Stopping power

Stopping power is the ability of a firearm or other weapon to cause a penetrating ballistic injury to a target (human or animal) enough to incapacitate the target where it stands. This contrasts with lethality in that it pertains only to a weapon's ability to incapacitate quickly, regardless of whether death ultimately ensues. Some theories of stopping power involve concepts such as "energy transfer" and "hydrostatic shock", although there is disagreement regarding the importance of these effects.

Stopping power is related to the physical properties of the bullet and the effect 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.

Stopping is usually caused not by the force of the bullet (especially in the case of handgun and rifle bullets), 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 damages parts of the central nervous system, such as the spine or brain. In response to addressing stopping power issues, the Mozambique Drill was developed to maximize the likelihood of a target's quick incapacitation.

A manstopper is any combination of firearm and ammunition that can reliably incapacitate, or "stop", a human target immediately. For example, the .45 ACP pistol round and the .357 Magnum revolver round both have firm reputations as "manstoppers." Historically, one type of ammunition has had the specific tradename "Manstopper." Officially known as the Mk III cartridge, these were made to suit the British Webley .455 service revolver in the early 20th century. The ammunition used a 220 gr cylindrical bullet with hemispherical depressions at both ends. The front acted as a hollow point deforming on impact while the base opened to seal the round in the barrel. It was introduced in 1898 for use against "savages", but fell quickly from favour due to concerns of breaching the Hague Convention's international laws on military ammunition, and was replaced in 1900 by re-issued Mk II pointed-bullet ammunition.

Some sporting arms are also referred to as "stoppers" or "stopping rifles." These powerful arms are often used by game hunters (or their guides) for stopping a suddenly charging creature, like a buffalo or an elephant.

History
The concept of stopping power appeared in the 19th Century when colonial troops (e.g. American in the Philippines during the Moro Rebellion, British in New Zealand during the Land Wars), engaging in close action with native tribesmen, found that their pistols were not able to stop charging warriors. This led to larger caliber weapons (such as the .45 Colt being returned to service, and the .45 ACP being developed), intended to stop opponents with a single round.

During the Seymour Expedition in China, at one of the battles at Langfang, Chinese Boxers, armed with swords and spears, charged the British and Americans, who were armed with guns. At point blank range, one British soldier had to empty four bullets into a Boxer before he stopped, and the American Captain Bowman McCalla reported that single rifle shots were not enough, multiple rifle shots were needed to halt a Boxer. Only machine guns were effective in stopping the Boxers.

British troops used expanding bullets against native tribesmen in the Northwest Frontier of India, and in the Sudan (see The River War by Winston Churchill). Hence Britain voted against a prohibition on their use at the Hague Convention of 1899, although the prohibition only applied to international warfare.

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 tissue. These two effects are typically referred to as permanent cavity (the track left by the bullet as it penetrates flesh) and temporary cavity, which, as the name implies, is the temporary (instantaneous) displacement caused as the bullet travels through flesh and is many times larger than the actual diameter of the bullet. These phenomena are unrelated to low-pressure cavitation in liquids.

The degree to which permanent and temporary cavitation occur is dependent on the mass, diameter, material, design and velocity of the bullet. This is because bullets crush tissue, and do 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 cause 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-alloy core with a thin gilding 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 causing more permanent cavitation as the tissue is crushed and accelerated into other tissues by the bullet, causing a shorter and wider wound channel.

Bullets are constructed to behave in different ways, depending on the intended target. Different bullets are constructed variously 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, tumble upon impact, fragment upon impact, or 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; gilding metal jacket which is a copper alloy of nickel and zinc to promote higher velocities; pure copper; copper alloy of bronze with tungsten steel alloy inserts to promote weight.

Some bullets are constructed by bonding the lead core to the jacket 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 dual 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 with the target. This fragmentation is intended to create more trauma to the target, and also to reduce collateral damage caused from ricocheting or overpenetrating of the target and the surrounding environments such as walls. Fragmenting rounds have been shown to be unlikely to obtain deep penetration necessary to disrupt vital organs located at the back of a hostile human.

