Forensic Ballistics

Four different productions of rifling

Standard (rectangular) profile of rifling, displaying sharp, angular lands and grooves.

• Broach cut: uses a single point tool, one groove at a time.

• Electrochemical etching: uses an electrolyte, passed through grooves in a tool, in contact with inside of barrel, producing barrel grooves.

Polygonal rifling

• Hammer forged: method where the barrel is shaped by hammering steel around a mandrel, creating the desired rifling pattern.

• Button formed: a process that involves forcing a hard metal button through the bore of the barrel which displaces material to form the rifling grooves.

Draw and label the nomenclature of a bullet

Test firings from a suspect weapon

• Water tanks are most commonly used.

• Also, a box of rubber tyre pieces can be effective.

What is the NABIS database and what items must be recorded on it?

It is the National Ballistics Intelligence Service, items that must be disclosed:

• All firearms of 1968 Firearms Act coming into police possession.

• All firearms that may have been potentially used in a crime.

• All firearms and imitation firearms, inc. blank cartridges, and deactivated firearms, used in a crime.

• Any licensed firearms reported lost or stolen.

• Any surrendered firearms that match database.

• All electronic stun guns, inc. incapacitants, and noxious sprays subject to S5 1968 Act.

Factors affecting path of a projectile, and difference between perfect and non-perfect ballistic arcs

Factors = gravity, angle of launch (elevation), velocity, air density, projectile shape, projectile stability.

Perfect ballistic arc = little/no air resistance

Non-perfect ballistic arc = end of curve is more ‘truncated’

What is the mach number?

Defines how many times speed of sound that projectile is moving at. Breaking the sound barrier exerts a significant drag force on projectile, due to pressure changes.

Relationship between sectional density and impact velocity.

Bullets with a high S will carry more impact energy at a given range, not losing velocity as easily as a low S projectile.

Ballistic coefficient

A measure of a projectile's ability to overcome air resistance in flight, defined by its sectional density and ballistic shape. Higher values indicate a more aerodynamically efficient projectile.

Difference between spin and twist.

Spin refers to the rotation of a projectile around its axis, while twist refers to the rate of rifling in a gun barrel that imparts this spin on the bullet.

Two main types of wind deflection experienced by a projectile.

Aerodynamic = caused by wind flow over projectile, in flight, effect is small and only shows at long ranges.

Windage = deflection caused by constant wind pressure during projectile flight, much more pronounced effect on overall trajectory.

Causes of low spin rate, experienced by projectiles.

• Low muzzle velocity

• High muzzle velocity

• Defective rifling

Three types of projectile’s instabilities

Yaw = lateral movement of nose of bullet away from line of flight

Precession = refers to rotation of bullet around CoM

Nutation = small circular movement at bullet tip, due to tip not being round

Shooting reconstruction vs shooting re-enactment

Reconstruction = utilising information derived from physical evidence at scene, inferences drawn to test various theories.

Re-enactment = demonstration of a previously existing reconstruction based on conjecture, rather than scientific principle.

Perforation vs penetration

Perforation = bullet remains intact, passing through medium, identifiable entry and exit wounds.

Penetration = bullet remains intact, does not pass through medium, an entry, but no exit wound.

Possible indicators of a use of a firearm

• Bullet(s), cartridges(s)

• GSR deposits, subject to confirmation tests

• Wipe ring around hole

• Size and shape of hole

• Evidence of high energy penetration and damage highly local to hole, depends on material

Metal penetrations/impacts

• Usually leaving a neat margin on entry side of hole

• Exit wound will look like a ‘truncated’ funnel

• Will just be a dent on metal surface, if metal plate is thick, paint or other coatings may be disrupted.

Wood and ‘frangible’ (surfaces which easily break up, crumbling, i.e., concrete) penetrations/impacts

• A neat entry hole

• Cratering and splintering on exit side, widely distributed debris

• Wipe ring towards entry side

Glass penetrations/impacts

• Concentric, circular ring cracks

• Relatively straight radial cracks

Fabric penetrations/impacts

• GSR present at short firing ranges

Microscopic examination shows:

• Synthetic fabrics: individual fabrics melting, resulting from frictional heating.

• Natural fabrics: reveals fibres with shredded and frayed ends around hole.

