firearms related impression evidence-Mark

Question 1

barrel manufacture - step 1

Answer 1

barrels start out as a metal bar of either chrome-molybdenum or stainless steel

stainless steel is more rust resistant and tends to resist copper fouling better whilst cro-mo steel is more traditional and can be used at far lower environmental temps

standard barrels tend to be made in cro-mo, specialist barrels in stainless steel

Question 2

barrel manufacture - step 2

Answer 2

hole needs to be drilled through the length of the bar - deep hole drilling

cutting bit has a single edge which is self-centering

barrel blank is centred in a lathe and spun at high speed

oil is forced down a channel in the cutting tool to lubricate the cutting process and force out the swarf created

hole drilled is undersized to allow for the rifling process and reaming that follow

this is now a rifle blank

Question 3

barrel manufacture - step 3

Answer 3

drilling produces significant marks useful to firearms examiner

chips produced by drilling often nick the surface before lubricants flush them from the hole, adding to the randomness of the marks

drilling marks run perpendicular to the axis of the bore, leaving little chance of carry-over of marks from one barrel to the next

Question 4

reaming

Answer 4

reamer is mounted on the end of a long tube through which coolant oil is pumped, but at far lower pressures than are used in the gun drill

reamer is rotated and the barrel is pulled over the reamer at about one inch a min

reaming reduces roughness inherent in drilled holes

surface of the barrel hole is smooth, yet reaming can leave markings that may be passed to a fired bullet

Question 5

button rifling

Answer 5

involves the button, tool with helical grooves machined into it, being pulled or pushed through the barrel blank

as it passes through the blank, the rifling grooves are printed into the bore, elongating the new barrel and hardening the bore as it goes

Question 6

cut rifling

Answer 6

uses a very sharp tool to individually cut each groove into the barrel

longer process than button rifling

cut rifled barrels can be either single point cut, one groove at a time, or they might be broached

broaching is quicker but requires more expensive tooling, the quality may also be lower

usually cut rifled pistol barrels are only broached.

Question 7

broaching

Answer 7

metal chips binding against the barrel before being flushed cause unique marks on the lands due to broaching

a handful of manufacturers use broaches that broach the lands as well as cut the grooves

if aggressive enough, this type of broaching can remove nearly all traces of drill and reamer marks

Question 8

rifling

Answer 8

drill and reamer marks are not removed by button rifling as they are with cut rifling

the marks are pressed into the grooves and the highly polished button does not produce significant major unique marks unless the button is damaged

Question 9

hammer forging

Answer 9

starts with a barrel blank that is about 30% shorter than the final size and has a hole that’s 20% larger than the desired size

a mandrel is inserted into the barrel

mandrel has the negative image of the barrel it has the grooves carved in relief on the outside of the mandrel

mandrel is attached to a long thin steel rod and inserted into the barrel

barrel is surrounded by hammers which apply pressure to shape from opposite sides

barrel rotated a fraction of a turn and then pressure is again applied and the process is repeated until the desired shape is reached

hammer forging produces a type of rifling called polygonal rifling

produces a very straight barrel with a work-hardened bore surface

marks of interest to the firearm examiner consist almost entirely of drill and reamer marks from upstream operations and any marks resulting from chambering and crowning operations

Question 10

what is the most common polygonal patterns

Answer 10

6/right

8/right

Question 11

cut/broach and button rifling

Answer 11

considered conventional rifling techqniues

transition from a land to a groove is very distinct and the lands and grooves are flat to slightly curved

Question 12

polygonal rifling

Answer 12

takes on a shapre that is sometimes referred to as hills and valleys

gradual transition prevents firearm examiners from measuring the individual rifling elements in polygonal rifles barrel

Question 13

chambering

Answer 13

use of a calibre specific reamer, extremely sharp machine tool that cuts the chamber

depth of cut is checked constantly as it has to exact if the barrels to be safe when installed in the rifle

mass produced rifles will have a factory chamber based on standardised spec but custom reamer markers cater to the custom rifle and wildcatting markets

barrel becomes a prohibited item and can only be held on a firearm certificate

Question 14

impression evidence analysis on projectiles

Answer 14

bullet weight
composition of bullet
base description
type and position of cannelures
manufacturer
general rifling characteristics
-calibre
-number of land and groove impressions
-direction of twist
-width of land and groove impressions
bullet design
extraneous marks due to firing
suitability for comparison purposes

