Stopping power

For the concept in nuclear physics, see stopping power (particle radiation).

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. This term is not a euphemism for lethality; it refers only to a weapon's ability to incapacitate quickly, regardless of whether death ultimately results. 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.

Most 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.

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.

Dynamics of bullets

A bullet will destroy or damage any tissues which it penetrates creating a wound channel. It will also cause nearby tissue to stretch and expand as it passes through. 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 is dependant on the weight, diameter, material, design and velocity of a bullet. This is because bullets actually crush tissue, not cut it. A bullet constructed with a half diameter ogive designed meplat and hard, solid copper alloy material will crush only the tissue directly in front of the bullet. This type of bullet (monolithic-solid rifle bullet) is conducive to more temporary cavitation as the tissue flows around the bullet causing a deep and narrow wound channel. A bullet constructed with a two diameter, hollow point ogive designed meplat and low antimony lead core with a thin gliding metal jacket material will crush tissue in front and to the sides as the bullet expands. Due to the energy expended in bullet expansion velocity is lost more quickly. This type of bullet (hollow-point hand gun bullet) is conducive to more permanent cavitation as the tissue is crushed and accelerated into other tissues by the bullet, causing a shorter and voluminous wound channel.

Bullets are constructed to behave in many different ways depending on the intended target. Bullets are constructed to: not expand upon impact; expand upon impact at high velocity; expand upon impact, expand across a broad range of velocities; expand upon impact at low velocity, fragmented upon impact and disintegrate upon impact.

To control the expansion of a bullet, meplat design and materials are engineered. The meplat designs are: flat; round to pointed depending on the ogive; hollow pointed which can be large in diameter and shallow or narrow in diameter and deep and truncated which is a long narrow punched hole in the end of a monolithic-solid type bullet. The materials used to make bullets are: pure lead; alloyed lead for hardness; gliding metal jackets which is a copper alloy of nickel and zinc to promote higher velocities; pure copper; copper alloy of bronze and tungsten steel alloy inserts to prompt weight.

Some bullets are constructed by bonding the lead core to the jack to promote higher weight retention upon impact causing a larger and deeper wound channel. Some bullets have a web in the center of the bullet to limit the expansion of the bullet while promoting penetration. Some bullets have duel cores to promote penetration.

Bullets that might be considered to have stopping power for dangerous large game animals are usually 11.63 mm (.458 caliber) and larger, including 12-gauge shotgun slugs. These bullets are monolithic-solids; full metal jacketed and tungsten steel insert. They are constructed to hold up during close range, high velocity impacts. These bullets are expected to impact and penetrate, and transfer energy to the surrounding tissues and vital organs through the entire length of a game animal’s body if need be.

Bullets with sufficient stopping power for humans are generally large caliber, 9.07 mm (.357 caliber) handgun bullets of hollow point design. Pre-fragmented bullets such as Glaser Safety Slugs and Magsafe ammunition are designed to fragment into birdshot on impact of the target. This is intended to create more trauma to the target; prevent over-penetrating of the target, walls and the surrounding environments causing collateral damage and prevent ricocheting.

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 will likely 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 process.

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 ballistic gelatin, in which temporary cavitation causes radial tears where the gelatin was stretched. Although such tears are visually engaging, some animal tissues, (other than bone or liver) are more elastic than gelatin . 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.

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

Neurological effects

The only way to completely incapacitate a person or other animal is to damage or disrupt their central nervous system (CNS) to the point that they become paralyzed, lose consciousness, or die. Bullets can achieve this directly or indirectly. If a bullet causes sufficient damage to the brain or spinal cord immediate loss of consciousness or paralysis respectively can result. However, these targets are relatively small and mobile, making them difficult to hit even under optimal circumstances.

Bullets can indirectly disrupt the CNS by damaging the cardiovascular system so it can no longer provide enough oxygen to the brain to sustain consciousness. This can be the result of bleeding from perforation of a large blood vessel or blood-bearing organ or the result of damage to the lungs or airway. 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, though with rapidly decreasing effectiveness as the victim begins to black out.

Unless a bullet directly damages or disrupts the central nervous system, a person or animal will not be instantly and completely incapacitated by physiological damage. However, bullets can cause other disabling injuries that prevent specific actions (a person shot in the femur cannot walk) and the physiologic pain response from severe injuries will at least temporarily disable most (but not all) individuals.

Recent work published by scientists M Courtney and A Courtney provides compelling support for the role of a ballistic pressure wave in creating remote neural effects leading to incapacitation and injury. This work builds upon the earlier works of Suneson et al. where the researchers implanted high-speed pressure transducers into the brain of pigs and demonstrated that a significant pressure wave reaches the brain of pigs shot in the thigh. These scientists observed neural damage in the brain caused by the distant effects of the ballistic pressure wave originating in the thigh.