Physical
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 liver or lung will be similar, with the lung shot having the added effect of reducing blood oxygenation; these effects however are generally slower to arise than damage to the heart. A hole through the brain can cause instant unconsciousness and will likely kill the recipient. A hole through the spinal cord will instantly interrupt the nerve signals to and from some or all extremities, disabling the target and in many cases also resulting in death (as the nerve signals to and from the heart and/or lungs are interrupted by a shot high in the chest or to the neck). By contrast, a hole through an arm or leg which hits only muscle will cause a great deal of pain but is unlikely to be fatal, unless one of the large blood vessels (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 identical to living tissue. Studies on the effects of bullets typically are based on experiments using 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 bruise. Some speculation states that nerve bundles can be damaged by temporary cavitation, creating a stun effect, but this has not been confirmed.

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 ranges roughly from 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.

Core-locked rifle bullets that strike a major bone (such as a femur) can expend their entire energy into the surrounding tissue, causing it to take on a gelled consistency as the cellular structure is destroyed. The struck bone is commonly shattered at the point of impact.

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, including physical, physiological, and psychological effects.

Neurological
The only way to immediately incapacitate a person or animal is to damage or disrupt their central nervous system (CNS) to the point of paralysis, unconsciousness, or death. 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 extremely difficult to hit even under optimal circumstances.

Bullets can indirectly disrupt the CNS by damaging the cardiovascular system so that it can no longer provide enough oxygen to the brain to sustain consciousness. This can be the result of bleeding from a 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 10–15 seconds of wilful action, though with rapidly decreasing effectiveness as the victim begins to lose consciousness.

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 physiological pain response from severe injuries will temporarily disable most individuals.

Several scientific papers reveal ballistic pressure wave effects on wounding and incapacitation, including central nervous system injuries from hits to the thorax and extremities. These papers document remote wounding effects for both rifle and pistol levels of energy transfer.

Recent work by Courtney and 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
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 effectiveness quality of any firearm or bullet; there are many documented instances where people 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 effects.

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 force trauma. It is easier for someone to feel when they have 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 the psychological effects of being shot. During the colonial era when native tribesmen came into contact with firearms for the first time, there was no psychological conditioning that being shot was fatal, and most colonial powers eventually sought to create more effective man stoppers.

Therefore, such effects are not as reliable as physiological effects at stopping people. Animals will not faint or surrender if injured, though they may become frightened by the loud noise and pain of being shot, so psychological mechanisms are generally less effective against non-humans.

Penetration
According to Dr. Martin Fackler and the International Wound Ballistics Association (IWBA), between 12.5 and 14 inches (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 inches (305 to 457 mm).

A penetration depth of 12.5 to 14 inches (318 and 356 mm) may seem excessive, but a bullet sheds velocity—and crushes a narrower hole—as it penetrates deeper, while losing velocity, so the bullet might be crushing a very small amount of tissue (simulating an "ice pick" injury) during its last two or three inches of travel, giving only between 9.5 and 12 inches of effective wide-area penetration. Also, skin is elastic and tough enough to 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 hollow point 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, while retaining enough velocity to generate a large diameter hole through tissue. An extreme example where penetration would be important is if the bullet first had to enter and then exit an outstretched arm before impacting the torso. A bullet with low penetration might embed itself in the arm whereas a higher penetrating bullet would penetrate the arm then enter the thorax where it would have a chance of hitting a vital organ.

Overpenetration
Overpenetration is when a bullet passes through its target and out of the other side, potentially going on to damage something or someone else. Over-penetration also denies the bullet its ability to fully "dump" its energy inside the intended target.

Other hypotheses
These hypotheses are a matter of some debate among scientists in the field:

Energy transfer
The energy transfer hypothesis states that the more energy that is transferred to the target, the greater the destructive potential.

