Ballistic Injuries

12th July 2023

As with blast injuries, the injuries sustained from the gunshot are not unique but the mechanisms involved are complex; blood loss, tissue damage, fracture and infection can usually be managed but the overall management of the injury may be complicated by complex factors involved.

Wounds are typically classified as low velocity (<2000ft/s or < 600m/s) or high velocity (>2000ft/s or 600m/s) but other factors include the physical characteristics of the projectile including deformation and fragmentation, kinetic energy, stability, entrance profile, the path traveled through the body, tissues, and organs involved and the exit wound if one exists.

There are a number of myths surrounding ballistic injuries which also need to be debunked.

 

Firearms

Most ballistic injuries in both civilian and military environments are from ‘small arms ‘ classified as <0.6 / 15mm calibre, which include handguns, rifles and carbines.  Although handguns are typically low-energy weapons with muzzle velocities below 420m/s they are the most frequently used firearm in fatal and non-fatal injuries (1-3)

Carbines (shorter rifles), can also be low-velocity such as the sporting .22 (330m/s) or high velocity; the M4 carbine has a barrel length of 14.5 inches and a muzzle velocity of upward of 838m/s using a NATO 5.56 or 7.62 round.

Full-length rifles such as the M16 with a 20 inch barrel fires the 5.56 at a muzzle velocity of 960m/s

Assault rifle bullets typically can over-penetrate the victim if they are within a range of a few hundred meters (4, 5) and often remain intact (4-7)

 

Anatomy of the Round

The round consists of the bullet at the front which is pressed into the casing.   The casing is filled with the propellant, typically nitrocellulose, which is ignited by the primer when struck by the firing pin.

On ignition the primer produces an intense flame which enters the main chamber of the round, through the firing hole, igniting the propellant.   This violent ignition can produce temperatures as high as 5,200oC and pressures as high as 340psi which propels the bullet forward. (4)

The expanding force of the propellant accelerates the bullet along the barrel, therefore the longer the barrel, the greater the velocity of the bullet (7).   Depending on the characteristics of the bullet, it will continue to accelerate even after it has left the barrel. (8)

Once in flight, the inertia acts on the centre of mass which lies along its line of flight.  The opposing forces or air resistance act on the centre of pressure, which lies in front of the centre of mass.  As these opposing forces do not act on the same point there is inherent instability causing the bullet to tilt or “yaw”.

To minimise this, the bullet has greater stability if it is long and thin as will as rotating which is achieved by helical grooves on the bore of the barrel of the firearm ‘rifling’.

This rotational force on a bullet that is unstable would describe a spiral shape in the air called precession.  This is coupled with a second, higher frequency but lower amplitude force (nutation) which causes the bullet to actually spin in a rosette pattern a bit like a spinning top.  (3)

 These motions reduce over time to a negligible pattern over a greater distance. (8, 9)

 

Anatomy of the Bullet

Bullets are composed primarily of lead or steel (combined with other metals to achieve a desired hardness) and covered with a jacket typically made of cupronickel or a gilding metal such as copper or zinc.  The jacketing is required because at velocities over 600m/s lead is stripped from the bullet in the barrel.

The ‘Full Metal Jacket’ completely covers the tip of the lead core and is generally used in assault rifles. 

Partial metal jacketing with a cavity at the tip (hollow point) cause the bullet to ‘mushroom’ on impact or once inside the body.   Interestingly this ammunition is considered inhumane and was made illegal in warfare by the Hague Peace Conference in 1899 due to the unnecessary suffering caused by the wounding yet hollow point bullets are routinely used by law enforcement due to their reduced likelihood of over-penetrating the target and causing injury or death to persons behind the target.

Where some full metal jacket bullets are manufactured by extruding a brass disc over the lead core from the tip, leaving the base exposed.   Partial metal jackets are base extruded – the brass is formed over the base, converging at the tip but not sealing the tip. 