Question 15

physical features

Answer 15

weight
measured calibre/diameter
composition
jacket type
magnetic properties
length
colour/finish
base construction
base shape
nose construction
nose shape
cannelures

Question 16

weight

Answer 16

weight of a bullet is measured in grains using a balance

weight of a relatively intact bullet is indicator of the type of cartridge that may have contained bullet

Question 17

determining bore/calibre

Answer 17

bore diameter of the barrel of a rifled firearm is defined as the diameter of the circle formed by the tops of the lands inside the barrel

a portion of the mass of a fired bullet is extruded into these grooves

diameter of fired bullet will approximate the larger groove diameter and will always be greater than the bore diameter of the firearm

the measured diameter of fired bullets is taken from one groove impression to one on the opposite side of the bearing surface

if there are an off number, the measurement is taken from the edges of the pair

base of the bearing surface of a bullet is used bc it is generally more protected when a bullet impacts with an object

often the bearing surface ends at the base of the entire bullet, although with boat tailed bullets this would not be the case

for bullets flattened or distorted, it may be possible to measure the circumference and calculate an approximate diameter

Question 18

solid bullets

Answer 18

swaged and cast lead

single metal

Question 19

bullet jackets

Answer 19

jacketing metals

Question 20

bullet cores

Answer 20

heavier metals

Question 21

bullet coatings

Answer 21

coated/washed/plated copper, brass, nylon

Question 22

sabots

Answer 22

plastic sleeve or adaptor surrounding a sub-calibre bullet that allows it to be fired in a larger calibre firearm

projectile never directly touched the barrel and the sabot typically falls away after firing

because the bullet never directly engages the barrel, rifling impressions will not be impressed on the bullet but rather the sabot

Question 23

full metal jacket

Answer 23

encloses the entire bullet, exception of base

military purposes

Question 24

semi-jacketed

Answer 24

partially encloses with exception of exposed soft point or hollow point

Question 25

total metal jacket

Answer 25

fully encloses bullet core

Question 26

nose construction

Answer 26

soft point bullets provide for exposure of a portion of the core at the nose of a jacketed bullet

jacketed hollow point bullets have a cavity in the nose to facilitate expansion

semi-jacketed hollow point bullets have an exposed portion of the lead nose that contains a cavity

metal point bullets consist of a standard lead bullet with a harder metal jacket over the nose to enhance bullet penetration

solid nose construction of a single metal or alloy

Question 27

general rifling characteristics

Answer 27

calibre
number of land and groove impressions
direction of twist
land and groove impression dimensions

Question 28

measuring lands and grooves

Answer 28

micrometre
callipers
glass measuring reticule
electronic reticule
digital measuring equipment and software
machinists scale

Question 29

examiners notes should reflect

Answer 29

number of visible land and groove impressions
number of measurable land and groove impressions
measurements of useable land and groove impressions

Question 30

recovered firearm without related evidence

Answer 30

obtain test bullets for later comparison by test firing the recovered firearm

Question 31

recovered firearm with related evidence

Answer 31

obtain test bullets from evidence firearm and compare to recovered bullet to determine if the recovered firearm fired the evidence bullet

Question 32

recovered bullets without related firearm

Answer 32

perform intercomparison to determine if recovered bullets are related to a single firearm

Question 33

class characteristics

Answer 33

measurable features of a specimen that indicate a restricted group source

result from design factors and are therefore determined prior to manufacture

general rifling characteristics of the barrel of a firearm are class characteristics

Question 34

individual characteristics

Answer 34

marks produced by the random imperfections or irregularities of tool surfaces

produced incidental to manufacture and are caused by use, corrosion or damage

unique to a particular tool and distinguish it from all other tools

Question 35

sub class characteristics

Answer 35

discernible surface features of an object that are more restrictive than class characteristics in that they are:

• produced incidental to manufacture
• significant because they relate to a smaller group source, that is, a subset of the class to which they belong
• identifiable within a time frame since manufacturing processes change over time