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Stopping power, also called terminal ballistics, is how bullets kill or incapacitate people and animals. The context may be self-defense, military, or hunting. This article assumes the target is human, but it applies also to animals.
Most theories about stopping power rely on impressive-sounding yet meaningless terminology--such as "energy transfer" and "hydrostatic shock"--to hide the fact that they have minimal basis in reality. Such strange, elaborate theories have only formed around firearms. People do not often claim that a particular knife or club has an "80% chance of a one-hit stop," or "transfers 400 foot-pounds of energy to the target." Popular entertainment portrays guns; if you're shot, you fall over dead. People infer that guns must be mystical weapons, capable of doing things no other weapon can, and so may require an equally mystical system for explaining the effects.
What bullets do
A bullet will destroy or damage any tissues which it penetrates, leaving a hole behind. It will also usually cause tissue to be propelled radially outward, entirely analogous to a splash in water.
These three things, penetration, hole, and stretch, are the main effects of being struck by a bullet. How wide and how deep a hole the bullet makes will be referred to as permanent cavitation, and the extent to which the hole is stretched outwards by "splash" will be called temporary cavitation, since these terms are favored by the real experts in the terminal ballistics field.
The depth to which the bullet penetrates is based on the size, shape, velocity, weight, and deformation of a bullet. Narrower diameter, more hydrodynamic (like aerodynamic, but in water) shape, higher velocity, heavier weight, less deformation, or any combination thereof will increase the penetration.
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 both the width of the hole, and the distance which this hole is stretched.
This is because bullets actually crush tissue, not cut it. A bullet with a rounded or sharp 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, and also propels uncrushed tissue with a higher velocity, but loses velocity more quickly in the process.
The velocity of a round also determines how efficiently it crushes tissue. A rounded bullet at a very high velocity may make a wider permanent cavity than a flatnosed bullet at low velocity. In general, a more hydrodynamically shaped bullet gives tissues more space to move to the sides, and a lower velocity bullet gives the tissue more time.
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 larger diameter hole deeper.
None of these processes are static in any way. 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 more shallowly.
Effects of permanent cavitation
The effects of a permanent cavity are fairly obvious. A hole through the heart can cause cardiac arrest, and will allow blood to escape. A hole through the brain can cause instant unconsciousness, and is quite likely to cause eventual death as well. A hole through an arm or leg which hits only muscle will hurt quite a lot.
Effects of penetration
If the bullet doesn't hit it, it doesn't damage it. If a bullet stops penetrating before hitting a particular organ or blood vessel, it won't hit or damage it, thus a certain amount of penetration is desirable.
Effects of temporary cavitation
When people study the effects of bullets in a tissue simulant, such as gelatin, this is what they usually concentrate on, as the effects of temporary cavitation are most pronounced in these mediums; pot roasts and other hunks of meat can be "exploded" by a powerful bullet, and gelatin usually exhibits radial tears where it was stretched by the bullet's passage. One theory of stopping power even states that temporary cavity alone is responsible for incapacitating people.
In reality, temporary cavitation has relatively little effect on living tissue. It is, as stated before, simply the radial stretching of the permanent cavity as tissue is thrown outwards by the bullet's passage. For most low and medium velocity bullets, such as those fired by handguns, the temporary cavity is not large enough to do anything more than slightly bruise elastic tissues. If the bullet passes through an inelastic organ, however, such as the kidneys, spleen, pancreas, liver, or brain, the tissue will tear as it stretches, and damage to these organs will be greater. Some high powered rifle cartridges, however, do stretch elastic tissues far enough that significant bruising and tearing can occur.
Some sources also state that nerve bundles can be damaged by temporary cavitation, creating a stunning effect, but this has not been borne out, to the best of my knowledge, by any testing.
When a very powerful temporary cavity intersects with the spine, however, the resulting blunt trauma can slam the vertebrae together hard enough to either sever the spinal cord, or in many cases 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, Special Agent Gordon McNeill was struck in the neck by a high-velocity .223 bullet fired by felon 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 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 psychological factors of incapacitation (covered below). This is not a good reason to choose a bullet which generates a large temporary cavity over one which performs better in other categories.
Effects of fragmentation
Some bullets, either because of their high velocity, or because of their design, will fragment to some degree. Prefragmented bullets such as Glaser Safety Slugs and Magsafe ammunition, are designed to completely disintigrate into birdshot and jacketing pieces on impact with anything, including a person. This is intended to achieve several things in most cases; it prevents the bullet from penetrating walls and hitting someone on the other side, it prevents the bullet from ricocheting and continuing to travel for a long distance, it prevents the bullet from penetrating a person and striking someone else, and it "transfers energy rapidly." "Energy transfer" is simply a buzzword, and does not describe any actual process by which bullets wound people.
What does happen in this case is the individual fragments, lacking both mass and velocity after the bullet has broken up, create a mass of tiny permanent cavities rather than one big one. Each fragment also generates only a minimal temporary cavity, so tearing is at a minimum.