Evidence from a firearm user incl. impacts on shooter

• Flash burns on user and target, inc. GSR present perhaps.

• Burns to fabrics and flammable surfaces.

• Imprints and impressions, inc. ricochet damage.

• Bullet/fragment wounds, inc. debris launched by bullet.

• Hearing damage.

Impacts on shooter:

• Slide bite: mark between thumb and index finger, on hand holding weapon.

• Hammer bite: external hammer can pinch or abrade web between thumb and fingers. May also be DNA left on hammer.

• Recoil marks: shoulder and upper arm, of gun user.

• Scope bite: the impact of a rifle scope on the shooter's face or brow during recoil.

Trajectory finding tools

String trajectory lines: (use of rods and lines)

• Inexpensive, easy to use.

• Jury easily understands it.

• Not rigid, so must be kept tensioned, requiring anchoring.

• Line passed through centre of bullet hole (using centring cones), then extended to show bullet paths.

• Different colours for different shots/weapons.

Laser trajectory lines:

• Useful for non-penetrating impacts, as laser inserted into hole, beam illustrating bullet path.

• Left in place, not obstructing same way as rods and lines.

• Hard to photograph, not effective for long range, as divergent.

Gunshot residue (GSR) brief analysis + its two main forms

• Can predict muzzle to target distances.

• An inanimate object, like chalk dust.

• May leave permanent human tissue markings - “tattooing”

2 main forms:

• Particulate - microscopic particles from discharged firearms.

• Precipitate - swab with distilled water, extract particles.

Main sections of 1968 Firearms Act

S1 = Firearms

S2 = Shotguns

S5 = Prohibited weapons

Definition of a firearm

A lethal barrelled weapon, from which a shot, bullet, projectile, missile, with a KE of more than one joule at muzzle of weapon, can be discharged.

Definition of ammunition

• Includes bullets, grenades, bombs, missiles. Any item designed to be used in a firearm or weapon to discharge a projectile.

S1 Firearms - Interpretation

• Covers most bolt action rifles

• As long as barrel is longer than 30cm, and overall length longer than 60cm.

• Muzzle loaded guns do not need to adhere to above rules.

  S1 also includes:

• Pistols, revolvers, and semi-automatic firearms.

S2 Firearms - Interpretation

• A smooth-bore, non air gun, which has a barrel more than 60cm, does not have any barrel with a bore exceeding 2 inches in diameter.

• Either has no magazine, or a non-detachable magazine, incapable of holding more than two cartridges.

• Typically have a box-lock mechanism.

• Not a revolver gun.

S5 Prohibited Firearms - Interpretation

• Military and police hardware, incl. automatic weapons, handguns, anti-tank rockets, grenade launchers, semi-automatic weapons.

• Any ‘disguised’ firearm to look like other objects.

• Prohibited ammo: armour-piercing rounds, incendiary + explosive rounds, expanding ammo for pistols.

Special exemptions for possessing a S1 and/or S5 Firearm

• Police permit

• Registered dealers, ballistic experts.

• Slaughter of animals

• Sports, athletics activities.

Internal Ballistics - Lock Time + trigger and sear safety

• Time between activation of firing mechanism and ignition of primer charge.

• Trigger mechanism, consisting of a system of levers (sears), which hold firing pin under pressure, until trigger operated.

• Trigger mechanism is often culprit in “accidental” shootings.

• Lock time process: when firearm is ‘cocked’, firing pin is compressed and mechanically held by sear, energy stored within spring released when trigger is pulled, activating primer.

• Trigger safety: function physically blocking external trigger action, activated by button or lever behind trigger.

• Sear safety: function that blocks action of one or more of trigger sears, closer to firing pin. Comprises of grip safety and fire selection levers.

Safety failure tests

• Trigger pressure tests - dropping firearm, impact velocity at all three planes of firearm (top, back, side).

Ignition time definition + what it is dependant on

Is the time from activation of primer charge to initial movement of projectile.

Dependant on:

• Primer type

• Propellant design

• Chamber dimensions

• ‘Tightness’ of bullet in cartridge neck

Ignition time sequence of events

• When firing pin contacts primer, it deforms soft primer cup, crushing primer mixture against an internal anvil.

• Impact causes primer to detonate.

