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 suspect’s 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Extractor: the mechanism in a firearm by which a cartridge of a fixed case in withdrawn from the chamber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 shotgun’s 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 firearm’s 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 Ejector
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 Come in a variety of different shapes
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. Bullet & Cartridge Comparisons
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 Face
Microscopic 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 other’s 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