The results of Suneson et al. were confirmed and expanded upon by a later experiment in dogs which "confirmed that distant effect exists in the central nervous system after a high-energy missile impact to an extremity. A high-frequency oscillating pressure wave with large amplitude and short duration was found in the brain after the extremity impact of a high-energy missile . . ." Wang et al. observed significant damage in both the hypothalamus and hippocampus regions of the brain due to remote effects of the ballistic pressure wave.

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 (stun grenades) 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.

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) may seem excessive, but 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 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 make a large enough hole through it.

Overpenetration

Overpenetration is often emphasized by those that prefer 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 serious 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%.

Other hypotheses of stopping power

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

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.

Energy transfer

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

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.

However, it remains a general physical principle, that when a volume of energy is transferred from one medium to another, the greater the volume of energy, the greater the destructive potential.

In ballistics, energy is a function of velocity and weight. Generally speaking, bullets which impact a target with greater energy cause greater damage. A bullet with too little energy might not penetrate the target - although in the case of a living target they may suffer blunt force trauma, possibly resulting in internal injury solely through the force of the impact.

Overpenetration is detrimental to stopping power with regard to energy since a bullet that passes through the target has not completely shed all of its stored energy. However, the increased tissue damage as well as the creation of an exit wound (and increased blood loss) resulting from a bullet passing through a person also affect whether the target is likely to be incapacitated. It should be noted that bullets that pass out of the body may still injure people nearby.

Rifles commonly propel bullets at at least 2-3 times the velocity of the most powerful pistols. Such bullets have more kinetic energy (kinetic energy is proportional to the square of the speed). Bullets not intended to expand such as the 5.56 x 45 mm NATO, M855 Ball Round, may cause much more tissue damage as a result, of expansion or fragmentation.

As discussed above, there are many factors that affect "stopping power." Energy transfer is undeniably related to destructive potential; however, the importance of energy transfer in determining the stopping power of bullets (when compared to other factors like location of the wound and bullet size) is not agreed upon.

It is a general principal of physics that the force between the bullet and tissue is equal to the bullet's local rate of energy of energy loss, dE/dx (the first derivative of the bullets kinetic energy with respect to position). The ballistic pressure wave is proportional to this retarding force (Courtney and Courtney), and this retarding force is also the origin of both temporary cavitation and prompt damage (CE Peters).

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 hollow point 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 9 mm 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, 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.

Some CCW holders in the United States have elected since 2006 to switch from carrying hollow-point bullets and especially 10 mm caliber weapons with perceived higher one-shot stopping power to instead favor carrying smaller caliber weapons after the conviction of retired school teacher Harold Fish in Arizona for second degree murder during a self-defense shooting. His conviction for killing a homeless man with a history of mental instability who attacked him while hiking on a remote trail was obtained through a jury trial by stressing Fish overreacted through choosing to use the increased stopping power of 10 mm hollow point bullets. State law in Arizona has subsequently been changed, such that the state now has the burden to prove that a self defense shooting was not in self defense, whereas the burden previously before the Fish incident was that the shooter on trial had to prove that the shooting was in fact done in self defense. Meanwhile, many CCW holders have elected to switch to carrying handguns loaded with FMJ bullets in calibers smaller than 10 mm. A choice often advocated for selecting the correct stopping power in CCW training classes is to select to use the exact same type of bullets (FMJ or hollow point), in the exact same caliber that are used by the local police, to avoid being accused of overreacting during any self defense post-incident trial.

Big Hole School

This school of thought says that the bigger the hole in the target the higher the rate of bleed-out and thus the higher the rate of the "magical" one shot kill. In this theory the bullet does not pass entirely through the body so that it incorporates the energy transfer and the overpenetration ideals. The people that like this theory cite the .40 S&W round to be the best bet, arguing that it has a better ballistic profile than the .45 but a lot more stopping power than a 9x19mm Parabellum.

External links

• Firearms Tactical Institute
• Wound Ballistics
• Reality of the Street? A Practical Analysis of Offender Gunshot Wound Reaction for Law Enforcement
• Handgun Wounding Factors and Effectiveness
• Ballistic gelatin testing done by "GoldenLoki"
• The AR-15.com Ammo Oracle
• B&T Ammo Labs
• Several journal articles by Martin Fackler, M.D.
• Tactical Forums
• What We Didn't Know Hurt Us (PDF)
• Virtual Naval Hospital - Emergency War Surgery
• One Shot Drops - Surviving the Myth
• Ammunition for the Self-Defense Handgun - anonymous
• Terminal effects of the Mk 7 .303 British cartridge [1]
• Stoppingpower.info