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Ch 15 – Firearms, Tool marks, and Other Impressions Techniques for rifling a barrel. Class and characteristics of bullets and cartridges. Comparison microscope.

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Presentation on theme: "Ch 15 – Firearms, Tool marks, and Other Impressions Techniques for rifling a barrel. Class and characteristics of bullets and cartridges. Comparison microscope."— Presentation transcript:

1 Ch 15 – Firearms, Tool marks, and Other Impressions Techniques for rifling a barrel. Class and characteristics of bullets and cartridges. Comparison microscope for bullets and cartridges. Caliber Vs. gauge. Distance of a target from a fired weapon. Lab tests for determining an individual has fired a weapon – limitations of present techniques.

2 Restoring an obliterated serial number. Proper collection and preservation of firearm evidence. Comparing suspects tool to a tool mark. Significance of class and individual characteristics to the comparison of impressions. Common field reagents for enhancing bloody footprints. TUTORIAL/GUNS/GUNINTRO.html

3 Firearms: a discipline mainly concerned with determining whether a bullet or cartridge was fired by a particular weapon. Not to be confused with ballistics, which is the study of a projectile in motion Grooves: the cut or low-lying portions between the lands in a rifled Rifling: the spiral grooves that are formed in the bore of a firearm barrel to impact a spin to the projectile when it is fired

4 Bore: the interior of a firearm Lands: the raised portion between the grooves in a rifled bore Caliber: the diameter of the bore of a rifled firearm. The caliber is usually expressed in hundredths of an inch or millimeters, e.g..22 caliber and 9 mm

5 Gauge: size designation of a shotgun, originally the number of lead balls with the same diameter as the barrel that would make a pound. For example, a 12-gauge shotgun would have a bore diameter of a lead ball. 1/12 pound in weight. The only exception is the.410 shotgun, in which bore size is 0.41 inch Breechblock: the rear part of a firearm barrel

6 Ejector: the mechanisms in a firearm that throws they cartridge or fired case from the firearm Extractor: the mechanism in a firearm by which a cartridge of a fixed case in withdrawn from the chamber Distance determination: the process of determining the distance between the firearm and a target usually based on the distribution of powder patte3rn or the spread of a shot pattern

7 Greiss Test: a chemical test to develop patterns of gunpowder residues around bullet holes Choke: an interior constriction placed at or near the muzzle end of a shotguns barrel for the purpose of controlling shot dispersion

8 Ch15 - Impression Evidence Tool Marks

9 Impression Evidence Any mark made in a softer surface by a harder implement or tool –tracks in mud –pry mark in a door frame –fingerprint in wax –bite mark –markings found on bullets & cartridge cases

10 Imprints Tire tracks & shoe prints most common kinds of imprint evidence found in a crime lab Mass production of tire treads & shoe soles produces only class evidence –tread pattern –trademark logos Individualizing potential comes from random characteristics acquired by use & wear

11 Toolmarks Any implement can be a tool for the purpose of leaving a mark –crowbars –tire irons –screwdrivers –metal pipes –heavy flashlights –fingernails

12 Types of Impressions Impressions made by striking, pressing or imprinting –leave a single point impression impressions made by cutting, sawing, dragging or prying –leave characteristic striations indicative of movement across a surface

13 Striation Patterns Pattern of striations produced by the edges of a tool can be traced back to microscopic irregularities in the sharp edges or surfaces intended for cutting

14 Striation Patterns Mass machine manufacturing of tools causes them to be very similar when they leave the factory –the greater the wear patterns the higher the potential for individualization

15 Striation Patterns Tool marks are often compared with another mark made by the suspect tool rather than the tool itself –suspect tools are never fitted into the tool mark –generation of the reference mark becomes a potential source of error different marks may be produced depending on the angle & amount of pressure used the substrate used may alter the appearance of the tool mark

16 Striation Patterns A reference mark made by a tool dragged at various angles

17 Tool Mark Examination Magnification is usually needed to reliably compare the minutiae of marks & striations Comparison Microscope –Allows two samples side by side in the same microscopic field

18 Gross Bullet Comparison Photo shows 4 different bullets Test (reference) & evd (evidence) bullets have been aligned so that their land impressions match up at the dividing line. A greater magnification would be used for striae comparison

19 Tool Mark Comparison Examples Comparison of two marks made by an axe blade.

20 Tool Mark Comparison Examples Comparison of a reference mark (left) to an evidence prymark left on a cash register. With larger tools and especially if the mark is on an edge, a comparison scope may not be necessary.