Energy is a function of mass and the square of velocity. Generally speaking, it is the intention of the shooter to deliver an adequate amount of energy to the target via the projectile/s. Projectiles such as rifle bullets, high velocity handgun bullets and shotgun slugs can over-penetrate. Projectiles such as handgun bullets and shot can under-penetrate. Projectiles that reach the target with too low a velocity or bird shot may not penetrate at all. All the above conditions affect energy transfer.

Over-penetration is detrimental to stopping power in regards to energy. This is because a bullet that passes through the target does not transfer all of its energy to the target. Despite decreased tissue damage due to loss of transferred energy on an over-penetrating shot, the resulting exit wound would cause increased blood loss and therefore a decrease in blood pressure in the victim. This effect on both persons and game animals is likely to be incapacitating over the length of the entire shooting event.

Under-penetration is also detrimental to stopping power. Projectile/s that do not transfer enough energy to the target may fail to create a fatal wound cavity. Also vital organs may not be reached, thereby limiting the amount of tissue damage, blood loss, and/or loss of blood pressure.

Non-penetration of projectile/s may only deliver enough energy to create bruising, punctures and or blunt force trauma. All of which may result in internal injury solely through the force of the impact but not stop the target. A notable example of projectiles designed to deliver stopping power without target penetration are Flexible baton rounds (commonly known as "beanbag bullets"), a type of reduced-lethality ammunition.

As mentioned above, there are many factors that affect "stopping power." Within this theory energy transfer is related to destructive potential; however, the importance of energy transfer in determining the stopping power of projectiles (when compared to other factors like location of the wound and wound cavity size) is a controversial topic.

The force exerted by a projectile upon tissue is equal to the bullet's local rate of kinetic energy loss, with distance $$\mathrm{d}E_k/\mathrm{d}x$$ (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).

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. It is argued that evidence of such shock can be seen in ultra-high-speed images of supersonic bullets passing through various objects such as fruit; the fruit explodes due to the shock waves caused by the bullet passing through at high speed. Proponents of the theory contend that damage to the brain from hydrostatic shock from a shot to the chest occurs in humans with most rifle cartridges and some higher-velocity handgun cartridges.

Knockback
The idea of "knockback" is a subset or simplification of energy transfer theory, and states that a bullet of sufficient caliber at sufficient speed which transfers all its energy to a subject has enough force, by sheer momentum of the bullet, to stop forward momentum of an attacker and knock them backwards or downwards. The idea was first widely expounded in ballistics discussions during American involvement in Philippine insurrections and, simultaneously, in British involvement in the Caribbean, when front-line reports stated that the .38 caliber revolvers carried by U.S. and British soldiers were incapable of bringing down a charging warrior. Thus, in the early 1900s, the U.S. reverted to the .45 Colt in single action revolvers, and later adopted the .45 ACP cartridge in what was to become the M1911A1 pistol and the British adopted the .455 Webley caliber cartridge in the Webley Revolver. The larger cartridges were chosen largely due to the Big Hole Theory (a larger hole does more damage), but the common interpretation was that these were changes from a light, deeply penetrating bullet to a larger, heavier "manstopper" bullet.

Though popularized in television and movies, and commonly referred to as "true stopping power" by novice or uneducated proponents of large powerful calibers such as .44 Magnum, the effect of knockback from a handgun and indeed most personal weapons is largely a myth. The momentum of the so-called "manstopper" .45 ACP bullet is approximately that of a 1 lb mass dropped from a height of 11.4 ft. Such a force is simply incapable of arresting a running target's forward momentum. In addition, bullets are designed to penetrate instead of strike a blunt force blow, because, in penetrating, more severe tissue damage is done. A bullet with sufficient energy to knock down an assailant, such as a high-speed rifle bullet, would be more likely to instead pass straight through, while not transferring the full energy (in fact only a very small percentage of the full energy) of the bullet to the victim.

