If you ask ten shooters what the term “stopping power” means, you will probably get several different answers, but most will center around the same basic idea: how effective a cartridge is at stopping a threat.

Ask those same shooters how stopping power is measured and the answers will begin to diverge. Some will point to muzzle energy. Others will talk about bullet diameter, penetration, expansion, momentum, sectional density, or temporary wound cavity. Ask which handgun cartridge has the most stopping power and you are liable to trigger a dissertation about how .45 ACP won two world wars, how 10mm is the answer to everything short of an angry moose, or how the added capacity of 9x19mm simply gives you more chances to stop the threat.

We have been talking about “stopping power” for well over a century, yet the term has never developed into a single, universally accepted scientific measurement.
Terminal-ballistics researchers can measure penetration, expansion, retained weight, velocity, wound dimensions, and the ability of a projectile to reach and damage important structures. What they cannot do is assign a cartridge one universal number that tells us whether a specific attacker will immediately stop fighting when struck.

That should tell us something.

The problem is not necessarily that stopping power does not exist. The problem is that we have spent decades talking about it as though it were a property contained entirely within the cartridge.

It is not.

Stopping power is better understood as the combined ability of the shooter, firearm, ammunition, circumstances, and the person being shot to produce one specific outcome:

The threatening action stops.

That is an important distinction. We are talking about stopping an attack, not necessarily killing the attacker. Those outcomes sometimes overlap, but they are not the same thing.
A person can suffer a fatal wound and remain capable of purposeful action for some period of time. Another person may stop immediately despite suffering no physical injury at all.

If the purpose of a defensive firearm is to stop a threat, then perhaps the better question is not:

Which cartridge has the most stopping power?

Perhaps the better question is:

What actually causes an attacker to stop?

Where Did “Stopping Power” Come From?

The phrase “stopping power” is not unique to firearms.

In technical fields, similar language has long been used to describe the ability of a material or system to slow or stop motion or energy. In firearms discussions, however, the phrase eventually became a broad description of a weapon or cartridge’s ability to halt an opponent.

The word stop is important.

The concern was not necessarily whether a weapon killed an opponent instantly. The concern was whether it prevented that opponent from continuing to fight.

This became especially important during the late 19th and early 20th centuries, when military forces were evaluating the effectiveness of handgun cartridges.

American experience in the Philippines is frequently cited in this discussion. Reports from the Philippine-American War and later conflicts involving Moro fighters contributed to dissatisfaction with the .38 Long Colt service cartridge. There were reports of determined attackers continuing to fight despite being struck multiple times.

It does not take much examination of the cartridge to understand why its performance was called into question. The military .38 Long Colt load pushed a roughly 150-grain bullet at around 750 to 775 feet per second, producing only about 200 foot-pounds of muzzle energy. By modern standards, that puts it in roughly the same energy class as many .380 ACP loads.

As the chart below illustrates, the .38 Long Colt was substantially less powerful than the larger handgun cartridges that would become associated with the stopping-power debate.

That does not prove that muzzle energy alone determines effectiveness. Later in this article we will explain why it does not. What it does demonstrate is that the Army was asking a relatively modest handgun cartridge to reliably stop highly determined attackers under extremely difficult circumstances.

The U.S. military temporarily returned some older .45-caliber revolvers to service, and subsequent testing and pistol trials eventually contributed to the development and adoption of the .45 ACP cartridge and the M1911 pistol.

That history helped establish a debate that never really went away.

The original argument over .38- and .45-caliber service handguns eventually evolved into .45 ACP versus 9x19mm. Later came .40 S&W versus 9mm, and 10mm Auto versus nearly everything else. Different cartridges rise and fall in popularity, bullet technology improves, firearms change, and new cartridges enter the market, but the underlying argument remains remarkably consistent:

Which cartridge is most effective at stopping an attacker?

More than a century later, we still do not have a simple answer.

Part of the problem is that we naturally gravitate toward things we can measure.

Modern terminal-ballistics testing gives us an enormous amount of objective information about projectile performance. Using calibrated ballistic gelatin and high-speed photography, we can watch a bullet expand, deform, yaw, fragment, and decelerate as it travels through a standardized test medium. We can measure impact velocity, penetration depth, recovered diameter, retained weight, and other characteristics.

Those measurements are valuable because they allow one projectile to be compared with another under controlled and repeatable conditions.

Marketing has eagerly embraced those measurements as well.

Slow-motion video of a hollow-point expanding in clear gelatin is visually impressive. A recovered bullet expanded to more than half an inch photographs beautifully next to an unfired cartridge. Penetration depths, expanded diameters, retained-weight percentages, and muzzle-energy figures also provide wonderfully convenient numbers for advertisements and charts.

The problem is not that those measurements are meaningless.

The problem is that they measure the projectile, not the stop.

Suppose a bullet penetrates 14 inches into calibrated gelatin and expands to 0.62 inch. We now know something useful about how that projectile behaved under those test conditions.

But how does that 0.62-inch diameter translate into the probability of stopping a particular attacker?

