firearms - weapons and ammunition

 

 

original @ Gothamist

 

This section will outline the main types of weapons likely to be encountered in clinical and forensic practice, and the ammunition that these weapons utilise.

Small arms refer to those weapons that are designed to be held and fired from the hand or the shoulder, and include handguns and rifles, as well as shotguns. In broad terms, weapons can be divided into rifled firearms, and smooth bore weapons.

The British Crime Survey 2004/5 recorded 10,979 firearm offences in England and Wales, and firearms were involved in 1,206 serious violent incidents (other than homicide). They were used in 73 (8.5%) of homicides.

pinterest - ballistics

 

 

rifled firearms

35 remington caliber, microgroove rifled barrel, manufactured by the marlin firearms company. It shows a 20 land and groove barrel with a right hand twist.

Source: Wikipedia

 

Revolvers, pistols and rifles are called ‘rifled’ weapons because of the design of their barrel. The inner surface has a series of parallel grooves cut in a spiral fashion along the length of the barrel (separated by ‘lands’), the effect of which is to impart spin on the bullet along its long axis.

This spin gives the projectile axial stability in flight and leads to improved accuracy over mid range to longer distance targets. 

The passage of a bullet through such a barrel imparts an imprint of the barrel surface configuration (the number of lands and grooves, and the way in which they ‘twist’) on the bullet, and if recovered may allow firearm examiners to deduce the type and make of weapon from which it was fired.

 

Characteristics of bullets and cartridge cases utilised by firearms examiners (Source: Pounder 1993)

  • Bullet class characteristics
    • number of lands and grooves (usually 4-6, range 2-22)
    • diameter of lands and grooves
    • width of lands and grooves
    • depth of grooves
    • degree of twist (number of cms of bore required for one complete rifling spiral)
    • direction of rifling twist (e.g. right/ clockwise)
  • Individual bullet characteristics
    • imperfections of grooves (most pronounced in lead bullets)
    • imperfections of lands (most pronounced in jacketed bullets)
  • Cartridge case markings
    • type of breech block marking
    • size, shape and location of extractor marks
    • size, shape and location of ejector marks
    • size, shape and location of firing pin marks (most important identifying marks in rimfire cartridge cases)

 

The calibre of the weapon is a function of the diameter of the bullet, the cartridge case length and/ or the cartridge case.

 

Browning 9 mm pistol

Source: Wikipedia

 

Pistols are loaded with ammunition via a magazine (the ‘clip’) that is located in the handle/ grip. The first bullet in the magazine is loaded manually by ‘cocking’ the weapon. This is achieved by pulling back on the ‘slide’, and then releasing it.

Movement forward loads the first round into the barrel. When the first round has been fired, by squeezing the trigger, the barrel recoil causes the slide to move backwards and the spent cartridge to be ejected. A new bullet is then loaded into position.

 

Enfield revolver No. 2 Mk. 1 .38

Source: Wikipedia

 

In contrast, bullet cartridge cases (‘rounds’) are loaded into the revolving chamber of a revolver at each squeeze of the trigger. The hammer of single action revolvers must be manually cocked before each round is fired, whilst in a double action weapon, squeezing the trigger not only cocks the weapon but also rotates the cylinder onwards so that the next cartridge is in place and ready to be fired.

 

 

 

 

British Lee-Enfield R.I.C. Carbine .303

Source: Adams Guns

 

Rifles are designed to be fired from the shoulder, and are classified as either ‘rim fire’ or ‘centre fire’ weapons. This describes the type of ammunition fired, and in particular where in the cartridge case the primer is located i.e. in the cartridge case rim or centre respectively. There are many types of rifle – pump action, lever action, bolt action, single shot and automatic loading etc.

 

Bren gun

Source: Wikipedia

 

Machine guns and sub-machine guns fire rifle and handgun ammunition, respectively, and can be fully automatic or semi-automatic. Semi-automatic handguns, particularly pistols are also increasingly being used.

 

how a revolver works

 

 

how a 1911 pistol works

 

See also an interactive animation of how a Glock pistol works, with the moving parts highlighted.

how an AK47 assault rifle works

 

 

rifled firearm ammunition

 

Rim fire ammunition

Source: Wikipedia

 

Rifled weapons

The bullet cartridge, or ‘round’ consists of a formed metallic case (usually a brass alloy), explosive primer, gunpowder (the propellant) and a bullet (the projectile).