• Resultant hot gases pass through flash hole and ignite the main propellant charge in cartridge case.

Detonate vs deflagrate + what is determined by burn rate

Detonate = explode, deflagrate = burning, at a controlled rate.

Burn rate is determined by grain size and shape, primer type, chamber dimensions.

Barrell time definition + what it is dependant on

Barrel time = time from first movement of projectile to when projectile exits muzzle.

Dependant on:

• Projectile shape, size (caliber), mass

• Internal barrel friction

• Rifling type + type of twist

• Chamber pressure + propellant burn rate

Sequence of events for spring operated air weapons + pre-charged pneumatic air weapons

Spring operated:

• Lock time

• No ignition time, but compression time.

• Barrel time

Pre-charged pneumatic:

• Lock time

• No ignition time, but valve operation time

• Barrel time

Home Office Test

• Three different masses of pellets used: heavy, medium, light

• 10 of each pellet type are fired, across a chronograph, velocities recorded.

• Muzzle velocity recorded, energies for each pellet calculated, using KE equation.

Muzzle crown deflection

‘Muzzle crown’ = flat end of barrel

• Any damage to this could potentially deflect bullet’s path

• Crown damage can cause anomalous markings + scratches on bullet with forensic sig.

• Leads to unusual scratches on bullets that do not come from rifling.

What makes a firearm sound, from loudest to quietest

• Pressure wave from rapidly expanding gases, going supersonic

• Supersonic crack if bullet goes supersonic

• Mechanical action noise of firearm itself

• Any other flight-related noises, e.g., air friction

• Impact sound - varies on type of projectile

How sound suppressors work + what they are more effective on

• Slowing gases leaving muzzle

• Allowing expansion of gases + applying turbulence to gas flow within a controlled chamber (suppressor), slowing down gases below speed of sound

• S.S. more effective on subsonic ammo, than supersonic ammo.

Sound suppressor types + diagrams

Can = hollow can-shaped extension on muzzle, not as effective

Baffle = can with partitions for pressure wave to impact + transfer energy (losing velocity)

Reflex = can with baffles + additional internal complexity, providing expansion chambers behind baffles for pressure waves to impact and transfer more energy to

Active = reflex suppressor with spring-loaded baffles which further transfer energy away from pressure wave

Recoil reducers

Under two categories:

• “Muzzle brakes” - external accessories + most effective

• “Recoil compensators” - integral to firearm + less effective

Work by redirecting propellant gases to counter recoil of firearm

Recoil greatest for larger-caliber, slow moving, less for fast, small-caliber projectiles.

Recoil reducers increase noise of firearm + gas blast is directed back towards firer.

Flash hiders function

• Physically hiding flash using a cone-shaped device, on end of barrel.

• Or, by dispersing flash upwards or sideways via a series of fingers or a tube with longitudinal cuts. This is to expose the flash to oxygen, allowing flash to burn quicker.

Shotgun chokes, formation + function

Choke = constriction in end of a shotgun’s barrel, used to tailor shot pattern for different purposes.

• Chokes formed by squeezing end of bore down over mandrel, threading barrel + screwing in an interchangeable choke tube.

• Consists of a conical section, smoothly tapering from bore diameter down to choke diameter, followed by a cylindrical section of choke diameter.

• As column of shot in barrel reached the choke in barrel, diameter of shot column is reduced, thus elongating its length.

• Results in outer layers of shot in column being given inward acceleration, delaying spreading of shot after leaving barrel, reducing its dispersion.

Typical primer charge for centre fire and rim fire primers + properties of it

= lead styphnate

• A high primary explosive, can be detonated by friction, impact, or flame.

• Relatively heat insensitive, does not react with metals.

• Highly sensitive to static charge.

Definitions: propellants, detonation, deflagration

Propellants = energetic materials which deflagrate in a way, for their energy to propel out of barrel.

Detonation = supersonic process, propagates through shock.

Deflagration = subsonic process, propagates through thermal conductivity.

Disadvantages of black powder

• Is hygroscopic, very effectively absorbing moisture, stopping it from burning.

• Unstable when hot, has a low ‘cook off’ temp.

• Sensitive to static.

• Can produce highly toxic by-products, i.e., sulfuric acid, corrodes weapon.

