Fire arms manufacture

Question 1

basic parts that may leave ballistic marks on ammunition

Answer 1

• rifling
• firing pin
• breech fave
• spent casing (ejected)
• trigger

Question 2

barrel

Answer 2

• chamber
• free bore
• throat
• rifling
• muzzle
• crown

Question 3

barrel manufacture (1)

Answer 3

• barrels start out as a metal bar either of chrome-molybdenum or stainless steel
• stainless steel (specialist barrels) more rust resistant and resist copper douling
• cro-mo (standard barrels) shot/used at far lower environmental temps.

at this stage the barrel is a steel rod

Question 4

barrel manufacture (2)

Answer 4

• a hole needs to be drilled through the length of the bar (deep hole drilling)
• cutting bit has a single edge which is self centred. barrel blank is centred in a lathe and spun at a high speed, oil is forced down to lubricate the process and force out the swarf created

at this stage the hole is drilled undersized to allow for the rifling process and reaming. this is now a rifle blank

Question 5

barrel manufacture (3)

Answer 5

• drilling produces significant marks useful to the firearms examiner
• 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 6

reaming

Answer 6

• barrel makers further process the bore by drilling to smooth the surface, correct minor variations in straightness and establish the final bore diameter
• reamer’s mounted on end of long tube through which coolant oil is pumped, but far lower pressures than used in gun drill. reamer is rotated about 200rpm and barrel is pulled over reamer at one inch per minute

reaming reduces roughness in drilled holes. the surface of the barrel hole is smooth yet reaming can leave marking that may be passed to fired bullet.

Question 7

rifling

Answer 7

button rifling- tool with helical grooves machined into it, being pulled or pushed through the barrel blank. the rifling grooves are printed into the bore, elongating the new barrel and hardening the bore.

cut rifling- uses a very sharp tool to individually cut each groove into the barrel. cut rifled barrels can either be single point cut (specialist weapons), one groove at a time, or they might be broached (mass produced). broaching is quicker but requires more expensive tools and lower quality.

Question 8

rifling (2)

Answer 8

• metal chips binding against the barrel before being flushed cause unique marks on the lands due to broaching.
• drill and reamer marks are not removed by button rifling as they are with cut rifling

Question 9

hammer forging

Answer 9

• starts with barrel blank that is 30% shorter than final size and a hole that’s 20% larger than final size
• mandrel is inserted into the barrel
• the barrel is then surrounded by hammers which apply pressure to shape from opposite sides
• the barrel is then rotated a fraction and the pressure is again applied
• this is repeated until the desired shape is reached.

hammer forgive produces a type of rifling called polygonal rifling. (6/right and 8/right are common)

Question 10

hammer forging (2)

Answer 10

it produces a straight barrel with a work-hardened bore surface. marks of interest to the examiner consistency of reamer and drill- upstream operations and any marks from chambering or crowning operations

Question 11

cut/broach and button rifling

Answer 11

• considered conventional rifling techniques

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

Question 12

polygonal rifling

Answer 12

• takes on a shape which is referred to as ‘hills and valleys’
• the gradual transition brevets firearm examiners from measuring the individual rifling elements in a polygonal rifled barrel

Question 13

chambering

Answer 13

• involves the use of calibre specific reamer
• tool that cuts the chamber
• the depth of the cut is checked constantly
• mass produced rifles will have what is known as a factory chamber

at this stage in the uk the barrel becomes a prohibited item and can only be held on a firearm certificate

Question 14

impression evidence analysis on projectiles

Answer 14

the examination process usually includes the following:

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

Question 15

the primary physical features of bullets include:

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

The weight of a bullet is normally measured in grains using a balance.

The weight of a relatively intact bullet is a partial indicator of the type(s) of cartridge that may have contained the bullet.

Weight is one of the factors used to determine cartridge type(s).

Question 17

determine bore/calibre

Answer 17

• defined as the diameter of the circle formed by the tops of the lands inside the barrel (doesn’t include grooves)
• the diameter of a fired bullet will approximate the larger groove diameter and will always be greater than the bore diameter of the firearm.

Question 18

bore/calibre diameter of fired bullet

Answer 18

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

• If there are an odd number of groove impressions, the measurement is taken from the edges of a pair of impressions.
• For bullets that are severely flattened or distorted, it may be possible to measure the circumference and calculate an approximate diameter.

Question 19

composition

Answer 19

Solid Bullets.
-Swaged and cast lead.
-Single metal (e.g., steel, aluminium, zinc, copper alloys, and brass).
Bullet Jackets.
-Jacketing metals (e.g., copper alloys, brass, aluminium, nickel, and steel).
Bullet Cores.
-Heavier metals (e.g., lead and steel).
Bullet Coatings.
-Coated/washed/plated copper, brass, nylon (Nyclad), Teflon®.
Sabots.
-Plastic sleeve or adaptor surrounding a sub-calibre bullet that allows it to be fired in a larger calibre firearm. The projectile never directly touches the barrel

Question 20

jacket type

Answer 20

• Full metal jacket - encloses the entire bullet, with the exception of the base. Normally used for military purposes
• Semi-jacketed – partially encloses the bullet with the exception of an exposed soft point or a hollow point.
• Total metal jacket – fully encloses a bullet core.

Question 21

nose construction

Answer 21

• Soft point bullets - exposure of a portion of the core at the nose of a jacketed bullet.
• Jacketed hollow point bullets- cavity in the nose to facilitate expansion.
• Semi-jacketed hollow point bullets- exposed portion of the lead nose that contains a cavity.
• Metal point bullets- (capped bullets) standard lead bullet with a harder metal jacket over the nose to enhance bullet penetration.
• Solid nose -constructed of a single metal or alloy.

Question 22

general rifling characteristics

Answer 22

• calibre (bore diameter),
• number of land and groove impressions,
• direction of twist,
• land and groove impression dimensions.

Question 23

measuring land and grooves

Answer 23

Using a comparison or stereomicroscope, land and groove impressions can be measured using a variety of equipment:
Micrometre
Callipers
Glass measuring reticule
Electronic reticule
Digital measuring equipment and software
Machinist’s scale

Question 24

examiners notes of measuring land and groove impressions should reflect

Answer 24

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

Question 25

damaged bullets

Answer 25

In some cases, bullets bear no microscopic marks of value for comparison purposes because they are fragmented, distorted, deformed, or otherwise damaged.

There may be retrievable information if all of the following are present:

One or more measurable land impression
One or more measurable groove impression
Sufficient bullet remains to determine a measured calibre

fragment with only one land and one groove impression visible (the minimum number for this to work).

Land impression width to be .055 inches and the groove impression width to be .130 inches.

Question 26

class characteristic to individual

Answer 26

Class characteristics – Measurable features of a specimen that indicate a restricted group source. They result from design factors and are therefore determined prior to manufacture. General rifling characteristics of the barrel of a firearm are the class characteristics of interest.

Individual characteristics – Marks produced by the random imperfections or irregularities of tool surfaces. These random imperfections or irregularities are produced incidental to manufacture and/or are caused by use, corrosion, or damage. They are unique to a particular tool and distinguish it from all other tools. In the context of this module, the tool is the interior of the barrel of a rifled firearm.

Question 27

sub class characteristic

Answer 27

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

Question 28

consecutive matching striae

Answer 28

quantitative method of describing an observed pattern match.