21 Tool Mark Comparison Examples Matching evidence (L) and reference (R) prymarks

22 Impression & Print Testimony Essentially opinion testimony –rests heavily on experience & expertise of the examiner

23 Firearms A Brief Overview

24 Types of Firearms Firearms can differ in many components –the way bullets are loaded & chambered –the cocking mechanism –the firing action –the ejection of used cartridges A main distinction is which components are automated & which are manual

25 Types of Firearms May be classified by their state of automation –single-shot (manual) –semiautomatic –automatic May also be classified by aspects of their physical design –Rifled –Smoothbore

26 Single-shot (Manual) Few weapons are operated completely manually –a single bullet being loaded by hand after each shot Lever-action or bolt-action weapons (rifles) –require the manual activation of a lever or turnbolt to extract & eject expended cartridges cock firing mechanism load a live round into the firing chamber

27 Semiautomatic (Self-loading) Use the recoil energy from a fired bullet to –extract & eject expended cartridge casing –to load a new round into the firing chamber –The trigger must be activated manually for each shot fired Two Types (usually revolver type pistols) –Single-action must be manually cocked before firing –Double-action cocked by the pull of the trigger

28 Automatic Fully automatic weapons will continue to fire & expend casings as long as the trigger is depressed and ammunition available

29 Rifled Firearms Contain a set of spiraling lands (raised ridges) & grooves (complementary depressions) –give a bullet rotational motion which helps it fly straighter & farther Types of rifled firearms –Pistols (handguns) –Rifles –Machine & submachine guns

30 Pistols (Handguns) Designed to be fired with one hand –originally designed for cavalrymen who could only spare one hand from the reins of the horse There are some single-shot pistols used for specialized target shooting Most pistols have revolving cylinders (revolvers) –can fire more than one shot before reloading –many are semiautomatic

31 Rifles Designed to be fired with two hands, usually from the shoulder There are some small bore sporting rifles that are single shot Most are semiautomatic or automatic –redirect the energy from either the bullet recoil or hot propellant gases to extract & eject the cartridge casing, then cock & load a new bullet –Assault rifles may be used either in semi-automatic or automatic mode

32 Machine & Submachine Guns Machine guns have a heavy recoil –designed to be fired from some type of sturdy mounting Submachine guns are designed to be fired while being held in the hands

33 Smoothbore Firearms Shotguns are the only smoothbore small arm currently in use Fire multiple small projectiles (pellets or shot) so accurate aiming not an issue –barrels smooth rather than rifled –barrels shaped or indented to control the spread of the shot as it exits the muzzle choke

34 Shotguns May be single or double-barreled –allows the two muzzles to be set for delivery of the ammunition at different distances or spread patterns Repeating shotguns may be reloaded –manually with a pump action or bolt action –semiautomatically using the same mechanisms as a semiautomatic rifle

35 A Colt.45

36 The Rifled Barrel Has a series of spiral cut-outs that run the length of the barrel –depressions grooves –raised portions lands

37 Rifled Barrel The # of lands & grooves; their direction & rate of twist are characteristic of a particular product from a specific manufacturer –class characteristics Examples –.32 caliber Smith & Wesson revolvers 5 lands & grooves twisting to the right –.32 caliber Colt 6 lands & grooves twisting to the left

38 Barrel Manufacture Before 1940, grooves cut one at a time by drawing a scraper repeatedly down the inside of the barrel as it was rotated Three modern methods of rifling –broaching –button rifling –hammer forging