Modern propellant types + constituents

Single base - nitrocellulose (NC)

Double base - nitrocellulose (NC) + nitro glycerine (NG)

Triple base - nitrocellulose (NC), nitro glycerine (NG), and nitroguanidine (NGu)

Nitrocellulose (NC):

• Created by reaction between between cellulose + nitric acid.

• NC is oxygen deficient (negative oxygen balance), combusts to CO, rather than CO2.

Nitro glycerine (NG):

• Highly shock sensitive, prone to accidental detonation, unless stabilised in ethanol/acetone.

• Efficient combustion process, practically ‘smokeless’.

• Has a positive oxygen balance, it does not require O2 in air to undergo combustion.

Nitroguanidine (NGu):

• Highly insensitive, but energetic high explosive.

• Reduces muzzle flash from propellant, as well as flame temp.

• NGu reduces barrel erosion in firearms.

• Has a negative oxygen balance, produces some C (soot).

Heat effects in primers + propellants

• Propellant burning is a highly exothermic reaction.

• Heat build up has effects:

  • Barrel distortion: poor accuracy, barrel blockage.

  • Cook off: residual heat present in chamber prematurely ignites primer/propellant of next chambered round.

  • Evaporation of lubricants - jamming of firearm’s action.

Propellant additives + functions

Reaction rate modifiers

• Changes propellant’s burning rate.

Flash reducers

• Reduces flash, increases smoke production.

De-coppering agents

• Slow copper build up in rifling.

Wear reducers

• Lower wear on inside of barrel.

Stabilisers + plasticisers

• Helps form propellant grains.

Gunshot residue (GSR) + its sources

= any particles or residues discharged from a firearm after trigger has been pulled, may include chemicals from primer, propellant, oxidisers, reducing agents, sensitisers, binders.

Sources:

• Residues formed through explosive reaction of primer compounds.

• Material originating from bullet jacketing or coating.

• Material eroded from cartridge case, primer cup.

• Material originating from interior of firearm chamber + barrel, can include biological material.

GSR formation + appearance

• Form due to rapid cooling of discharge gases + solids matter, originating from reacted components of primer, propellant, as well as metallic compounds of ammo and firearm.

• Some of gases condense in form of spheres, can interact with solid residue materials, forming complex mixtures + aggregate forms.

• May show as spherical particles or irregular shapes.

Collecting GSR: human sampling

Primary sample area = hands

• Sample from subject prior to handcuffing

• Subject under visual observation before collection

• Subject can not wash hands

• No bathroom visits, if unavoidable, no washing hands

• No hands in pockets

• No removal of clothing

• No fingerprint taking before GSR collection

Greiss Test for GSR

• Presumptive chemical test, suggesting presence of organic nitrile compound.

• Test for explosive materials + propellants, turns brown/orange in presence.

• Can provide false positives.

Sodium Rhodizonate Test for GSR

• Presumptive test, suggesting presence of lead.

• Can be performed after Greiss Test, involves spraying solution of reagent in distilled water onto test surface.

• Produces a red/pink stain, suggesting presence of lead.

• Area treated with dilute HCl, stain turns blue if presence of lead.

• A positive test from both Greiss Test AND Rhodizonate is strong evidence that a firearm has been discharged.

  • Independently, not strong enough tests for confirmation.

Primer component functions

Lead styphnate = acts as an explosive, driving ignition process.

Barium nitrate = provides oxygen to support combustion.

Antimony sulphide = acts as fuel ignition process, as a ‘frictionater’ during ignition.

SEM-EDX advantages + process

• Non-destructive, minimal sample preparation.

• Highly effective, rapid automated systems.

• Electron gun fires beam of electrons at a high voltage through a vacuum.

• Electromagnetic lenses focus beams onto surface of object.

• Finely focused beam scanned across a sample to cover area of interest.

• Electrons scattered by different amounts, scattered electrons hit detector, recorded.

Classifications of GSR particles

1.) Particles classified as ‘characteristic of GSR’ have compositions rarely found from other sources.

2.) Particles classified ‘consistent with GSR’ have compositions that are also found in particles from number of relatively common, non-firearm sources.

3.) Particles classified as ‘commonly associated with GSR’ have compositions that are also commonly found in environmental particles from numerous sources.