The other way fragmentation can occur is in very high velocity bullets, usually those fired by rifles. Lead is not a particularly strong metal, thus when a softpoint or hollowpoint is very violently deformed, as with the high velocities which modern rifles generate, some lead fragments can be sheared away and create small holes surrounding the main one. In this case, the main mass of the bullet generates a substantial temporary cavity, and can cause a truly massive amount of tearing as the perforated tissue is stretched. Many people who have hunted are well aware of this effect.
What people do when shot
What a person does when shot depends on a very large number of factors. A person can be incapacitated by either the psychological or the physiological effects.
Psychological effects
Emotional shock, terror, and surprise can cause a person to faint dead away when shot. This is the likely reason for most "one-shot stops," and not any intrinsic quality of any one bullet.
The realization that one has been shot, or at least shot at, is also often enough to cause a person to give up or flee.
Pain is another psychological factor. Having holes put in your body hurts quite a bit, after all, and can quite possibly be enough to dissuade a person from doing anything but screaming.
If a person is sufficiently enraged, determined, drugged-up, or just plain mean, however, they can simply shrug off any psychological effects of being shot, so they should not be counted on to stop an attacker.
Physiological effects
The only way to physiologically stop a person is to damage or distrupt 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. These targets are very small, well-protected, and mobile, however, making them difficult to hit even under optimal circumstances; it is not unheard of for a bullet to skim along the skull, just under the skin, or even be stopped outright in the case of very weak calibers. Similarly, a bullet which passes through the neck or upper chest might not have the velocity to break the spine and damage the spinal cord itself.
Indirectly, bullets can damage the CNS by way of bleeding. This is accomplished by putting a large enough hole through a vital enough 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 a person's higher brain functions will usually shut down in a life-or-death situation, this figure might actually be a bit low.
Major blood vessels include the superior aorta, inferior aorta, vena cava, brachial arteries, femoral arteries, carotid arteries, and jugular veins.
Major blood-bearing organs include the heart, kidneys, and liver.
Unless a bullet strikes and damages a CNS structure, there is absolutely no physiological reason for a person 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.
Ammo selection
The main difference between bullets is how deep they penetrate, and how large they expand (if they expand at all). A wider bullet crushes a larger hole in anatomical structures it strikes solidly, and has a better chance of grazing a structure on a very near miss. A deeper penetrating round has a better chance of getting to those structures in the first place.
Caliber, weight, gun model, etc., don't really matter on their own. If a 35 grain .25 ACP round and a 300 grain .44 magnum round exhibit the same expanded diameter, penetration depth, and cavitation in properly calibrated tissue simulant (10% ballistic gelatin), they will have the same effect in soft tissue and be equally effective at wounding a person (not that such a scenario is likely)!
Penetration requirements
According to Dr. Martin Fackler and the IWBA, between 12.5" and 14" (31.8-35.6 cm) of penetration in calibrated tissue simulant is optimal, and penetration is one of the most important factors when choosing a bullet (and the number one factor, no matter what, is shot placement); if the bullet penetrates less it is inadequate, and if it penetrates more, it is satisfactory. The FBI's penetration requirement is very similar at 12" to 18".
12.5"-14" might seem like a lot, but 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 indeed for its last two or three inches of travel, giving you between 9.5" and 12" of effective penetration if the bullet meets their guidelines. Tests have also shown that human skin can resist penetration by projectiles about as much as 2" of muscle tissue, by virtue of its high tensile strength and elasticity, further cutting this down to 7.5" to 10".
There is also the fact that you might not be able to score a solid, unobstructed, frontal torso hit. You might need to shoot someone from the side, through their bicep, to prevent them from harming someone else, or you might need to shoot an attacker through their hand or forearm if they are pointing a weapon at you.
The IWBA's and FBI's penetration guidelines are to ensure that your bullet can reach a vital structure from most angles, and retain enough velocity (not energy!) to punch a large enough hole through it.
Overpenetration
Overpenetration is exaggerated by those who advocate shallow-penetrating "rapid energy transfer" bullets. Tests have shown that human skin, on the entry side, resists penetration about as much as 2" of muscle, and skin on the exit side is the equivalent of 4"! A bullet would need to penetrate greater than 14" of tissue simulant to have a chance to completely perforate an 8" thick torso.
Even if the bullet does penetrate a person through-and-through, it would probably have very little velocity left by that point, and pose a reduced risk to those downrange. When you consider than police officers miss entirely with 90% of their shots in real shooting incidents, it is very, very unlikely that a bullet will hit someone after going through an attacker.
Other Theories of Stopping Power
When the goal is not to inform, but to astound and impress, or to sell a particular brand of ammunition, many less than scientific theories can develop.
