ballistics and mechanisms of tissue wounding

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Ballistics and mechanisms of tissue wounding

Laith A. Farjo, M.D., Theodore Miclau, M.D.

Department of Orthopaedic Surgery, University of California, San Francisco, San Francisco General Hospital, San Francisco, California, U.S.A.

Summary’

The wounding potential and mechanisms of tissue damage of firearms are reviewed. Internal ballistics is the study of projectile flight within a firearm while external ballistics describes projectile flight through air to the target. Terminal ballistics characterizes the final effects of the bullet after it has impacted its target. Upon impact, three tissue phenomena are noted: sonic wave formation, temporary cavitation, and permanent cavitation. Subsequent tissue damage is dependent on the characteristics of the firearm that fired the projectile (e.g. rifle versus handgun), the nature of the projectile itself (e.g. fully jacketed versus expanding bullet), and the attrib- utes of the target tissue (e.g. tissue elasticity).

Keywords: ballistics, firearms, bullets, gunshot wounds

Introduction

Ballistics is the study of the firing, flight, and effects of projectiles. It may be classified into three subgroups: internal ballistics, or the study of projectile firing; external ballistics, or the description of projectile flight; and terminal ballistics, or the science of the projectile effect on the target. There are many variables that describe the wounding potential of firearms, including weapon type and design, bullet type, and target tissue characteristics.

1 Abstracts in German, French, Italian, Spanish and Japanese are printed at the end of this supplement.

Internal ballistics

Internal ballistics describes projectile flight within the weapon. Despite advances in bullet design and the introduction of more powerful and fully automatic weapons, firearms continue to operate on long-estab- lished principles (1). The firearm is loaded with a cartridge that contains explosive primer, gunpowder, and the bullet. After the trigger is released, it drives a firing pin into the cartridge which contains the primer at its base. The spark created by this process ignites the gunpowder, which then propels the bullet down the barrel. During its path in the barrel of the weapon, the bullet begins to spin as it traverses grooves machined into the barrel. These grooves are termed the ‘rifling’ of the weapon.

There are three basic determinants of the exit velocity of the bullet. The first determinant is the mass of the bullet: the greater the bullet’s mass, the harder it is to propel with the same amount of gunpowder (2). The second determinant is the amount of gunpowder in the cartridge: the larger the amount of gunpowder, the greater the explosion. ‘Magnum’ versions of weapons are those which utilize a bullet of the same size and weight, but contain more gunpowder (3). The amount of gunpowder placed in a cartridge is limited, however, to the strength of the barrel and the amount of recoil produced (excessive recoil may make a weapon diffi- cult to use) (4). The final determinant of muzzle or exit velocity is the length of the barrel. As soon as the bullet exits the firearm, the gaseous pressure that propelled it forward quickly dissipates. The bullet then decelerates as it faces the effects of atmospheric drag (1). This is one of the reasons that rifles, with their longer barrels, are able to attain significantly higher velocities than handguns.

Favjo and Miclau: Ballistics and tissue wounding

External ballistics

External ballistics describes the flight of the projectile through the atmosphere as it travels towards its target. In this phase, the bullet is in a state of deceleration due to the drag effect of the atmosphere. The amount of drag in air is directly related to bullet speed; faster bullets are retarded proportionally more than slower bullets (2). The spin imparted by the rifling of the barrel helps to stabilize the bullet during its course. Heavier bullets tend to be more stable in flight (1). In addition, the bullet undergoes several complex motions during its path. ‘Yaw’, describing the angle of the long axis of the bullet with respect to its flight path, occurs as the bullet rocks back and forth on its centre of gravity (Fig. 1) (5). In air, the yaw angle is only l-3 degrees (6). Yaw becomes more significant once the target is struck.

Terminal ballistics and tissue wounding

One of the classical determinants of tissue wounding is the release of kinetic energy to the target (4). Kinetic energy (KE) is defined as

KE = MV2

where M = mass and V = velocity. The total energy released to the target can therefore be represented by

A KE = KEentry - KEexit

From this formula, it is evident that a higher entry KE (e.g. by increasing bullet mass or velocity) has a higher potential for wounding (7). However, if the exit kinetic energy is still high, then relatively minor tissue damage may result. For example, a bullet that passes cleanly through the target and exits with a significant velocity will not cause as much damage as a bullet with similar entry kinetic energy that completely decelerates and comes to rest within the target (8). In addition, projectiles with a higher impact velocity have a greater drag within the tissues, and hence have a greater A KE (1).