The "knockback" effect is however commonly "seen" in real-life shootings, and can be explained by physiological and psychological means. Humans encountering a physical hit, be it a punch or a bullet, are conditioned to absorb the blow by moving in the same direction as the force. The physical effect against a non-penetrating weapon is to reduce the force felt by the blow, and in addition, retreating from an attack increases the distance such an attack must cover, which in the case of non-projectile weapons such as fists or a knife, places the target out of range of further attack. In addition, there is a theoretical sociological explanation, that in modern civilization, with far greater separation by most individuals from violence, hunting, and combat, normal individuals may simply recoil, buckle, or fall backward when hit by a bullet, even when in pure physiological terms they are perfectly capable of continuing to charge.

Although knockback is not possible with a handgun bullet, it can be an actual effect occurring in reaction to being hit by a massive slug, such as a rubber bullet or sandbag fired from a shotgun. The dynamics of a slug round are quite different than penetrating bullets; the projectile is here designed not to penetrate but instead to strike a hard, blunt force blow, and as the momentum carried by a shotgun cartridge is greater than practically any production handgun cartridge, the force imparted is comparable to a hard punch and is capable, by physics, of affecting a person's forward motion. In any case, due to conservation of momentum, the gun's recoil is always larger than the bullet's knockback, as some momentum of the bullet is lost during flight, and if the bullet penetrates through the target it will not have imparted all its momentum into the target.

One-shot stop
This hypothesis, promoted by Evan P. Marshall, is based on statistical analysis of actual shooting incidents from various reporting sources (typically police agencies). It is 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 predominantly used 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.

Since 2006, after the conviction of retired school teacher Harold Fish in Arizona for second degree murder during a self-defense shooting, some CCW holders in the United States have elected to switch from carrying hollow-point bullets, and especially 10mm Auto caliber weapons with perceived higher one-shot stopping power, to carrying smaller caliber weapons. Fish's conviction for killing a homeless man with a history of dangerous violent behavior and mental instability who attacked Fish while hiking on a remote trail, was obtained through a jury trial by stressing that 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. The conviction has since been thrown out by the Arizona Court of Appeals. CCW training classes often advise the use of bullets that are identical to those used by local police, in type (FMJ or hollow point) and caliber, to prevent an overreaction prosecution.

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 aforementioned "one shot stop". According to this theory, as the bullet does not pass entirely through the body, it incorporates the energy transfer and the overpenetration ideals. Those that support this theory cite the .40 S&W round, arguing that it has a better ballistic profile than the .45, and more stopping power than a 9×19mm Parabellum.

The theory centers on the "permanent cavitation" element of a handgun wound. A big hole damages more tissue. It is therefore valid to a point, but penetration is also important, as a large bullet that does not penetrate will be less likely to strike vital blood vessels and blood-carrying organs such as the heart and liver, while a smaller bullet that penetrates deep enough to strike these organs or vessels will cause faster bleed-out through a smaller hole. The ideal may therefore be a combination; a large bullet that penetrates deeply, which can be achieved with a larger, slower non-expanding bullet, or a smaller, faster expanding bullet such as a hollow point.

In the extreme a heavier bullet (which preserves momentum greater than a lighter bullet of the same caliber) may "overpenetrate", passing completely through the target without expending all of its kinetic energy. So-called "overpenetration" is not an important consideration when it comes to wounding incapacitation or "stopping power", because: (a) while a lower proportion of the bullet's energy is transferred to the target a higher absolute amount of energy is shed than in partial penetration, and (b) overpenetration creates an exit wound.

Other contributing factors
As mentioned earlier, there are many factors, such as drug and/or alcohol levels within the body, body mass index, mental illness, motivation levels, body part strike (e.g. "armpit hold") which may determine which round will kill or at least catastrophically affect a target during any given situation.