Hydrostatic Shock
This theory of wounding states that the more "hydrostatic shock" a bullet generates in a target, the greater the effect. The term "hydrostatic shock" itself is quite nonsensical. Hydro - water; static - remaining still; shock - a rapid, violent motion; hydrostatic shock - water that moves while remaining still. Meanwhile, we hear the sound of one hand clapping.
That's a relatively small problem, however. The big problem is that temporary cavitation (which is what "hydrostatic shock" actually refers to) does not usually play a big part in actual terminal ballistics. A large temporary cavity does have a very impressive effect in inelastic test mediums, such as pot roasts (muscle tissue loses almost all of its elasticity when dead), watermelons, and clay, but that is because in those mediums, temporary cavity becomes permanent.
This theory likely arose from the observation that very high velocity softpoints used for hunting produced very dramatic effects in animals, while muzzleloaders, revolvers, and other moderate velocity guns did not. Comparative testing of these firearms in an inelastic medium reveals that the high-powered rifle does indeed produce more temporary cavitation. But correlation is not causation.
In that one example, temporary cavity actually does contribute to the dramatic "liquifying" effects of high velocity softpoints in living tissue, but it is not temporary cavity alone; it is the synergistic effect of partial fragmentation perforating tissues, and temporary cavity tearing the weakened tissue. Lower velocity bullets do not normally fragment, and even if they did (as is the case with some rounds), the temporary cavitation is insufficient to cause significant tearing.
The temporary cavitation theory is only partially correct. A better rule of thumb to go by is if a standard softpoint fragments more than 10% at its impact velocity, temporary cavitation is likely enough to cause a synergistic effect.
Energy Transfer
The energy transfer theory states that the more "energy" which is "transferred" to the target, the greater the effect.
What is deadlier, a 50 oz. (1.42kg) basketball thrown at a relatively modest 59.4 mph (95kph), or a 1 pound (454 gram) brick falling from a height of 14 feet, 3 inches (4.36 meters)? Most will answer the brick. More people are killed by falling bricks than thrown basketballs, right?
Well, the aforementioned basketball has the kinetic energy of a .45 caliber (11.43mm), 230 grain (14.9 gram) bullet at 850 feet per second (259 m/s); about 369 foot-pounds (500 joules). The brick has the kinetic energy of a .177 caliber (4.5mm) steel BB at 500 feet per second (152 m/s), a mere 3.2 foot-pounds (4.32 joules). Either is quite capable of damaging a human head, or bouncing off with minimal injury. Identical effect, yet very disparate amounts of "energy transfer."
As stated earlier, real wounding effect is dependent upon the caliber, shape, velocity, weight, and deformation of a bullet. Energy covers only two of those factors, weight and velocity. It is better to consider how large a hole a bullet makes than how much "energy" it "transfers," or how fast it "dumps" that energy. If two bullets expand to the same size and penetrate to the same depth, they will have nearly identical wounding effects, no matter what the other numbers are.
One Shot Stop
This is possibly the worst theory of any. It states that numbers in a book, with no hard evidence provided, somehow show the percent chance of incapacitating a person with one bullet. It is allegedly based on studies of real shooting incidents in which particular brands of ammunition were used.
Consider a scenario in which a particular serial killer has two guns, a .25 ACP and a .44 magnum, both loaded with a very new type of ammunition. He injures 6 people by shooting them once in the right arm with the .44, and they all run away. He then finds 6 more victims and kills each one by shooting them through the right eyeball with his .25. From this anecdotal data, the only possible conclusion is that the .44 magnum round has a 0% chance of a one shot stop, while the .25 ACP cartridge has a 100% chance!
If you look at the data, high-velocity 9mm hollowpoints tend to have the highest one-shot stop percentage. Is this due to some mystical, intrinsic property of those bullets, or is it simply due to the fact that the 9mm has been the preferred caliber by many police departments, thus many of the shots made with those 9mm rounds were made by police officers?
And that is assuming the percentages are based on actual data. Analysis by several unbiased, third party statisticians have shown that the numbers have little to no basis in reality.
Summary
Bullets make holes in people.
If a bullet makes a hole in a central nervous system structure such as the brain, it is extremely likely to stop them. If it hits a major blood vessel or blood-bearing organ, it is fairly likely to stop them within a minute or less. If it does not hit a vital organ, it can only stop them by psychological means. Thus, shot placement is the biggest factor in stopping power.
If a bullet does not penetrate deeply enough to damage a vital structure, it does not damage that structure. Thus, penetration depth is the second most important factor to stopping power.
If a bullet does not make a large enough hole through what vital structures it hits, it might not cause rapid enough bleeding to incapacitate a foe in good time. Thus, hole size (a factor of velocity, penetration depth, and expanded diameter) is the third most important factor for stopping people.
If a bullet has extra features, such as "talons" on certain bullets, secondary fragmentation, or significant temporary cavitation, that increases the wounding effect. Thus, bonus features of the bullet are in last place.
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