Stopping power

From Wikipedia, the free encyclopedia

You have new messages (last change).
For the concept in nuclear physics, see stopping power (particle radiation).
This article or section does not cite its references or sources.
Please help improve this article by introducing appropriate citations. (help, get involved!) This article has been tagged since July 2006.

Stopping power is a colloquial term used to describe the ability of a particular weapon to stop the actions of an individual by means of penetrating ballistic injury. Some theories of stopping power involve concepts such as "energy transfer" and "hydrostatic shock", although there is disagreement about the importance of these effects. Obviously, stopping power is related to the physical properties of the bullet and the effects it has on its target, but the issue is complicated and not easily studied. Critics contend that the importance of "one-shot stop" statistics is overstated, pointing out that most gun encounters do not involve a "shoot once and see how the target reacts" situation.

Many ballistic and forensic experts claim that "stopping power" does not exist, especially with handgun bullets. Stopping is caused not by the force of the bullet, but by the damaging effects of the bullet which are typically a loss of blood, and with it, blood pressure. More immediate effects can result when a bullet strikes a critical organ such as the heart or damages the central nervous system such as the spine or brain. The myth that a human is thrown back by the force of a bullet is false, as Newton's third law dictates. The target and the shooter receive equal and opposite forces, meaning that for a gun to knock a target back, its recoil would have to knock the shooter back an equal amount, which is undesirable.

Contents

[edit] History

The concept of "stopping power" appeared in the late 19th Century when colonial troops (American in the Philippines, British in New Zealand) engaged in close action with native tribesmen found that their pistols were not able to stop charging warriors. This led to larger caliber weapons being developed to stop opponents with a single round, without necessarily killing them.

[edit] Dynamics of bullets

A bullet will destroy or damage any tissues which it penetrates, leaving a hole behind. It will also cause nearby tissue to stretch and expand as it passes. These two effects are typically referred to as permanent cavitation (the hole left by the bullet) and temporary cavitation (the tissue displaced as the bullet passed).

The degree to which permanent and temporary cavitation occur depend on the size, shape, and velocity of the bullet. Wider diameter, blunter shape, higher velocity, or any combination thereof will increase the width of the permanent and temporary cavitation. This is because bullets actually crush tissue, not cut it. A bullet with a rounded or a sharply pointed tip will crush only the tissue directly in front of a small portion of its diameter; tissue closer to the edge of the bullet will simply "flow" around it and be pushed outwards. A blunter, flatter bullet uses more of its face to crush tissue, but loses velocity more quickly in the process. The depth of cavitation is based on the same properties, but in a slightly different way. In this case, it is narrower, more rounded (i.e. more hydrodynamic) bullets which are able to penetrate more deeply into tissue.

How much velocity a bullet retains during penetration is very important. A bullet which starts at a high velocity but loses its velocity quickly during penetration will crush a relatively large diameter hole at first, but the permanent cavity will quickly narrow deeper in. A projectile which retains velocity better (usually a heavier weight one of the same caliber) might make a smaller hole than the faster, lighter bullet at first, but retains velocity better as it penetrates, crushing a deeper hole of larger diameter. Due to their general design, rifle rounds, which typically have more mass and vastly higher velocities than handgun rounds, cause much more temporary cavitation.

None of these processes are static. As a bullet penetrates, it inevitably loses velocity (and, in the case of expanding bullets, deforms). This means that the diameter of the temporary and permanent cavities will gradually get narrower as the bullet penetrates deeper. In the case of expanding bullets, such as hollowpoints, the wider diameter and blunter shape temporarily crush a wider hole and generate a larger temporary cavity, but the bullet loses velocity even faster, penetrating less.

Some bullets, either because of their high velocity or intended design, will fragment to some degree. Prefragmented bullets such as Glaser Safety Slugs and Magsafe ammunition are designed to completely disintegrate into birdshot and jacketing pieces on impact with anything, including a person. This is intended to achieve several things:

  1. preventing the bullet from penetrating walls and hitting someone on the other side,
  2. preventing the bullet from ricocheting and continuing to travel for a long distance,
  3. preventing the bullet from overpenetrating a person and striking someone else.

In this case the individual fragments create a mass of tiny permanent cavities rather than one large one, and each fragment generates only a minimal temporary cavity. Fragmentation can occur with very high velocity bullets, e.g. those fired by rifles. Lead is a malleable metal; when a softpoint or hollowpoint bullet is violently deformed fragments can shear off and these will create small holes around the main one.

[edit] Physical effects

Permanent and temporary cavitation cause very different biological effects. The effects of a permanent cavity are fairly obvious. A hole through the heart will cause loss of pumping efficiency, loss of blood, and eventual cardiac arrest. A hole through the brain can cause instant unconsciousness and is quite likely to kill the recipient. A hole through an arm or leg which hits only muscle, however, will cause a great deal of pain but is unlikely to be fatal, unless a large blood vessel (femoral or brachial arteries, for example) is also severed in the act.