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It should be noted, however, that transfer of kinetic energy is not the singular determinant of tissue wounding. Interactions between the projectile and tissue play a major role in influencing the resultant amount of de- struction (6). Three tissue phenomena are noted when the target is struck: 1) sonic pressure waves, 2) permanent cavity formation, and 3) temporary cavitation (Fig. 2) (1,5,6,9-11).

Sonic waves

A sonic pressure wave, travelling at approximately 4800 feet/second (the speed of sound in water), precedes the projectile following impact (9,12). Harvey, in a series of elegant experiments performed in 1947, showed that these pressure waves, although producing pressures up to 117 atmospheres, lasted only a few microseconds. He was able to isolate the sonic pressure wave from the temporary and permanent cavity effects and show that it did not have any significant disruption capacity on tissue, including beating frog hearts (9). Indeed, modern lithotriptors produce stronger sonic pulses without any significant soft tissue effects (11). Recently, investigators have re-examined the issue of sonic wave contribution to wounding, demonstrating that sonic waves may con- tribute to the disruption of neural function, even at sites of the body distant to that impacted by the bullet (13- 16). However, their data has been criticized because they failed to completely isolate the effects of the sonic wave from the temporary cavity (11).

Permanent cavity

The permanent cavity produced by bullet entry consists of that tissue which is crushed by the bullet. Fackler et al. have demonstrated that different firearms can produce markedly different permanent cavities depending on the interaction of the bullet with the tissues (Fig. 3) (6,lO). His experiments were performed by firing vari- ous weapons into gelatin blocks and then examining the

Line of

Yaw angle 2 Fig. 1: Yaw is the angle between the long axis of the bullet and its path of flight. In this figure, the yaw angle is exaggerated for illustrative purposes. It typically averages 1-3 degrees for most bullets travelling through air (6).

Injury 1997, Vol. 28, Suppl. 3

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---_-. /” Sonic Wave . Temporary Cavity

---___ --%._

-%

Permanent Cavity

High Pressure

Zone

Fig. 2: Tissue pressure phenomena caused by bullet entry. All of the illustrated effects are variable, depending on tissue characteristics and bullet mass, velocity, and deformation. The sonic wave precedes the bullet at the speed of sound in tissue at approximately 4800 feet/set- ond. A localized area of high pressure is noted at the bullet tip (9). The temporary cavity expands radially out- ward behind the bullet and the permanent cavity is the tissue that has been crushed by the bullet.

resultant damage. The volume of the permanent cavity was shown to be influenced by several factors.

As the projectile strikes its target, the stabilization effect of its spin is quickly overcome by the density of the tissue. The bullet can yaw to 90 degrees and often may come to rest at a 180 degree angle to its initial path (1). The yaw growth in the target is directly related to the yaw at entry (17). This process of tumbling within the

target significantly increases the destructive capacity of the projectile. As the bullet yaws within the target tissue, it increases its effective diameter, with a maximum reached at 90 degrees of yaw, This explains why a de- ceptively small entrance wound can coexist with massive internal damage. Different bullets yaw at different tissue depths; for example, faster, more stable bullets tend to yaw deeper into the target (1).

The ratio of bullet size to velocity also influences tissue effects. A large, relatively slow projectile tends to crush tissue in its path with little radial dispersion of energy (temporary cavity formation). A small, fast bullet, on the other hand, tends to crush little tissue and cre- ate more transient radial tissue stretching (6). Because of these phenomena, it is evident that two bullets of similar kinetic energy can have markedly different wounding potentials. Bullet deformation and fragmentation are significant contributors to permanent cavity formation. Bullet design has a major influence on these effects and will be discussed below.

Temporary cavity

The temporary cavity is caused by the release of energy into structures adjacent to the bullet path causing a radial stretch of these tissues. The cavity is created behind the path of the bullet, creating pressures between 4 atmospheres and subatmospheric pressure (9). The size of this cavity can be up to 30 times the volume of the missile (6).

The effect of the temporary cavitation will depend on the elasticity of the tissue struck. Highly elastic tissue

I - I ?A32 mm NATO

Vet-2830 fh, (ES2 mts) Wt-15Ogr (0.72m) FMC

Psrmenent Csvity

Fig. 3: Wound profile (lo), created by firing a 7.62 mm NATO full metal jacketed bullet at 2830 feet/second into or- dinance gelatin. Note the yaw of the bullet to 90 degrees in tissue, then coming to rest at 180 degrees to its entry path. This tumbling markedly increases the dimension of the permanent cavity. Another factor in wounding is that the depth of maximal yaw, 28 cm in this case, may occur after the bullet has already passed through a thin target, such as a human body struck directly anteriorly, or a limb. Also note the radial expansion of the temporary cavity around the bullet path.