The primer usually contains heavy metal compounds, such as those of lead, barium and antimony, and detonates the gunpowder when struck by the firing pin.

 

 

 

Eley rim fire .22

Source: Eley

 

In rim fire rounds, the primer is located in the cartridge case rim, and in centre fire rounds it is recessed in the centre of the cartridge. In general rim fire ammunition is used in handguns, whilst centre fire ammunition is utilised in rifles.

 

.303 centre fire

Source: Wikipedia

 

Video demonstration of how a bullet works

 

wounds and injuries - implements and weapons

 

The following video examines a handgun. When you watch it, consider what sort of injuries it might cause when fired at a person.

 

a gun for everyone: infographic

see the full-sized version of this at online schools.org

smooth bore weapons

 

 

Pump action Winchester Model 1897 Shotgun 12 Gauge full choke

Source: Adams Guns

 

As the name suggests, these weapons do not have rifling of their barrel, and are designed for hunting game at closer distances than rifles. They fire multiple missiles, in the form of ‘shot’ (pellets) from a smooth barrel, and again they can be of many different types – single shot, double barreled, pump action and auto loading etc.

Shotguns are not referred to in terms of calibre, but ‘gauge’. This is archaic term used to describe the quantity of lead shot capable of fitting in the diameter of the barrel, and has left us with the ’12 gauge’ to ’20 gauge’ shotguns etc. They may have 1 or 2 barrels.

As a shotgun is fired, a mass of pellets leaves the weapon, and given enough distance, will spread out in a cone-like fashion. The barrel dimensions, amongst other factors, affect the amount of spread and one way in which the spread can be controlled is by the amount of ‘choke’. This represents a partial constriction of the muzzle end of the barrel, and can be manipulated to reduce spread by means of narrowing the barrel diameter (by increasing the amount of choke). In some weapons barrels of varied choke can be interchanged, but on most it is fixed.

 

Standard shotgun bore diameter

Source: Winchester

smooth bore firearm ammunition

 

Shotgun ammunition

Shotguns fire pellets (or rarely a single pellet) that have been loaded into a ‘shell’. This consists of a metal (often brass) base with a plastic cylinder crimped shut at the end. The shot is separated from the gunpowder by plastic or cardboard ‘wadding’.

 

Cut-away of Winchester shotgun shell

Source: Winchester

 

At discharge, the wadding follows the mass of shot, and may follow the pellets into a wound track if at close range, or may strike the target surface just wide of the wound at increasing distances.

Eventually the wadding will fail to travel far enough and will fall to the ground. If a separate injury is caused by wadding, one may see a cross-shaped abraded area corresponding to the opened out wadding material.

 

Air guns

Air guns are weapons designed to use the expanding force of compressed air to propel a projectile down a rifled barrel. They are often used by children as 'toys', and are frequently the cause of injury. Aslan et al (2006) report that airgun injuries occur frequently, such that there are 30,000 air weapon injuries in the US each year.

Fatalities have been recorded, and Monticelli et al (2002) note that the majority of these cases involve children under the age of 16 years (boys>girls), and that the most common parts of the body to be penetrated by the air pellets are the eye, temple or forehead, but pellets are also known to have penetrated the nasal septum. Milroy et al (1998) indicate that a person dies from air powered weapon injuries each year in the UK.

 

captive bolt pistols/ humane killers

 

Captive bolt pistols are variously described as ‘tools’, ‘industrial instruments’ or firearms, used in the ‘stunning’ of large food animals during the slaughtering process. 

 

Cash Special penetrating-type captive bolt pistol

Source: Accles and Shelvoke Limited, Birmingham, UK

 

 

Cash Knocker percussion stunner

Source: Accles and Shelvoke Limited, Birmingham, UK

 

‘Penetrating-type’ models are available that propel a hollow-tipped bar or ‘bolt’ into the head in order to produce ‘insensibility’ prior to a definitive killing procedure such as severing the jugular veins.

Fatalities associated with captive bolt pistols have been reported in the literature, but appear rare. Although accidents and suicide are well described, few case reports of homicide exist.