The Open Tip Match bullet is a competition bullet that is also base-extruded; the tip is ‘almost’ completely sealed but not quite.   It is illegal for hunting because it behaves like a hollow point and causes unnecessary suffering, but it is not classified as a ‘hollow point’ so is used in warfare by NATO military personnel.   So it behaves like a hollow point which is illegal.  But it isn’t a hollow point so it isn’t illegal.  Even though it has a hollow point…

Other bullets are designed to fragment or even explode but are rarely seen in civilian or military situations.

Full Metal Jacket

Hollow Point

Open Tip Match

The physical properties of the bullet also have a function on tissue damage; the AK-47 round (7.62×39mm) has a weight of 7.9-10.0g and has a tendency to yaw late, enter the body head-on and leave a more linear wound track through the body.  Conversely, the light AK-74 round (5.45×39mm) has a weight of 3.43-5.2g and tends to yaw early, resulting in more severe injury, a wider injury tract and less likely to over-penetrate.

The M193 ball round is a 5.56x45 NATO standard bullet but is considerably more fragile than the similar SS109/M855 NATO and is likely to fragment in soft tissue, possibly remaining in the body even when no bone is hit. (4, 6, 10-16).

 

Velocities

The wounding capacity of the bullet is not solely related to its kinetic energy at impact but rather a multitude of factors including:

  • the mass, shape and construction of the bullet

  • the bullets trajectory, stability and entry profile

  • the distance and path the bullet has traveled within the body

  • variations of tissue density within the path of travel

  • fragmentation and deformation

But certainly, velocity is a significant factor due to the Kinetic Energy (KE) of the projectile, determined by the formula  = ½ Mass x Velocity^2.   Before World War II bullet and weapon construction focussed on mass, favouring heavier projectiles of large calibre.  Although a projectile’s velocity and mass are inseparable, a greater mass only increases the KE linearly whereas an increase in velocity increases the KE exponentially.   For example, if the mass of the bullet is doubled, the KE is doubled.    If the mass is increased three times, the KE increases three times.    But if the velocity is doubled, the KE is increased by a factor of 4 (2x2).  If the velocity is increased three times, the KE is increased by a factor of 9 (3x3).  As such, modern firearms focus on lighter, spin-stabilised projectiles which are fired at greater velocities.

Projectiles of the same calibre and mass at higher impact velocities generally create a larger entry wound and larger wound channel (17), increased incidence of indirect bone fracture (18), and increased severity of tissue damage (16, 19, 20).

Low-velocity wounds, from handguns, for example, are generally considered to be less severe and more common in civilian settings.  But to only focus on muzzle velocity (the speed at which the bullet exits the barrel) is misleading because the striking velocity does not necessarily correlate with the resulting amount of tissue damage. (10, 21-23) and there is evidence that the wounds caused by military rifles potentially can cause no greater tissue damage than low-velocity weapons (6, 10, 13, 24-29), and neither should one assume that low-velocity bullets cause only superficial injury (25, 30).

The Gunshot Wound

Once the bullet has penetrated the skin at speed, its small cross-sectional area causes comparatively little damage immediately in its path unless it impacts directly with a major blood vessel or organ.  The significant damage is caused by the transference of the bullet’s kinetic energy displacing surrounding tissues causing crushing, stretching, and tearing injuries to soft tissue.

This rapidly expanding cavity is temporary and will rebound, imploding on itself causing a secondary insult on the tissue. 

The volume of the temporary cavity is directly proportional to the kinetic energy of the bullet (8, 31) and also the shape of the bullet:  In one study, wound profiles from high-velocity bullets in gelatine models appeared to be approximately 17-30 times that of the diameter of the bullet (12) with 5.56 rounds causing cavities of 24 times that of their diameters in another study (15).  Because the temporary cavities are largest when the bullet reaches a maximum yaw of 90o (travelling sideways) (10, 25, 26-28, 32-35) the length of the bullet is possibly more of a significant factor in predicting the amount of internal damage from temporary cavitation.   In experiments with steel balls (which cannot yaw as their profile is identical in all orientations, temporary wound cavities were found to be only 12.5 their diameter (25).  