False positives of GSR

• Car air bags

• Car brake pads

• Fireworks + pyrotechnics

• All aspects containing elements of barium, antimony, lead.

Short range shot injuries

Tattooing:

• Hot gunshot residues can embed into human tissues at close ranges, pattern of which can be analysed.

• Pattern left on skin known as ‘tattooing’ or ‘stippling’ (easily mistaken for gunshot wounds).

Gas Injection Trauma:

• Contact shot to head leads to a distinctive star shaped (stellate) splitting of skin.

• Propellant gases are ‘injected’ between dermis and cranium, and with no where else to go, bursts out surrounding wound.

• Dark metallic combustion deposits in wound.

Muzzle Imprint:

• A mark or impression left on the skin by the muzzle of a firearm when fired at very close range.

Skull impacts wounds + ‘gutter’ wound

Skull impacts:

• Result in ‘bevelling’ only reliable way of identifying entry and exit wounds.

• Shape of bullet hole in bone indicates angle of impact.

• Bullet usually remains intact, after impact through skull.

‘Gutter’ wound:

• If projectile impacts surface of skull at a short range, not penetrate, a tangential impact.

Comminuted fracture + skull cap wounds

Comminuted fracture:

• Angled impacts can be deflected within cranial activity, causing a comminuted fracture, diametrically opposite point of entry.

• Bullet is recoverable, these fractures mistaken for exit wounds - ‘pseudo exit wounds’.

Skull cap:

• Skull cap (calvaria) susceptible to ‘popping off’, following high velocity trauma, due to large increase in pressure, generated inside skull, as high energy transfer to brain material, brain material pushed out radially from projectile’s path.

Permanent wound cavity

= most significant mechanism in overall extent and severity of wounding

• Path of tissue destroyed by transit of bullet, formed by crushing of soft tissues, tearing by shrapnel/fragments.

• Cavity does not close completely after passage of projectile, leaves internal tissues exposed to atmospheric contamination.

• Permanent cavity size somewhat closely related to bullet’s caliber.

• Destroyed tissue must be excised, to avoid necrosis.

• Bullets = dirty = increased infection etc.

Temporary wound cavity + elasticity

• Opens up by transit of bullet through tissues.

• Soft tissues damaged slightly, natural tissue elasticity causes this cavity to close after bullet passes through.

• Cavity formed by projectiles importing radial acceleration to tissue, forcing out laterally.

• KE passed into moving elastic tissue transformed into strain energy. Strain energy eventually released, causing it to retain back to original position.

• If penetrated tissue is highly elastic, pulsing of cavity occurs, until all of strain has been released, temporary cavity ceases to exist.

• Initially, temporary cavity is a vacuum, before air entering - suction of unwanted material could occur.

Low velocity-large caliber vs high velocity-small caliber wounds

Low velocity-large caliber:

• Small permanent cavity

• Temporary cavity is cylindrical + regular, does not extend beyond permanent cavity.

• Bullets tend to lose energy quickly, often not leaving body.

High velocity-small caliber:

• Large, irregular permanent wound cavity if bullet breaks up.

• Large temporary wound cavity widest in middle.

• Non-partitioned bullets tend to strip from core

  • May over penetrate, jacket remains in body.

Ballistic penetration simulant definition + reasoning of using it

= any material, either biological or synthetic, that is able to provide comparable and reproducible wound data, in relation to human body when penetrated by a projectile.

Reasoning:

• Overcome ethical issues associated with testing on human tissues.

• Give comparative penetration data between different projectiles.

• Understand potential wounding effects of projectiles.

• Accurate + reproducible data attained.

Desirable characteristics of soft tissue simulants + ballistic gelatin used

• Non-cadaveric, non-biological, so any natural biological variation can be avoided.

• Ethically sound

• Readily available, easy to handle, stable storage

• Similarity in deceleration + deformation behaviour of projectile between simulant and living tissue type.

• Extrapolation of temporary and permanent wound cavity diameter

• Reproducibility

Ballistic gelatin:

• Consists of collagen, extracted from bones, and sinews of pigs.

• Typically in two formulations:

  • 10% gelatin @ 4 degrees Celsius

  • 20% gelatin @ 20 degrees Celsius

• Density of ballistic gelatin temperature sensitive.