Favjo and Miclau: Ballistics and tissue wounding

such as lung will easily accommodate the stretching created by the temporary cavity. Other, less elastic structures, such as liver, or a structure contained by an in- elastic tissue, such as the brain, may be seriously dam- aged by this wave. Investigators have shown that a temporary cavity produced in close proximity to bone can cause it to shatter and propel many ‘secondary missiles’, thereby increasing tissue damage in a fashion similar to the permanent cavity (18).

Due to the variability of damage secondary to the temporary cavity, clinical judgment must be utilized in determining the actual extent of tissue damage. For the most part, the surgeon should concentrate on tissues that are clearly not viable and perform a re-exploration of the tissues that may have been injured by secondary effects. Certainly, with the often massive size of the temporary cavity produced by modern day firearms, exci- sion of the entire cavity is not only unwarranted, but also is often impossible (19).

Firearm types

There are three basic types of firearms: rifles, handguns, and shotguns. Rifles are the most powerful and are known for their longer barrels and higher velocities when compared to handguns. While great emphasis has been placed on the classification of a weapon as high or

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low velocity, the distinction is quite vague, with values ranging from 1100 to 2500 feet/second (2, 11). Most rifles generate muzzle velocities greater than 2500 feet/second. These high energy weapons are designed to be held with both arms, thereby increasing their tol- erable recoil energy limit. An M-16 military rifle, for example, generates a velocity of 3,240 feet/second for a 55 grain bullet (20). Rifles have increased wounding pow- er, increased accuracy at long distances, and the capacity for high speed automatic fire. These benefits are at the expense of difficulty in concealment because of increased weight and size and increased recoil which makes them harder to shoot.

Handguns are the least powerful of the firearms. Be- cause of their shorter barrels, they have decreased velocity, kinetic energy, and accuracy when compared to rifles. Typically, the maximum velocity attainable is approximately 1,500 feet/second (20). ‘Caliber’ refers to the diameter of the bullet, in thousandths of an inch. Hence, a ‘.357 magnum’ fires a 0.357 inch diameter projectile at 1450 feet/second, a higher velocity than a stan- dard .357 handgun. Handguns are available in two main designs - revolvers and pistols. Revolvers have a cylinder which usually holds six cartridges and requires re- peated pulling of the trigger to fire the weapon - the hammer is cocked either manually (single action) or au- tomatically by pulling the trigger (double action). Au- tomatic pistols contain a magazine of cartridges which

Bullet hatlmsnts

Parmsnsnt Cavity

‘

7.02mm SP vel - 2922 t/s (891 mfa) wt-tsogr (wgmj

Flnal wt -02.7 gr (5.M am) 98.4% fragmentation

mporaty Cevtty

cm

Fig. 4: Wound profile of a soft-point version of the 7.62 mm NATO bullet shown in Figure 3, fired at a similar velocity (10). Note the massive increase in permanent cavity formation caused by bullet deformation and fragmentation. The maximum dimension of this cavity is also formed much closer to the surface, 14 cm versus 28 cm, than the fully jacketed bullet.


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typically holds 9 to 19 rounds and can be fired by re- peatedly pulling the trigger. Handguns are favored in urban warfare because they are easily concealed and are effective at short ranges; most shootings occur within 7 yards of the target person (20).

Shotguns, depending on the range from the target and the size of the pellets fired, may cause minor to massive damage. A shotgun shell consists of a cylinder with primer and gunpowder at its base. The projectile por- tion of the shell, the pellets, are separated from the gunpowder by plastic or cardboard wadding. This wadding can frequently be found in wounds inflicted at short range. Shotguns are classified by ‘gauge’, which indi- cates the weight of the pellets fired by the weapon in fractions of a pound. For example, a 12-gauge shotgun shoots pellets that weigh 1/12th of a pound each. The barrel of a shotgun is smooth and is constricted by a ‘choke’ on the muzzle which narrows the exit diameter and therefore helps decrease the spread of the pellets in flight. ‘Sawing off’ the end of the shotgun serves both to increase the spread of the shotgun pellets for use in close-range combat and to make the weapon easier to conceal (21).