39 Broaching The diameter of the hole reamed in the steel will be the final diameter (caliber) of the barrel Grooves cut with a tool called a gang broach –consists of a series of circular cutting tools with projecting teeth positioned a regular intervals on a rod

40 Broaching The teeth cut away the metal in the grooves as the broach is forced through the barrel by hydraulic pressure Successive cutting disks increase in diameter until the desired groove depth is reached

41 Button Rifling (Swagging) The diameter of the original hole is drilled smaller than the final desired caliber Rifling button tool bears an exact negative impression of the lands & grooves needed

42 Button Rifling (Swagging) Tool is forced through the reamed hole simultaneously rifling & expanding the barrel No actual cutting of metals involved.22 caliber rifles are commonly made this way

43 Hammer Forging The hole is drilled slightly larger than the final desired caliber Barrel is hammered of a specially shaped mandrel to produce the rifling characteristics The mandrel is forced out the end of the barrel

44 Bore Diameter All firearms may be characterized by their bore diameter or caliber –the measure of the diameter in hundredths of an inch or in millimeters American & British weapons are normally given in hundredths of an inch (.22;.45) other countries are normally in mm (9 mm) The larger the number, the larger the diameter –.22 is smaller than.45

45 Bore Diameter of Rifled Barrels Measured from the tops of opposing lands when there is an even number of lands & grooves measured from a circle tangent to the tops of the lands if there is an odd number of lands & grooves

46 Shotguns Typically characterized by their gauge Gauge is a term which comes from the days when all firearms fired spherical lead balls –gauges were expressed as the number of appropriately-sized bullets that could be made from one pound of lead 10-gauge shotgun –10 lead balls of about the same diameter as the barrel could be made from a pound of lead

47 Shotgun Gauge The higher the gauge number, the smaller the diameter of the barrel –12-gauge shotgun has a bore diameter of inches –16-gauge shotgun has a bore diameter of inches Exception –.410-gauge shotgun has a barrel inches in diameter

48 Moving Components Some components of a firearms firing & cartridge ejection mechanism are of importance to the forensic examiner –the breechblock –the firing pin –the extractor –the ejector

49 Breechblock Supports base of the cartridge in the chamber Often finished by hand filing –characteristic striations on this part contact the back of the cartridge leaving individualizing markings on the cartridge base

50 Firing Pin (Primer Pin) Strikes the cartridge primer to initiate the firing process Finished on a lathe or filing by hand Can transfer striations to the soft metal of the primer cap

51 Extractor & Ejector Extractor –extracts a spent cartridge from the chamber Ejector –ejects the extracted cartridge from the side of the weapon body Metal parts of these mechanisms can leave individualizing markings on cartridge cases

52 Moving Components All of the moving components contact the cartridge rather than the bullet –can leave useful impressions on shotgun shell cartridges

53 Ammunition Construction

54 Bullet Morphology

55 Bullets When black powder was used as a propellant –projectiles had relatively low velocities –bullets were usually almost pure lead With smokeless powders –burn rate & heat production much greater –pure lead bullets foul rifling

56 Common Types of Bullets For Rifled Firearms Lead Alloy –lead hardened by addition of antimony Semijacketed –have a copper-alloy or aluminum jacket that covers part of the bullet surface usually nose is left exposed –nose configurations encourage penetration & expansion on impact »soft-point, hollow-point, bronze point, etc

57 Common Types of Bullets For Rifled Firearms Full jacketed –consist of a lead core surrounded by a jacket of copper-nickel alloy or mild steel –usually core is left exposed at the base of the bullet –Semiautomatic pistols use full jacketed bullets noses of bullets must slide up a ramp when rounds chambered

58 Bullet Shapes Influence aerodynamic properties –round 0r pointed noses reduce air resistance increases striking power –some have boattailed bases act as a rudder to reduce turbulance & decrease drag

