Internal ballistics and gunshot residue Flashcards
what is terminal ballistics
related to the interaction of the projectile with its target
external ballistics
Related to the passage of the expelled projectile through the air up until its interaction with the target
Considers drop, drift etc
internal ballistics
Related to all processes from the moment of initiation of a cartridge to the repulsion of the bullet from the barrel
4 parts of ammunition
case
bullet
propellant
primer
what is the case of ammunition usually made from?
brass
steel
aluminium
what is cartridge brass
70% copper
30% zinc
advantages of cartridge brass in cartridge cases
Excellent tensile strength – Resistant to bursting from gas pressure
Ductile – Case will expand slightly when fired giving a good gas seal against the chamber of the weapon
Reusable – Can be reshaped and reloaded very easily with minimal equipment needed
Reshape-able – Allow the production of so-called wildcat cartridges
Corrosion resistant – Long term storage
Soft enough not to damage working parts
give two alternative case materials
low carbon steel
aluminium
advantage of low carbon steel as a cartridge case
Very high tensile strength
disadvantages of low carbon steel as a cartridge case
Lower ductility than brass – Poorer gas seal
Prone to corrosion unless lacquered
Hard on working parts of a firearm
advantage of aluminium as a cartridge case
cheap
disadvantages of aluminium as a cartridge case
Relatively ductile but not all that strong
Generally limited to lower pressure rounds
Often coated (PTFE)
should not be reused
the bullet - lead
Relatively dense
Will expand into barrel rifling without causing damage
Deforms upon impact
the bullet - jacketed
Normally a lead core with a gilding metal jacket
Less deformation and greater penetration
Some include an internal penetrator
Some may also include a hollow point or ‘expanding’ tips
the bullet - solid
Solid metal such as soft copper or in some cases some very odd alloys
Uncommon – Specialist
give 8 examples of shapes and designs of bullets
lead round nose was cutter semi jacketed semi jacketed hollow point special jacketed hollow point full metal jacket semi wad cutter
mechanical/physical generation - air rifles
propulsion is provided by mechanically or physically
Sprung – Spring powered piston provides compression
Pre-charged – Either pre-charged HPA tank or CO2 bulb
chemical generation - conventional ammunition
A composition that will react rapidly to produce large quantities of hot gas
Solid or liquid compositions
Propulsion as a result of deflagration or low order explosion
Detonations extremely undesirable
what is black powder made up of
Potassium Nitrate – 75 parts
Charcoal – 15 Parts
Sulphur – 10 Parts
name 2 disadvantages of black powder
Produces vast amounts of smoke
Causes fouling of barrels and weapon components degrading accuracy comparatively rapidly
what is pyrodex a substitute of?
black powder
what does pyrodex contain?
Contain additional oxidisers in addition to other components such as potassium perchlorate
4 benefits of pyrodex
Less smoke – Clean burning gaseous exhaust
Less fouling – Less solid material formed
Less sensitive – Comparatively difficult to ignite
Not considered an explosive hence easily purchased and stored
what are modern propellants based upon
cellulose nitrate
discovered by Christian Schönbein in 1846
what is the nitration reaction for modern propellants
Conc. H2SO4 + Conc. HNO3 in 2:1 ratio
Addition of Cotton for 2-10 minutes
Resultant product washed and neutralised
Remaining acid contaminants may increase sensitivity and decrease longevity
Acid removal process vital to stability and many manufacturers take days over this process
propellants are never sold in the raw guncotton form
Most undergo a complex series of processes in order to obtain a more useable material which burns uniformly releasing gas as a controlled rate
These materials are generally formed by dissolving the NC in a solvent after the addition of various other materials designed to affect powder characteristics
These are then extruded through a die and are chopped into pellets or granules ready for sale
how are pellets typically sold
Propellants are typically sold for reloading as pellets, granules or powders of specific size and shape in 1lb (454g) tubs which retail for around £30-40 in UK
name 7 propellant additives
Additional energetic materials – To provide superior burn characteristics