Bullet design

As noted above, bullet interaction with tissue plays a major role in determining the wounding capacity of any given firearm. Military bullet design is governed by the Hague Convention of 1899 which mandated that these bullets be surrounded by a full metal jacket (1). This jacketing consists of a hard metallic outer coating, such as copper, which surrounds the softer inner bullet which consists of lead. This allows for higher weapon velocities, since unjacketed lead cannot be fired faster than 2000 feet/second because of stripping of the lead within the barrel of the gun (5). Because of the jacketing, these bullets will not fragment in tissue as much as an unjacketed bullet. Bullets available to civilians, however, do not have to be jacketed. Because bullet fragmentation can have a significant adverse effect on tissue, it is often quite possible to see much more devastating wounds with weapons fired from civilian firearms (Fig. 4).

Bullet manufacturers have introduced new bullet designs which are designed to either deform or fragment within the target. These designs provide maximum de- liverance of kinetic energy and increase in the size of the permanent cavity. Hollow-point bullets expand to double the bullet diameter on impact (22, 23). Shot shells such as the ‘Glasser Safety Slug’ contain pellets in the bullet tip which disintegrate upon impact. Bullets such as the ‘Devastator’ (utilized in the President Reagan assassination attempt) explode upon impact and can produce massive internal damage. The Safety Slug and Devastator were developed for use by law enforcement

officials in situations where innocent bystanders near the target victim may be endangered by passing of the bullet through the intended target. Little is known about the risk of in-vivo retention of undetonated exploding bullets; however, extreme care must be taken during their removal because simple manipulation can cause them to detonate (22).

Concluding remarks

The primary factors in tissue wounding include the amount of kinetic energy delivered to the subject and tissue-bullet interaction. Sonic waves produced by projectile impact do not significantly affect most organ systems. The size of the permanent cavity produced, which is the most significant component of tissue damage, de- pends on bullet energy, tumbling, expansion, and fragmentation within the target. The temporary cavity produced by bullet impact can be large, but for most elastic tissues causes only minor damage.

Of the three basic firearms, rifles have the greatest wounding capacity at long distances. Shotguns are ca- pable of producing massive wounds at close ranges. Handguns, particularly with the advent of deforming and fragmenting bullets, can also induce massive wounding and can be easily concealed.

Future research in ballistics will include the mathe- matical and statistical modelling of bullet flight and tissue damage, the characterization of the true injury caused by the temporary cavity and sonic waves, the further study of alternative projectile systems (such as fragmentation devices), the definition of the biological response to projectile wounding, and the continued evaluation of the newest available firearms in modern day civilian and military warfare (17,24,25).

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DeMuth WE, Jr. Ballistic characteristics of ‘magnum’ sidearm bullets. J Trauma 1974;14:227-9.

DeMuth WE, Jr. Bullet velocity and design as determinants of wounding capability: an experimental study. J. Trauma 1966;6:222-32.

Hollerman JJ, Fackler ML, Coldwell DM, Ben-MenachemY. Gunshot wounds: 1. Bullets, ballistics, and mechanisms of injury. Am. J. Roentgenol; 1990;155:685-90.

Fackler ML. Wound ballistics. A review of common mis- conceptions JAMA 1988;259:2730-6 [published erratum appears in JAMA 1988 9;260(22): 32791.

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Cheng XY, Feng TS, Liu YQ, et al. Wounding properties of steel pellets with different velocities and quality on soft tissue of dogs. J. Trauma 1988;28:S33-6.

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Swan KG, Swan RC. Principles of ballistics applicable to the treatment of gunshot wounds. Surg. Clin. North Am. 1991;71:221-39.

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Address for correspondence: Theodore Miclau, M.D., San Francisco General Hospi- tal, 1001 Potrero Avenue, 3A36, San Francisco, CA94110 USA

10. Fackler ML, Bellamy RF, Malinowski JA. The wound profile: illustration of the missile-tissue interaction. J. Trauma 1988;28:S21-9.

11. Fackler ML. Gunshot wound review. Ann. Emerg. Med. 1996;28:194-203.

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13. Suneson A, Hansson HA, Kjellstrom BT, Lycke E, Seeman T. Pressure waves caused by high-energy missiles impair res- piration of cultured dorsal root ganglion cells. J. Trauma 1990;30:484-8.

14. Suneson A, Hansson HA, Seeman T. Peripheral high- energy missile hits cause pressure changes and damage to the nervous system: experimental studies on pigs. J. Trau- ma 1987;27:782-9.

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16. Suneson A, Hansson HA, Lycke E, Seeman T. Pressure wave injuries to rat dorsal root ganglion cells in culture caused by high-energy missiles. J. Trauma 1989;29:10-8.

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18. Amato JJ, Syracuse D, Seaver PR, Jr., Rich N. Bone as a secondary missile: an experimental study in the fragmenting of bone by high-velocity missiles. J. Trauma 1989;29:609- 12.

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ballistics and mechanisms of tissue wounding

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