firearms related impression evidence-Mark Flashcards
barrel manufacture - step 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
barrel manufacture - step 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
barrel manufacture - step 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
reaming
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
button rifling
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
cut rifling
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.
broaching
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
rifling
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
hammer forging
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
what is the most common polygonal patterns
6/right
8/right
cut/broach and button rifling
considered conventional rifling techqniues
transition from a land to a groove is very distinct and the lands and grooves are flat to slightly curved
polygonal rifling
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
chambering
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
impression evidence analysis on projectiles
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
physical features
weight measured calibre/diameter composition jacket type magnetic properties length colour/finish base construction base shape nose construction nose shape cannelures
weight
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
determining bore/calibre
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
solid bullets
swaged and cast lead
single metal
bullet jackets
jacketing metals
bullet cores
heavier metals
bullet coatings
coated/washed/plated copper, brass, nylon
sabots
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
full metal jacket
encloses the entire bullet, exception of base
military purposes
semi-jacketed
partially encloses with exception of exposed soft point or hollow point
total metal jacket
fully encloses bullet core
nose construction
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
general rifling characteristics
calibre
number of land and groove impressions
direction of twist
land and groove impression dimensions
measuring lands and grooves
micrometre callipers glass measuring reticule electronic reticule digital measuring equipment and software machinists scale
examiners notes should reflect
number of visible land and groove impressions
number of measurable land and groove impressions
measurements of useable land and groove impressions
recovered firearm without related evidence
obtain test bullets for later comparison by test firing the recovered firearm
recovered firearm with related evidence
obtain test bullets from evidence firearm and compare to recovered bullet to determine if the recovered firearm fired the evidence bullet
recovered bullets without related firearm
perform intercomparison to determine if recovered bullets are related to a single firearm
class characteristics
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
individual characteristics
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
sub class characteristics
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
