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 regarding the importance of these effects.
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.
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 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 quick incapacitation of an attacker.
A manstopper is a generic term used to describe almost any combination of firearm and ammunition that can reliably be expected to incapacitate, or "stop" a human target immediately. For example, the .45 ACP pistol round and the .357 Magnum revolver round have a firm reputation 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 grain 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 Conventions 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.
The degree to which permanent and temporary cavitation occur is dependent on the weight, 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 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 more voluminous 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 and 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.
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 slight bruise. Some speculation states that nerve bundles can be damaged by temporary cavitation, creating a stunning 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.
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 seconds of willful 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.
A number of papers in the peer-reviewed journals suggest ballistic pressure wave effects on wounding and incapacitation, including remote neural effects. These papers are mainly concerned with velocities of rifle bullets, but the energy transfer and pressure waves produced are within the regime of pistol bullets.
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.
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's 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. 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.
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.
Even if the bullet does completely penetrate a person, it will have a very reduced velocity and probably will no longer be ballistically stable. Missing the intended target altogether, hence 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 at which they were fired. 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 typically only 15.8%. By either measure, the vast majority of bullets were not stopped by hitting perpetrators, but ultimately encountered some other object. The propensity of these "stray" bullets to pass through windows, walls, car doors, etc. and possibly injure bystanders is a concern, and falls into the overall risk/benefit calculation when considering how much penetration is enough.
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 that shot must have been fired from in front of the limousine rather than from behind, where Lee Harvey Oswald was claimed to have fired from the Texas School Book Depository, and thus implying a second assassin. However, it was recently shown to a large television audience by Penn and Teller on May 9, 2005 on their Showtime network program, Bullshit!, that when a simplified 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 can produce unexpected results.
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 mass. 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 regards 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. Bullets that pass out of the body may still injure people nearby.
Rifles commonly propel bullets to speeds of 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 than the tissue damage caused by fragmenting or expanding bullets fired at handgun velocities.
As mentioned 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 a controversial topic.
It is a general principal of physics that the force exerted by a bullet upon tissue is equal to the bullet's local rate 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).
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 the momentum of a 1 pound (0.45 kilogram) mass dropped from a height of 11.4 feet (3.47 meters) onto a surface. It should be noted that momentum is very different from kinetic energy; an equivalent kinetic energy of a typical .45 ACP bullet is that of a 60 pound (27 kilogram) mass dropped from a height of 10 feet (3 meters). 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 power of a shotgun cartridge is greater than practically any production handgun cartridge, the force imparted is comparable to a hard punch and is capable, by sheer physics, of doubling up, hence knocking down, someone.
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 mental instability who attacked him 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. 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.
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 doesn't 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. It is important to note however, that 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 in the target.