 

penetrating skull injury - captive bolt pistol

 

Penetrating head injury - captive bolt pistol

 

clinical documentation of gunshot wounds

 

9mm handgun - source: Chicken Billy

 

The documentation process is of crucial importance. Handwritten notes and diagrams are best supported by photographs (with and without a scale), and the use of pre-printed body diagrams may also be beneficial.

These methods of documentation must be made at the earliest opportunity before surgical intervention, for example, or the ‘cleaning up process’ in the emergency department are carried out, after which features of the wound may be irretrievably lost.

Any clothing or trace evidence identified and recovered should be kept in a locked and secured place until they can be personally handed to a police officer/ forensic scientist (or forensic physician), and the ‘chain of evidence’ preserved.

Doctors in the UK have recently been issued guidance by the General Medical Council (GMC) regarding the reporting of persons injured by gunshot wounds to the police (GMC 2003).

In essence, all such injuries should now be reported, so that the police can then assess the risk posed to the public, staff and the injured person from a further attack. Access to the injured person by the police is, however limited where such access might hamper further treatment.

The guidelines also clarify situations where patient confidentiality might need to be broken, in cases where the public interest over-rides this ethical principle.

 

Factors to consider:

  • The condition of any clothing, and in particular the presence or absence of ‘trace evidence’ such as gunpowder residues etc
  • The features of the wound
    • The size, shape and associated features (caused by ‘secondary projectiles’)
  • The anatomical location of the wound (best measured from fixed anatomical landmarks such as the sternal notch, tip of the shoulder, angle of the jaw etc, rather than the umbilicus or nipples)
  • The location of the wound relative to the midline, and the crown and heels
  • The location and description of the exit wound(s) if present
  • The anatomical structures which may have been traversed
  • Distal pulses/ perfusion
  • Presence of fractures
  • Degree of wound contamination (e.g. by bits of clothing/ debris etc)

 

 

Ancilliary imaging techniques

The use of radiography when assessing wounds made by bullets and pellets is of vital importance. Good quality radiography can assist in the location of bullets and their fragments, as well as the number of projectiles in the body. It can also assist in the location of ‘bullet emboli’ that occur when, for example a shotgun pellet enters a large vessel, and is transported by the flow of blood to a part of the body distant to the gunshot wound or track.

Both anterior/ posterior and lateral films should be taken to give the best available information from which surgical or autopsy strategies can be planned.

 

Computer modelling and innovative imaging techniques

Subke et al (2002) describe a method of developing 3D ‘models’ of gunshot wound victims using animation programs (Poser ®) in order to reconstruct gunshot wound tracts and thus determine possible mechanisms.

The combined modelling of injury data and anatomical values (e.g. The range of movement for limb joints etc) allows for accurate reconstruction of events providing valuable information as to causation i.e. was the wound capable of being self inflicted or not?

In order to simulate the effects of gunshot wounds on the human head, and provide data that are comparable with wounds seen in practice, Thali et al (2002) have developed a ‘skin-skull-brain model’. This model incorporates a silicon cap (the artificial scalp); a layered polyurethane sphere (the artificial skull) and ballistic gelatin (the artificial brain).

This model allows for greater understanding of the ‘terminal ballistics’ of gunshots striking the body, and they hope that the use of this model will help to improve the accuracy of wound description by means of an increased understanding of the mechanisms behind the injuries.

The artificial skin incorporates synthetic fibres to simulate collagen and fat in the scalp, and the artificial skull is layered to represent the internal, external and diploe layers of the human skull.

The use of multi-slice Computed Tomography (MSCT) and Magnetic Resonance Imaging (MRI) for the autopsy documentation and analysis of gunshot wounds and related soft tissue and bone trauma is described by Thali et al (2003).

The technique identified the 3D reconstruction of facial features and the skull, visualisation of fracture patterns (including the inward bevelling of entrance wounds in the skull, and the outward bevelling of exit wounds), visualisation of brain contusions, lacerations and bullet tracts (with bullet fragments and bone splinters visible). In addition, bullets could be localised for autopsy retrieval, and peripheral effects of gunshot wounds documented, such as pneumothorax, gas embolism and blood aspiration into the lungs.

 

references

 

 

 

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