Consider a 9mm bullet which has a length of 16mm, at maximum yaw its profile is only 1.7 times longer than its diameter at 0o yaw.  The 5.56mm bullet has a length of 19.3mm giving a profile of 3.5 longer than its diameter, causing greater temporary cavitation at maximum yaw.

9mm bullet leaving an almost parallel wound track despite rotating 180o.

0.45 bullet leaving an almost parallel wound track despite rotating 90o.

Also entering the wound is the tailwind, dragged in by the vacuum immediately behind the bullet.

Imagine the turbulence you notice when a large vehicle drives passed you; the front of the vehicle is pushing air in front of it around vehicle which then converges turbulently behind it (due to the flat rear end of the profile.)  An aerodynamic sports car does not cause as much turbulence and causes less of a vacuum behind it.

Whilst the large vehicle may have a massive cross-sectional area, it is only travelling at 30 miles per hour (13m/s).    The bullet has a tiny cross-sectional area so displaces less air, but still has a flat rear end which will cause a vacuum behind its flight path and is travelling 60 times faster.  This vacuum, especially with high-velocity firearms, will drag air and debris into the wound.

If the bullet has passed through and exited, further air and debris is dragged into the wound from the exit wound if the temporary cavity is still expanding (4, 8, 29, 31, 36-39).

After the temporary cavity has collapsed, rebounding with diminishing amplitude, a smaller permanent cavity remains, (9, 28, 38, 40) primarily made up of tissue damaged tissue from the direct impact with the bullet.   The more the elastic capacity of the tissues is exceeded the larger the permanent cavity.

 

 Tissue density

Of course the human body is not of homogenous and of uniform density. Less dense and more elastic tissues such as skin and lung would expect to sustain little residual damage (4, 7, 31, 35, 40-44) where denser, less elastic organs such as the brain, liver and spleen, as well as fluid filled organs including the heart, bladder and digestive system will sustain much greater damager as the transferred kinetic energy exceeds the tissues elastic limits causing shearing and tearing.

Although capillaries are prone to rupture, larger arteries (unless directly struck) are remarkably resistant to indirect injury (8, 42).

The damage to bone depends upon the shape and energy of the bullet as well as the type of bone (comparing the dense, rigid femur to the narrower, more malleable rib, for example).  The clinical and radiographic appearance of bone damage is typically a punched out hole with sharp edges in small bore, high-velocity bullets where as larger, lower-velocity bullets are likely to cause more fragmentation (5, 7, 15).

These fragments can be propelled forwards due to the expanding forces of the temporary cavitation and behave like secondary missiles (6, 7, 9, 15, 29, 42, 45) although more commonly they retract quickly back to the parent bone as the cavitation collapses (4).

Contamination

Contrary to popular belief, the heat and air pressures exerted on the bullet as it discharges do not ‘sterilise’ the bullet (4, 8, 36, 46-49) and shotgun wadding has a particularly high association with wound contamination (47, 50).

Additional sources of infection come from clothing fragments, skin flora, bodily fluids and tissue of other casualties, airborne dust and pollutants, which are sucked into the wound.  Bowel injury has a significant risk of sepsis. (37, 51, 52)

Since 2017 the US CoTCCC has included prophylactic moxifloxacin as part of the Combat Wound Medication Kit (or “Pill Pack” ) as a point-of-injury treatment for casualties who are still able to fight and has reduced mortality and morbidity compared to those who did not receive immediate prophylactic antibiotic treatment. (53)

 

Exit wounds

Contrary to popular belief the exit wound is not necessarily larger than the entry wound; if the bullet loses enough kinetic energy, it may not exit the body at all.   If it does exit the body it may be as small as the entry wound if the bullet does not yaw and does not lose significant kinetic energy.

