certain makes but are not enough to ID a particular gun.

  17. Individual Characteristics • Imperfections in the manufacturing process make each barrel unique. • Rifled barrels, even if made in succession will NOT have identical striation (scratch-like marks).

  18. Bullet Comparisons • To match bullets to a gun, test bullets must be fired through a suspect barrel for comparison. • Goddard & ComparisonMicroscopes • Examined bullets side-by-side (to match striated markings).

  19. Bullet Comparisons

  20. Cartridge Markings All moving components contact the cartridge rather than the bullet can leave useful impressions on shell cartridges • Breechface marks • Firing pin impressions • Chamber marks • Extractor & Ejector markings

  21. Breechface Marks

  22. Actual Breech Marks

  23. Firing Pin Marks

  24. Chamber Marks

  25. Ejector Marks

  26. Other Factors • Perfect matches sometimes difficult b/c: • Presence of grit & rust in a barrel • Recovered bullets too mutilated or distorted on impact • A spent bullet’s weight can sometimes determine the gun make. • Microgrooves: 8-24 grooves; it’s not as common • General Rifling Characteristics File • FBI database of known land/groove width for all weapons.

  27. Smooth barrel Projectile NOT marked as it passes through Fire small lead balls or pellets contained within a shell. Characterized by: diameter of the shot size & shape of the wad Gauge: diameter of the barrel ( gauge  diameter) Shotguns

  28. Gunshot Residue (GSR) • GSR Sources: • victim, clothing or target • shooter’s hands • Gunpowder Chemistry • Major detectable elements are: lead (Pb), barium (Ba) & antimony (Sb) • Virtually all cartridge cases are made of brass (copper & zinc); also detectable.

  29. Griess Test • Tests for the presence of nitrates (partially burned or unburned gunpowder) • Swab of shooter’s hand • Must produce a pattern for a distance determination

  30. Results of GSR Hand Test • Negative results may be caused by: • Washing the hands • Shooter may have been wearing gloves • Lead free ammunition • A rifle or shotgun may not deposit GSR on hands

  31. GSR on the hand of a suicide victim, proving he was holding the weapon when it was fired.

  32. With a contact or very close range gunshot wound, it is possible to have blood spatter as well as GSR on the hand of the person firing the weapon.

  33. Trajectory

  34. DC Sniper

  35. SERIAL NUMBER RESTORATION • When a serial number is stamped into a gun, the metal underneath the number is compressed & hardened. • If the number is filed-off, the hardened area may still be present. • By using an acid solution the metal can be slowly eaten away. • In this process the softer metal will be eaten away first and the number may reappear.

  36. Before & After • Historically, serial numbers are successfully restored 63% of the time on steel & 54% of the time for aluminum or zinc.

  37. FIREARMS EVIDENCE COLLECTION • Make sure it is unloaded!!!!! • DO NOT put a pencil into a barrel • REVOLVERS • Indicate location of fired & unfired ammunition • AUTOMATICS • Check magazine for number of rounds • Fingerprint magazine • Place ID tag on trigger guard

  38. FIREARMS EVIDENCE COLLECTION • AMMUNITION • Write on base or nose • Package in pill box or envelope • Wrap in tissue to protect

  39. FIREARMS EVIDENCE COLLECTION • CLOTHING • Protect & preserve any residue • Air dry if wet • Package separately in paper bags • Establish CHAIN OF CUSTODY

  40. TOOL MARK IMPRESSIONS • Impressions • Cuts • Gouges • Abrasions

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