Hydrostatic shock

Hydrostatic shock, also known as hydro-shock, is the controversial concept that a penetrating projectile (such as a bullet) can produce a pressure wave that causes "remote neural damage", "subtle damage in neural tissues" and "rapid effects" in living targets. It has also been suggested that pressure wave effects can cause indirect bone fractures at a distance from the projectile path, although it was later demonstrated that indirect bone fractures are caused by temporary cavity effects (strain placed on the bone by the radial tissue displacement produced by the temporary cavity formation). Proponents of the concept argue that hydrostatic shock can produce remote neural damage and produce incapacitation more quickly than blood loss effects. In arguments about the differences in stopping power between calibers and between cartridge models, proponents of cartridges that are "light and fast" (such as the 9×19mm Parabellum) versus cartridges that are "slow and heavy" (such as the .45 ACP) often refer to this phenomenon. Martin Fackler has argued that sonic pressure waves do not cause tissue disruption and that temporary cavity formation is the actual cause of tissue disruption mistakenly ascribed to sonic pressure waves. One review noted that strong opinion divided papers on whether the pressure wave contributes to wound injury. It ultimately concluded that no "conclusive evidence could be found for permanent pathological effects produced by the pressure wave".

Origin of the hypothesis An early mention of "hydrostatic shock" appeared in Popular Mechanics in April 1942. In the scientific literature, the first discussion of pressure waves created when a bullet hits a living target is presented by E. Harvey Newton and his research group at Princeton University in 1947:

It is generally recognized that when a high-velocity missile strikes the body and moves through soft tissues, pressures develop which are measured in thousands of atmospheres. Actually, three different types of pressure change appear: (1) shock wave pressures or sharp, high-pressure pulses, formed when the missile hits the body surface; (2) very high-pressure regions immediately in front and to each side of the moving missile; (3) relatively slow, low-pressure changes connected with the behavior of the large explosive temporary cavity, formed behind the missile. Such pressure changes appear to be responsible for what is known to hunters as hydraulic shock—a hydraulic transmission of energy that is believed to cause instant death of animals hit by high-velocity bullets (Powell (1)). Frank Chamberlin, a World War II trauma surgeon and ballistics researcher, noted remote pressure wave effects. Col. Chamberlin described what he called "explosive effects" and "hydraulic reaction" of bullets in tissue. ...liquids are put in motion by 'shock waves' or hydraulic effects... with liquid filled tissues, the effects and destruction of tissues extend in all directions far beyond the wound axis. He avoided the ambiguous use of the term "shock" because it can refer to either a specific kind of pressure wave associated with explosions and supersonic projectiles or to a medical condition in the body. Col. Chamberlin recognized that many theories have been advanced in wound ballistics. During World War II he commanded an 8,500-bed hospital center that treated over 67,000 patients during the fourteen months that he operated it. P.O. Ackley estimates that 85% of the patients were suffering from gunshot wounds. Col. Chamberlin spent many hours interviewing patients as to their reactions to bullet wounds. He conducted many live animal experiments after his tour of duty. On the subject of wound ballistics theories, he wrote:

If I had to pick one of these theories as gospel, I'd still go along with the Hydraulic Reaction of the Body Fluids plus the reactions on the Central Nervous System. Other World War II era scientists noted remote pressure wave effects in the peripheral nerves. There was support for the idea of remote neural effects of ballistic pressure waves in the medical and scientific communities, but the phrase "hydrostatic shock" and similar phrases including "shock" were used mainly by gunwriters (such as Jack O'Conner) and the small arms industry (such as Roy Weatherby, and Federal "Hydra-Shok.")

Arguments against Martin Fackler, a Vietnam-era trauma surgeon, wound ballistics researcher, a colonel in the U.S. Army and the head of the Wound Ballistics Laboratory for the U.S. Army's Medical Training Center, Letterman Institute, claimed that hydrostatic shock had been disproved and that the assertion that a pressure wave plays a role in injury or incapacitation is a myth. Others expressed similar views. Fackler based his argument on the lithotriptor, a tool commonly used to break up kidney stones. A lithotriptor uses sonic pressure waves which are stronger than those caused by most handgun bullets, yet it produces no damage to soft tissues whatsoever. Hence, Fackler argued, ballistic pressure waves cannot damage tissue either. Fackler claimed that a study of rifle bullet wounds in Vietnam (Wound Data and Munitions Effectiveness Team) found "no cases of bones being broken, or major vessels torn, that were not hit by the penetrating bullet. In only two cases, an organ that was not hit (but was within a few cm of the projectile path), suffered some disruption." Fackler cited a personal communication with R. F. Bellamy. However, Bellamy's published findings the following year estimated that 10% of fractures in the data set might be due to indirect injuries, and one specific case is described in detail (pp. 153–154). In addition, the published analysis documents five instances of abdominal wounding in cases where the bullet did not penetrate the abdominal cavity (pp. 149–152), a case of lung contusion resulting from a hit to the shoulder (pp. 146–149), and a case of indirect effects on the central nervous system (p. 155). Fackler's critics argue that his evidence does not contradict distant injuries, as Fackler claimed, but the WDMET data from Vietnam actually provides supporting evidence for it. A summary of the debate was published in 2009 as part of a Historical Overview of Wound Ballistics Research.

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