FN 5.7mm bullet does not have enough mass or energy to penetrate completley but the velocity causes a much larger temprary cavity.

 

Shotguns

Shotguns fire a few to hundreds of pellets (shot) at a velocity of 300-450m/s.   The damage from shotgun injuries is based on the choke (a partial constriction at the end of the muzzle that widens or narrows the column of shot on exit) load, barrel length (a minimum of 24 inches in UK Law and 18 inches in US law), bore, wadding, powder charge, range and the type of ammunition used.

The most common types of shotgun ammunition is ‘birdshot’ which may contain hundreds of small pellets or ‘buckshot’, used for larger game, which may contain only a few larger, heavier pellets.

More esoteric ammunition such as slugs ( a singular large projectile) or flechettes (many tiny ‘darts’ or ‘needles’) can be used but these are rarely seen in either civilian or military shooting incidents.

One of the earliest papers describing a classification system of such injuries from the most extreme (Type III) to benign (Type 0) from 1963 (54) and remains unchanged:

  • Type III
    Under 3 yards (point blank) the size of the shot makes negligible difference to the wound pattern or extent of damage as the tight cluster of pellets essentially acts as a solid column destroying everything in its path. (5).   The presence of soot is evidence of a blast within 1 foot (4).

  • Type II
    Within 3 to 7 years (close range) are almost as severe, often consisting of several parallel wound channels with gross vascular damage and blood loss (5)

    Type II and III injuries are associated with higher rates of bone damage (32%-48%), major soft tissue injury (43%-59%), vascular injury (23%-35%) and peripheral nerve damage (21%-58%).  (50, 55-58).

  • Type I
    Long range (over 7 yards) yields widely scattered birdshot producing many small holes and rarely significant soft tissue injury (59)

  • Type 0
    At maximum range (beyond 20-50 yards, rapidly decelerating pellets create negligible damage (38, 60, 61). Fine birdshot may only yield superficial subcutaneous or superficial injury. Buckshot, in contrast, having the same mass as a 0.22 bullet, can still cause significant damage at greater ranges of up to 150 yards.


Conclusions


These are not new ideas: more than 60 years ago, Dzieman et al warned “the amount of debridement required for a bullet wound should depend more on the damage inflicted than the nature of the missile” (62). Lindsey concluded “Treat the wound, not the weapon” (44)

  1. Prioritise scene safety.
    Immediately after the recognition of gunfire, panic usually ensues.   Consider your safety first, then the safety of others and finally the safety of any casualty.

    Further Reading – Understanding adrenaline.

  2. Get down to skin
    The casualty of a gunshot has experienced a complex mechanism of injury that is impossible to asses without getting down to skin.   Entry wounds can be small and hidden in creases such as the groin or armpit.  If there is an entry wound there may be an exit wound.  If there is, it may not be adjacent to the entry wound.  There may not be an exit wound, but you will only know if you get down to skin.

    Consider these two case studies of casualties who were not fully examined.

  3. Treat gunshot injuries as any other injury.
    Stop all serious bleeding as a priority.  Manage open chest injuries as any other open chest injury.  Head injuries are usually fatal but provide Basic Life Support if resources and safety allow.  Immobilise bone and joint injuries as any other bone and joint injury.

    Do not overcomplicate the treatment unnecessarily.

  4. Recognise what equipment you need
    Guns and tourniquets go hand in hand on most on-line discussion forums but the injury patterns from terrorist attacks and mass shootings do not yield a high incidence of extremity injury.   Management of chest and abdominal injury is more likely.

    Further reading – Understanding injury patterns from terrorist events.

  5. All ballistic injuries require hospitalisation.
    Due to the sinister damage internally which cannot be assessed externally, the casualty will need imaging to identify internal injury and confirm the existsance of embedded projectiles.

  6. Contamination kills.
    Even if the casualty has a relatively simple in-and-out wound, without significant tissue damage, start prophylactic antibiotics early.

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