The effects of temporary cavitation are less well understood, due to a lack of a test material similar to living tissue. Studies on the effects of bullets typically experiment on gelatin, in which temporary cavitation causes radial tears where the gelatin was stretched. Although such tears are visually engaging, animal tissue is much more elastic than gelatin, and in most cases temporary cavitation is unlikely to cause anything more than a slight bruise. Some speculation states that nerve bundles can be damaged by temporary cavitation, creating a stunning effect, but this has not been confirmed experimentally.

One exception to this is when a very powerful temporary cavity intersects with the spine. In this case, the resulting blunt trauma can slam the vertebrae together hard enough to either sever the spinal cord, or damage it enough to knock out, stun, or paralyze the target. For instance, in the shootout between eight FBI agents and two bank robbers on April 11, 1986 in Miami, Florida, (see FBI Miami shootout, 1986) Special Agent Gordon McNeill was struck in the neck by a high-velocity .223 bullet fired by Michael Platt. While the bullet did not directly contact the spine, and the wound incurred was not ultimately fatal, the temporary cavitation was sufficient to render SA McNeill paralyzed for several hours.

Temporary cavitation can also cause the tearing of tissues if a very large amount of force is involved. The tensile strength of muscle is roughly 1 to 4 MPa (145 to 580 lbf/in²), and minimal damage will result if the pressure exerted by the temporary cavitation is below this. Gelatin and other less elastic media have much lower tensile strengths, thus they exhibit more damage after being struck with the same amount of force. At typical handgun velocities, bullets will create temporary cavities with much less than 1 MPa of pressure, and thus are incapable of causing damage to elastic tissues which they do not directly contact.

High velocity fragmentation can also increase the effect of temporary cavitation. The fragments sheared from the bullet cause many small permanent cavities around the main entry point. The main mass of the bullet can then cause a truly massive amount of tearing as the perforated tissue is stretched. Awareness of this effect and the suffering it causes is one of the arguments against high-velocity rifles being used in hunting. It might be noted that high velocity bullets are usually lighter in weight, and when striking even something as light as a twig will often break up.

Whether a person or animal will be incapacitated (i.e. 'stopped') when shot depends on a large number of factors, both physical and physiological.

[edit] Neurological effects

The only way to physiologically stop a person or other animal is to damage or disrupt their central nervous system (CNS) to the point that they fall unconscious or die. Bullets can achieve this directly or indirectly. If a bullet causes sufficient damage to the brain (particularly the cerebellum or brain stem) or cervical spinal cord, the CNS damage is direct and nearly instant. However, these targets are well-protected, very small, and mobile, making them difficult to hit even under optimal circumstances.

Indirectly, bullets can damage the CNS by way of bleeding. This is accomplished by putting a large enough hole through a vital blood vessel or blood-bearing organ. If blood-flow is completely cut off from the brain, a human still has enough oxygenated blood in their brain for 10 seconds of willful action. Considering that the higher brain functions will usually shut down in a life-or-death situation, this figure might actually be a bit high.

Unless a bullet directly causes damage to a CNS structure, there is absolutely no physiological reason for a person or animal to be instantly incapacitated, and unless the bullet crushes a large enough hole in a major blood vessel or a major blood-bearing organ, there is no physiological reason for them to be incapacitated at all.

[edit] Psychological effects

Emotional shock, terror, or surprise can cause a person to faint, surrender, or flee when shot or shot at. Emotional fainting is the likely reason for most "one-shot stops," and not an intrinsic quality of any firearm or bullet; there are many documented instances where suspects have instantly dropped unconscious when the bullet only hit an extremity, or even completely missed. Additionally, the muzzle blast and flash from many firearms are substantial and can cause disorientation, dazzling, and stunning effects; flashbangs and other less-lethal "distraction devices" rely exclusively on these.

Pain is another psychological factor, and can be enough to dissuade a person from continuing their actions.

Temporary cavitation can emphasize the impact of a bullet, since the resulting tissue compression is identical to simple blunt trauma. It's easier for someone to feel that they've been shot if there is considerable temporary cavitation, and this can contribute to either psychological factor of incapacitation.

However, if a person is sufficiently enraged, determined, or intoxicated they can simply shrug off any psychological effects of being shot; therefore, such effects are not as reliable as physiological effects at stopping people. Animals will not faint or surrender if injured, though they may be frightened by the loud noise and pain of being shot, so psychological mechanisms are even less effective against non-humans.

[edit] Industry penetration requirements

According to Dr. Martin Fackler and the IWBA, between 12.5 and 14 in (318 and 356 mm) of penetration in calibrated tissue simulant is optimal performance for a bullet which is meant to be used defensively, against a human adversary. They also believe that penetration is one of the most important factors when choosing a bullet (and that the number one factor is shot placement); if the bullet penetrates less than their guidelines, it is inadequate, and if it penetrates more, it is still satisfactory though not optimal. The FBI's penetration requirement is very similar at 12 to 18 in (305 to 457 mm).