Stabilisers – Increase shelf life and prevent decomposition
Plasticisers – Provide desirable mechanical properties
Flash suppressant materials – Minimise muzzle flash
Moderators/Deterrents – Alter burn rate
Colourants – Often purely marketing, the NC material itself is generally an unpleasant yellowish colour
Graphite – Lower sensitivity to static and allow the material to flow rather than caking in its packaging
single-base propellants
Consisting of a single energetic material
Cellulose nitrate
Dissolved in alcohol-ether
Extruded into flakes, tubes, cylinders or similar
multi-base propellants
Consisting of two or three energetic materials
Nitrocellulose with…
Nitroglycerine or 2,4 DNT – Double base
Nitroguinidine – Triple base
Used to increase power and enhance other characteristics
name 6 other components found in propellants
stabilisers plasticisers flash suppressants moderators/deterrents colourants graphite
why are stabilisers put in propellants
Normally compounds used to neutralise the acidic decomposition products of the energetic components
Extend shelf-life and prevent sensitisation
Ethylcentralite (3-Diethyl-1,3-diphenylurea) or diphenylamine most common examples
why are plasticisers put in propellants
Used to allow propellant to be extruded and cut more easily
May also lower ‘hygroscopicity’ (prevent from absorbing moisture)
Ethylcentralite, triacetin, 2,4 DNT
why are flash suppressants put in propellants
Minimise the flash that occurs as the weapon fires due to incomplete combustion
More common in military compositions for obvious reasons
Nitroguanidine, Ammonium or Potassium Salts
why are moderators/deterrents put into propellants
Used to allow regulation of the initial rate of combustion
May also regulate burn characteristics of the propellant allowing slower release of gas
Particularly important in rifles where rapid gas production may cause over-pressure
Ethylcentralite, 2,4 DNT
why are colourants put into propellants
May be used to impart a desirable colour for marketing purposes
May also be used for identification purposes
Variety of colours, although most are black or dark grey
why is graphite put into propellants
To improve propellant flow and prevent caking or clumping
Makes life easier for the reloader
Prevent static build up which could potentially initiate premature initiation
how does grain size affect propellants
affects rate of gas generation
small grain propellants
Fast burn – Pistols or short barrelled firearms
May cause overpressure in larger calibre rounds
Hand loaders take care with powders to ensure that potentially lethal mix ups don’t occur
medium grain propellants
Moderate burn – Rifles or large calibre pistols
Gas is produced more slowly over a longer period of time to account for the longer rifle barrel
large grain propellants
slow burn - large calibre rifles and canon
digressive burn
Rate of gas generation decreased with time
Rapid rise in pressure and higher peak
Cylindrical/flake
neutral burn
rate of gas generation constant
single perforation
progressive burn
Rate of gas generation increases with time
Perforated or star shaped grain
what is 1gram in grains
15.43gr
what is 1 grain in grams
0.0648g
what are the 2 variants of brass cup
Boxer primed – The most popular design
Berdan primed – Very much out of favour
primer construction
Contains priming composition
- Sometimes sealed with foil paper
- Boxer type also includes anvil device
Friction and shock sensitive
Allows initiation of main propellant charge
- Flash and hot material ignite propellant charge
- Three main sizes available for small arms
primer sizes
3 main primer sizes for boxed primed cartridges
- Small –.175 inches in diameter (Rifle and Pistol)
Used in small calibres rifle and pistols – i.e. .223Rem or 9mm - Large –.210 inches in diameter (Rifle and Pistol)
Larger calibre Rifles and pistols – i.e .243Win, or .45ACP - Shotgun –.209 inches in diameter (Standard and Magnum)
Generally, longer than a typical primer to fit into the longer primer pocket of shotshells
what two forms to primers come in
standard
rifle/magnum
latter pairing most appropriate for cartridges using propellant that is more difficult to ignite therefore requiring a more sustained ignition
primer design
Will reliably ignite when struck with a firing pin/striker – Low failure rate
Will reliably ignite the entire propellant mixture – To maximise efficiency
Will not ignite when primer or cartridge is dropped – So not that sensitive!