How does 14 inches of penetration tell us the exact moment someone will decide, or become physically unable, to continue an attack?

Those questions are much harder to answer.

We focus heavily on the projectile because it is something we can measure. Velocity, penetration, expansion, and retained weight can all be quantified and repeated under controlled conditions.

The rest of the system is nowhere near as cooperative.

We cannot put a reliable numerical value on an attacker’s determination, predict exactly how they will respond to being wounded, or know what anatomical structures a projectile will actually intersect in a real encounter.

That creates a peculiar problem in the stopping-power debate:

The easiest variables to measure are not necessarily the variables that have the greatest influence on the outcome.

That does not diminish the importance of ammunition testing. Quite the opposite. Objective testing tells us what a projectile is capable of doing and helps identify ammunition that performs consistently.

But ballistic testing describes capability, not destiny.

A gelatin block cannot tell us whether an attacker will surrender, lose mobility, suffer rapid neurological incapacitation, or remain capable of purposeful action until blood loss eventually causes unconsciousness.

Those outcomes depend on variables extending far beyond the cartridge.

We have become extraordinarily good at measuring bullets.

Stopping an attacker involves considerably more than the bullet.

How Does a Fight Actually Stop?

For the purposes of this discussion, the mechanisms that end an attack can be grouped into four broad categories:

1. Psychological
2. Neurological
3. Mechanical
4. Circulatory
    

These categories overlap.

A single gunshot may contribute to more than one mechanism. A wound that fractures bone may also cause significant blood loss. A painful injury may produce a psychological surrender. A person may stop fighting voluntarily seconds before physical incapacitation would have occurred anyway.

The important point is that an attacker stops because one or more of these mechanisms has changed their ability or willingness to continue.

The cartridge is merely one part of the process that may create those conditions.

Psychological Stops

The psychological component is probably the most variable and least predictable part of any violent encounter.

A person may stop because they see a firearm. They may stop because the first shot is fired. They may stop after being struck. They may stop because of pain, fear, surprise, or simply because they decide that whatever they intended to accomplish is no longer worth dying over.

In some defensive encounters, the presentation of a firearm may be enough to end the confrontation without a shot ever being fired.

That is not an endorsement of brandishing a firearm, firing warning shots, intentionally missing an attacker, or otherwise using a firearm irresponsibly. It is simply an acknowledgment of reality.

Sometimes a person stops because they choose to stop.

Consider the same principle in a more clearly criminal encounter. A criminal looking for an easy victim may make a similar calculation. The moment that person realizes their intended victim is armed and capable of resisting, the potential reward may suddenly no longer justify the risk. The attacker retreats, surrenders, or simply runs away.

In that situation, the firearm has accomplished its defensive purpose without terminal ballistics entering the equation at all.

No bullet expanded.

No wound cavity was created.

No kinetic energy was transferred to the attacker.

The caliber, bullet weight, penetration depth, and muzzle energy were irrelevant to the outcome because the attacker made the decision to stop before any of those characteristics could matter.

This creates an interesting problem when discussing stopping power.

If the purpose of a defensive firearm is to stop a threatening action, then an encounter that ends without a shot being fired is arguably a perfect stop. The desired outcome was achieved without anyone being wounded. Yet virtually every traditional measurement of stopping power would have nothing to measure because the cartridge itself never entered the equation.

That alone should tell us that stopping power cannot be exclusively a property of ammunition.

The opposite situation is equally important.

An attacker may be extremely determined to continue. Their mental state may be altered by fear, anger, adrenaline, drugs, alcohol, mental illness, or simply an unusually strong commitment to whatever they intended to accomplish. Someone may also understand that being shot does not necessarily mean they are immediately physically incapable of continuing and consciously choose to fight through an injury.

In those circumstances, psychological deterrence may contribute very little.

The attacker may not stop when confronted with a firearm. They may not stop when shots are fired. They may not even immediately stop after sustaining serious or ultimately fatal wounds.

At that point, the problem transitions from psychological willingness to physical capability.
The attacker is no longer being stopped because they have decided to quit. They must instead become physically incapable of continuing the threatening action through one or more of the other mechanisms discussed in this article: neurological disruption, mechanical damage, or loss of circulatory function.

This distinction also helps explain why anecdotal accounts of shootings can appear so contradictory.

One person may collapse or surrender after sustaining a relatively minor wound. Another may remain capable of purposeful action despite injuries that are considerably more severe. It is tempting to attribute those dramatically different outcomes entirely to caliber or ammunition performance, but the psychological response of the individuals involved may have been completely different.

Pain presents another complication.

Pain can certainly influence behavior, but pain is subjective. Individuals perceive and respond to it differently, particularly during periods of extreme stress. A painful injury that immediately convinces one person to stop may have little immediate effect on the behavior of another.

That makes psychological incapacitation potentially extremely effective while simultaneously making it extraordinarily difficult to predict. When it works, the stop may be immediate. It may even occur before the trigger is pulled.

But the defender cannot depend upon it.

