Myth 3: High-velocity projectiles cause greater injury and more traumatic wounding than low-velocity projectiles

The terms ‘‘high velocity’’ and ‘‘low velocity’’ as they relate to projectiles can be somewhat confusing.Consensus between US and European research does not occur in the literature, with varying definitions correlating to where the study was performed. The US literature designates high velocity as being be-tween 2000 and 3000 ft/s (610 – 914 m/s), whereas studies from the United Kingdom designate the line between low and high velocity projectiles as being 1100 ft/s (335 m/s), which is the speed of sound in air [2]. The earliest recognized entry of high-velocity projectiles having an association with increased wounding potential occurred during the Vietnam War. In 1967, Rich reported in the Journal of the American Medical Association that bullets fired from the M16 rifle inflicted tremendous tissue destruction and injuries upon enemy combatants [17].The muzzle velocity of the projectile shot from the M16 was 3100 ft/s. When coupled with erroneous information published by Rybeck in 1974 and in the 1975 edition of the ‘‘Emergency War Surgery’’manual regarding the size of the temporary cavity caused by the missile, this information led to the common misperception that high-velocity projectiles caused more significant injuries [2,18].Part of the confusion regarding the wounding potential of high-velocity projectiles is caused by misinterpretation of ballistic gelatin model studies.Ballistic gelatin is 10% to 20% gelatin refrigerated to 4 to 10°C and is used as the tissue model for ballistic studies [8,14]. The wound profile diagrams included in this article and others represent the findings of these studies. The validity of the ballistic gelatin model has been confirmed by comparison with human autopsies, although there is confusion in correlating these studies to living patients, because the human body is much more resistant to deformation than gelatin [1,3,14,19]. The effects of skin resistance, clothing, and resistance to separation of the fascial planes cannot be replicated in gelatin. Harvey and colleagues [20] evaluated the two types of pres-sure waves produced by penetrating objects in 1947:the sonic pressure wave and the temporary cavity[2]. The first wave is the sonic pressure wave, some-times referred to as the ‘‘shock wave,’’ and it relates the sound of the projectile striking the target. This wave transmits at the speed of sound (ie, approximately 4750 ft/s [1450 m/s]) and is traveling considerably faster than the projectile entering the target[2]. No temporary cavity is formed with the sonic pressure wave, and in that regard it is analogous to the lithotripsy devices used for renal calculi destruc-tion, with corresponding minimal risks for tissueinjury [15]. Although American and Swedish research-ers have tried to disprove Harvey’s conclusions,no definitive evidence suggests that his findings arein error, and additional studies by French andAmerican researchers support the original findingsof 1947 [2,4,7,21 – 25]. The secondary pressure wave, referred to as the temporary cavity, is formed when the penetrating projectile strikes tissue and the wave radiates away laterally.

After being struck by the projectile, the ballistic gelatin/tissue displays an obvious temporary cavity, which potentially injures tissues such as muscle, vessels, and organs. The clinical significance of this cavity is variable with no real consensus in the literature, and the temporary cavity caused by the M16 in animal laboratory models is much smaller than the approximate 18-cm temporary cavity seen in ballistic gelatin [14]. Dog models indicated that acute tissue injury secondary to temporary cavity formation sustained with high-velocity projectile strikes were no more than 5 cm and were able to resolve within 72 hours [14,26,27].The United States military conducted extensive research into the wounding patterns of projectiles,and the results are summarized in Figs. 4 and 5.Although traditional concepts of ballistics teach that impact kinetic energy is equal to one half the mass ofthe projectile times velocity squared (E = 1/2 MV2),the increased energy transmitted from a high-velocity projectile does not necessarily translate to increased wounding capacity. Military-style ammunition is de-signed to remain intact after striking the intended target, in accordance with the Hague Convention of 1899, and generally is high velocity in nature.Reviewing ballistic studies from the most recent‘‘Emergency War Surgery’’ manual shows that tissue damage in excess of the diameter of the high velocity projectile does not occur until approximately 15 cm into the body of the target when struck with a 7.62-mm NATO round and 25 cm into the body when struck with an AK47 round. The cause for this increase in tissue damage is the backward flip of these projectiles to a reverse trajectory once they strike the body. This change allows for a scenario in which all of the projectile’s potential energy may not be transferred into the target, which decreases the wounding potential and tissue damage. Depending on the site of impact, a high-velocity projectile actually could enter and exit the body before breakage or deformation of the projectile and effectively leave minimal tissue injury. Whereas 1/2 MV2 relates the potential for tissue injury, the determination of actual injury imparted is related to the construction of the projectile, such as a hollow point that allows expansion versus a bullet that resists deformation, and the projectile’s ultimate fate upon striking the target. Fackler [1,2,13] reported extensively that even experienced civilian practitioners who work in urban trauma centers are unable to delineate the injury pat-tern of low- versus high-velocity projectiles by clinical observation