Will not ignite when exposed to vibration during firing process – This is particularly relevant to high end big game rifles
Comparatively insensitive to extremes of environmental temperature
Efficiency must be retained at lower temperatures
Chemically stable – Will not decompose over time
Will not cause damage to firearm
Ideally non-toxic
name 3 early primer compositions
mercury fulminate
chlorate compositions
lead azide
mercury fulminate
An innocuous looking grey-white powder
Friction and shock sensitive and also toxic
Combustion products which contain Mercury have tendency to destroy the brass casing (as a result of amalgamation) preventing re-use
Additives
Commonly includes potassium chlorate (an oxidiser) to allow proper flame propagation
Some fulminate primers contain ground glass to improve sensitivity
chlorate compositions
Armstrong’s mixture – often pacified with boron carbide
Often described as unpredictable and potentially very shock sensitive
Hygroscopic – Therefore limited shelf life
Decomposes to metallic oxides and chlorides which convert to corrosive compounds that may cause severe damage to barrels over time and are thus obsolete
lead azide
Relatively stable
In general, less sensitive than fulminates
Reaction with copper leads to copper azide formation which is extremely sensitive to shock
Application in gas generators Airbags (Sodium Azide) but no longer common in primer compositions
name 3 modern primer compositions
lead styphnate
barium nitrate
antimony sulphide
lead styphnate
Shock sensitive primary initiator chemically related to both
TNT and TNP
Less sensitive than azides or fulminate although more sensitive to static
barium nitrate
Oxidising agent increasing burn rate and propagates flame/flash
antimony sulphide
May increase sensitivity
Fuel to aid in flame propagation producing longer flash
diazodnitrophenol or tetrazene derivatives - multi component non toxic SINTOX
Shock sensitive primary initiators
Less sensitive than azides, fulminate or Lead Styphnate
Less reliable and hence arguably less popular with firearms professionals
Non Toxic – Less liable to cause elevated environmental lead levels
zinc peroxide - multi component non toxic SINTOX
Oxidising agent increases burn rate and aid flame propagation
titanium - multi component non toxic SINTOX
Fuel – Used to prolong the flash
name a SINOXID variant
lead styphante
in SINOXID, the oxidiser may be replaced with what 3 possible things
Strontium compounds – Strontium Nitrate (Also found in fireworks)
Potassium (per)chlorates (Powerful oxidisers used in pyrotechnics)
Stable peroxides – Zinc Peroxide
in SINOXID, the antimony sulphide fuel can be possibly replaced with what 3 things
Calcium Silicide – Flammable but moisture sensitive
Aluminium – Prolongs flash and increases heat produced
Rim fire variants may include ground glass frictionators
primer initiation - cartridge discharge
The firing pin or striker hits the centre of the primer
The anvil and primer composition are forced together
This ignites the primary initiator
Initiation of other components
White hot particulate forced through flash hole
all within few ms
what is the temperature and pressure during primer initiation
temp - 1500 degrees
pressure - 1400psi
propellant initiation - cartridge discharge
As the primer derived hot gases enter the main cartridge body
The propellant will begin to burn rapidly
Producing vast quantities of gas
Temperature increases to ~3000⁰C+
Pressure in 105 kPa range (30,000+ PSI) although pressures of over 60,000 PSI are not uncommon in some calibres
Bullet is forced out down the barrel of the firearm and pressure begins to drop off
Bullet acceleration in the order of 10,000 G
GSR, smoke, gas, lubricant and particulate
Rapidly expelled down the barrel and out any available gaps in the firearm
This is the likely mechanism of GSR deposition
In the same way that even the most careful smoker will always smell of cigarette smoke, the firer of a weapon will invariably be covered in GSR
revolvers are ‘leaky’
Chambers/Barrel gap and rear of chamber
Note flash from cylinder gap
Covers shooters hands and surrounding areas in GSR material
Backwash will also cover clothing, face and hair
semi-automatics - particulate
SLP’s perhaps less intrinsically leaky in their design
There is no barrel/cylinder gap however, some of the deposition of material (other than backwash) may be the result of slide return and cartridge extraction
A secondary flash as barrel/chamber and slide separate may sometimes be seen and some GSR may also be deposited from the trigger area
Tendency to adhere to upper surfaces of hands, forearms, face and clothing
Possible directional bias
what are the two main categories of GSR
inorganic
organic
inorganic GSR
GSR generally derived from the detonation of the primer but may include shaved materials from bullet and barrel
Normally metallic multi-element particulate – Pb-Ba-Sb for example
Few alternative sources
Forensically very significant
organic GSR
GSR generally derived from the propellant and its additives
Normally carbon containing particulate
Unburned propellant in particular
Nitrates and Nitrites
Alternative sources
May be less forensically significant