12.5 and 14 in (318 and 356 mm) might seem like a lot until you consider that a bullet sheds velocity--and crushes a narrower hole--as it penetrates, so the bullet might be crushing a very small amount of tissue during its last two or three inches of travel, giving only between 9.5 and 12 in of effective penetration. Also, skin is elastic and tough enough that it can cause a bullet to be retained in the body, even if the bullet had a relatively high retained velocity when it hit the skin. About 250 ft/s (76 m/s) velocity is required for an expanded hollowpoint bullet to puncture skin 50% of the time.

The IWBA's and FBI's penetration guidelines are to ensure that the bullet can reach a vital structure from most angles, and retain enough velocity to punch a large enough hole through it.

[edit] Overpenetration

 The neutrality of this section is disputed.
Please see the discussion on the talk page.

Overpenetration is often exaggerated by those who advocate shallow-penetrating "rapid energy transfer" bullets. Tests have shown that human skin, on the entry side, can resist penetration as much as 2" (5 cm) of muscle, and skin on the exit side can be the equivalent of up to 4 in (10 cm). A bullet would need to penetrate greater than approximately 15 in (38 cm) of tissue simulant to have a chance to completely perforate a 9" (23 cm) thick torso, and would need to penetrate more than 17 in (43 cm) to actually pose a serous threat to people downrange.

Even if the bullet does completely penetrate a person, it will have a very reduced velocity and probably no longer be ballistically stable. Missing the intended target altogether, thereby leaving a full velocity bullet to harm whatever is in its path, is a much greater threat.

A hit on a less dense peripheral body area, such as a limb, does present a more serious risk of overpenetration however. Penetration of walls and other cover is also a consideration for police and urban use.

According to NYPD SOP-9 (Standard Operating Procedure #9) data, in the year 2000, only 9% of shots fired by officers engaged in gunfights actually hit perpetrators. In the same year, there were a total of 129 "shooting incidents" (including non-gunfights, such as officers firing at aggressive dogs, unarmed or fleeing perpetrators, etc.), 471 total shots fired by officers, 367 shots fired at perpetrators, and 58 total hits on perpetrators by police. So when non-gunfight shooting data is added, the rate at which police hit what they aim at in real life situations is still only 15.8%. Given that none of those misses resulted in injury to an innocent third party, it is very unlikely for someone to be injured by a bullet that passes through the intended target.

[edit] Other hypotheses of stopping power

These hypotheses are used mostly in marketing of bullets and firearms, and are not considered scientific.

[edit] Hydrostatic shock

Hydrostatic shock is a theory of terminal ballistics that wounding effects are created by a shock wave in the tissues of the target.

[edit] Energy transfer

The energy transfer hypotheses states that the more energy is transferred to the target, the greater the effect.

This theory is frequently referred to by Kennedy assassination theories, who cite the Zapruder film, which shows Kennedy's head recoiling backwards from a shot, as evidence that therefore, that shot must have been fired from in front of the limousine rather than from behind, where Lee Harvey Oswald was firing from the Texas School Book Depository, implying a second assassin. However, it has been repeatedly demonstrated, most recently to a large television audience by Penn and Teller on May 9, 2005 on their Showtime network program, Bullshit!, that when a simplified physical model of a brain inside a skull, composed of a melon wrapped with strapping tape, is shot in a similar fashion, the melon recoils backwards, towards the gun; evidence that the actual transfer of energy from a bullet passing through a complex object is much more complex than simple mathematical models based on oversimplified physical assumptions can predict, a priori.

[edit] One-shot stop

This hypothesis, promoted by Evan P. Marshall, is based solely on statistics, intended to be used as a unit of measurement and not as a tactical philosophy, as mistakenly believed by some. It considers the history of shooting incidents for a given factory ammunition load, and compiles the percentage of "one-shot-stops" achieved with each specific ammunition load. That percentage is then intended to be used with other information to help predict the effectiveness of that load getting a "one-shot-stop." For example, if an ammunition load is used in 10 torso shootings, incapacitating all but two with one shot, the "one-shot-stop" percentage for the total sample would be 80%.

Some argue that this hypothesis ignores any inherent selection bias. For example, high-velocity 9 mm hollowpoint rounds appear to have the highest percentage of one-shot stops. Rather than identifying this as an inherent property of the firearm/bullet combination, the situations where these have occurred need to be considered. The 9mm has been the preferred caliber of many police departments, so many of these one-shot-stops were probably made by well-trained police officers, where accurate placement would be a contributory factor. However, Evan P. Marshall's database of "one-shot-stops" does include shootings from law enforcement agencies, private citizens, and criminals alike.

Critics of this theory point out that bullet placement is a very significant factor, but is only generally used in such one-shot-stop calculations, covering shots to the torso.

[edit] External links

Retrieved from "http://en.wikipedia.org../../../s/t/o/Stopping_power.html"
In other languages