TERMINAL MECHANICS
Before providing an explanation of the JSWB IPT’s results, a brief discussion of small caliber, high velocity terminal ballistics is in order. The small caliber, high velocity bullets fired by military assault rifles and machine guns have distinct lethality mechanisms; conclusions provided here do not necessarily apply to low velocity pistol rounds, for example, which have different damage mechanisms. The performance of the bullet once it strikes the target is also very much dependent upon the bullet’s material and construction as well as the target: a bullet passing through thick clothing or body armor will perform differently than a bullet striking exposed flesh. This study focused on frontal exposed targets. Take an average M855 round, the standard round of “greentip” rifle ammunition used by US forces in both the M4 and M16 series weapons and in the M249 SAW. The 62-grain projectile has an exterior copper jacket, a lead core, and a center steel penetrator designed to punch through steel or body armor. An M16 launches the M855 at roughly 3,050 feet per second, and the M855 follows a ballistic trajectory to its target, rotating about its axis the entire way, and gradually slowing down. Eventually, the bullet slows enough that it becomes unstable and wanders from its flight path, though this does not typically happen within the primary ranges of rifle engagements (0-600m). (For more detailed ballistic discussion, see FM 3-22.9). Upon impacting the target, the bullet penetrates tissue and begins to slow. Some distance into the target, the tissue acting on the bullet also causes the bullet to rotate erratically or yaw; the location and amount of yaw depend upon speed of the bullet at impact, angle of impact, and density of the tissue. If the bullet is moving fast enough, it may also begin to break up, with pieces spreading away from the main path of the bullet to damage other tissue. If the target is thick enough, all of these fragments may come to rest in the target, or they may exit the target. Meanwhile, the impacted tissue  rebounds away from the path of the bullet, creating what is known as a “temporary cavity.” Some of the tissue is smashed or torn by the bullet itself, or its fragments; some expands too far and tears. The temporary cavity eventually rebounds, leaving behind the torn tissue in the wound track – the “permanent cavity.” It is this permanent cavity that is most significant, as it represents the damaged tissue that can impair and eventually kill the target, provided, of course, that the damaged tissue is actually some place on the body that is critical. This is where the balance of factors in bullet design becomes important. Volume of tissue damage is important – which might suggest high velocities to enable the bullet to tumble and fragment sooner, materials that cause the bullet to break up sooner, etc. – but it must also occur in critical tissue. If the bullet immediately breaks up, it may not penetrate through outer garments to reach tissue, or it may break up in muscle without reaching vital organs underneath. The projectile must have enough penetration to be able to reach vital organs to cause them damage. At the same time, it must not have so much penetrating capability that it passes completely through the target without significant damage – resulting in a so-called “throughand-through.” Energy expended outside the target is useless (incidentally, this is why “impact energy” is a poor measure of bullet comparison, as it does not separate energy expended in damaging the target from energy lost beyond the target). The ideal bullet would have enough energy to penetrate through any intervening barrier to reach vital organs without significantly slowing, then dump all of its energy into damaging vital organs without exiting the body. Unfortunately, design of such a bullet is nearly impossible in a military round, even if all human bodies were uniform enough to allow for such a thing. A round that reaches the vital organs of a 5-foot 6-inch 140-pound target without over-penetration is likely to react differently against a 6-foot 2-inch 220-pounder, even without considering target posture. To complicate matters, when hitting a prone firing target the bullet might have to pass through a forearm, exit, enter the shoulder, then proceed down the trunk before striking heart or spinal cord. A flanking hit would engage the same target through or between the ribs to strike the same vital regions. All these possibilities are encountered with the same ammunition. Ultimately, bullet design is a series of tradeoffs complicated by the need to survive launch, arrive at the target accurately, possibly penetrate armor, glass, or other barriers, and be producible in large quantities (1+ billion per year) at costs the military can afford.