59 Cannelures An important aspect of bullet morphology is the presence or absence or cannelures –circumferential grooved rolled into the surface of a bullet can contain lubricant can serve as seats for the crimped mouths of cartridge cases

60 Primers Since gunpowder is relatively stable, it I ignited via a more flammable or explosive primer Types of systems –rimfire cartridge primer placed in the rolled rim of the cartridge casing –centerfire cartridge

61 Boxer Primer System Consists of a metal cup which holds primer. A small anvil rests on top of primer material Firing pin detonates the explosive between cup & anvil Flame ignites propellent through a single flash hole

62 Berdan Primer System Incorporates a conical anvil as part of the base Primer cap is a small metal up containing a pellet of primer compound 2 or 3 small flash holes communicate the flame from primer

63 Cartridge Cases Cutaway photos of three kinds of cartridge cases

64 Cartridges Cases Made from brass –may be nickel-plated to prevent corrosion Come in a variety of different shapes –Revolvers fire straight rimmed cartridges –Self-loading pistols fire straight rimless cartridges Can have cannelures rolled into them to prevent the bullet from being pushed back into the case

65 Cartridge Cases Bullets may be held in place by criming or staking the mouth of the cartridge case into a cannelure on the bullet May have nformation stamped on the head of the cartridge –identify manufacturer –identify caliber

66 Shotgun Shells Most contain pellets which come in a variety of sizes –the smaller the number, the larger the pellet 000 buckshot pellets are 0.36 inches in diameter No. 12 birdshot pellets are 0.05 inches in diameter The load in each shotshel depends on the gauge of the shell & the size of the pellets –12-gauge No. 1 buckshot contain 16 pellets –16-gauge no. 1 buckshot contains 12 pellets

67 Shot gun Shells Shotgun pellets may be made of pure lead (drop shot), lead alloy (chilled shot) or soft steel Smaller size pellets are manufactured by pouring molten metal through a perforated plate & allowing it to fall some distance –solidifies as spheres Larger size are made by pressing lengths of wire between cup-shaped dies

68 Compression Formed Shotgun Shell Pellets are separated from the propellant by one or more overpowder wads –seal the propellant gases behind the shot

69 Firing A Shot ¶Chambered bullet ·Firing pin hits primer & sending a flash to ignite the powder ¸Bullet is propelled forward though the gun barrel as the spent cartridge case is slammed back against breechblock

70 Impression Evidence Bullet & Cartridge Comparisons

71 Interior View of a Rifled Gun Barrel Horizontal striations on the grooves (G) come from the rifling process Vertical striations on the lands (L) come from the initial reaming of the barrel These markings impress the bullet as it travels through the barrel

72 How A Bullet Acquires Rifling Impressions

73 Rifling Impression Patterns

74 Bullet comparisons Test bullet generated by firing into a water chamber or cotton box Bullets are then examined using a comparison microscope

75 Bullet Comparisons One bullet is rotated until a well-defined land or groove comes into view The other bullet is then rotated in search of a matching region Analyst must allow for distortion between bullets due to mutilation on impact

76 Cartridge Case Comparisions Marks which might be found on a spent cartridge

77 Cartridge Case Comparisons The hole in the center is the imprint from the primer pin Striation within circle are breechblock impressions

78 Breechblock Comparison Evidence Cartridge Reference Cartridge

79 Breechblock Comparison Breechblock FaceMicroscopic Comparison Not a match

80 Bullet Matches

81 Extractor Matches

82 Firing Pin Match

83 Some Rimfire Impressions

84 Automated Firearm Search Systems National computerized networks have been developed to allow labs to search each others data DRUGFIRE (FBI) –concentrates on cartridge markings although bullet striae can be stored as well IBIS (Integrated Ballistic Identification System)

85 Automated Firearm Search Systems IBIS (Integrated Ballistic Identification System) –developed by the Bureau of Alcohol Tobacco & Firearms (ATF) –incorporates two separate software programs Bulletproof –for bullet-specific markings Brasscatcher –for cartridge cases


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