You cannot know beforehand whether the person confronting you will flee at the sight of resistance, surrender after being wounded, or continue despite injuries that would cause someone else to immediately give up.

This is why the psychological stop occupies such a strange position in the discussion of stopping power.

It can produce the fastest and least physically destructive resolution possible, yet it is also the mechanism over which the defender has the least control. And once again, the cartridge alone cannot explain the outcome.

A .22 LR that is never fired may be sufficient to convince one attacker to flee. A much more powerful handgun cartridge may fail to psychologically deter another attacker even after that person has been wounded.

The cartridges did not somehow exchange their physical capabilities.

The people responded differently.

That variability is precisely why stopping power cannot be understood by studying ammunition alone.

Neurological Stops

The nervous system is the communication network that allows the brain to control the body.

The brain receives and processes information and sends electrochemical signals through the spinal cord and peripheral nervous system. Those signals control voluntary movement, transmit sensory information, and help regulate many of the involuntary processes necessary to sustain life.

Damage to that system can interrupt those communications.

A projectile can directly damage peripheral nerves, the spinal cord, or brain tissue. Neurological damage can also occur indirectly as surrounding tissue and bone are displaced, fractured, crushed, or otherwise damaged.

Not all neurological injuries, however, produce the same result.

Damage to a peripheral nerve may cause pain, numbness, weakness, or loss of function in the part of the body served by that nerve. A severed or severely damaged nerve controlling part of an arm, for example, may impair the use of that arm without preventing the person from remaining conscious, moving, or continuing purposeful action.

The central nervous system is different.

The brain and spinal cord serve as the primary control and communication pathways for the body. Severe physical disruption of certain critical structures can immediately interrupt the signals necessary for consciousness or purposeful movement. Unlike circulatory incapacitation, which generally requires some period of time for blood pressure and oxygen delivery to fall, sufficiently severe central nervous system disruption can produce an effect essentially at the moment the damage occurs.

This is the physiological basis for the familiar “lights out” description of neurological incapacitation.

But even here, location matters enormously.

The brain is not a single homogeneous organ in which damage anywhere produces the same result. Different regions perform different functions, and injuries that appear superficially similar can produce dramatically different outcomes. A projectile entering the skull does not automatically cause immediate unconsciousness or complete loss of voluntary movement. The result depends on the structures damaged and the extent of that damage.

The brainstem and upper spinal cord are especially significant because of their role in consciousness, basic life functions, and the transmission of motor signals between the brain and the rest of the body. Severe disruption of these critical central nervous system structures can result in extremely rapid loss of purposeful movement.

The difficulty is reaching them.

The spinal cord is relatively small and surrounded by the vertebral column. Depending on the direction from which a projectile enters the body, it may have to pass through substantial amounts of tissue and potentially other anatomical structures before reaching the spine. Even striking a vertebra does not necessarily mean the spinal cord itself will be completely disrupted.

The brain presents a different problem.

The head is comparatively small relative to the torso, and it is highly mobile. During a violent encounter, the attacker may be moving, the defender may be moving, or both may be moving simultaneously. Orientation can change in fractions of a second.

A hit somewhere on the head therefore does not automatically guarantee immediate incapacitation.

Once again, the effect depends on what anatomical structures are actually damaged.
Direct disruption of critical portions of the brain or upper spinal cord can produce the familiar “lights out” response associated with rapid neurological incapacitation. In some firearms training literature, this concept is illustrated using what is commonly called the “Fatal T,” depicted in the figures below.

The Fatal T should not be confused with a defined anatomical structure or a guarantee of incapacitation. It is a simplified training concept used to represent an approximate region of the face and skull associated with underlying central nervous system structures. Real anatomy is three-dimensional, varies between individuals, and changes in orientation as the head moves.
 

Frontal View

 
Profile View

The illustrations are therefore best understood as anatomical orientation aids rather than maps guaranteeing a particular outcome.

This distinction matters because neurological incapacitation occupies an unusual position in the stopping-power discussion.

If the central nervous system are physically disrupted, the resulting loss of purposeful action can be extraordinarily rapid. It does not necessarily depend upon the attacker recognizing that they have been wounded, responding to pain, deciding to surrender, or waiting for blood loss to reduce consciousness.

But the structures capable of producing that result are relatively small and well protected.
That makes neurological incapacitation potentially one of the fastest mechanisms for physically stopping purposeful action while simultaneously making it difficult to produce predictably in the chaotic circumstances of a defensive encounter.

This is one reason defensive shooting doctrine generally emphasizes increasing the probability of effective hits rather than relying upon the ability to strike extremely small anatomical structures.

There is also an important distinction between possible and predictable.

A projectile may damage a peripheral nerve. It may fracture a vertebra and damage the spinal cord. It may penetrate the skull and disrupt critical brain tissue. Each of those outcomes is possible, and each can contribute to stopping an attack.

But the shooter cannot precisely control the internal path of a projectile once it enters the body. Small differences in impact location, angle, anatomy, projectile behavior, and structures encountered along the wound path can produce very different results.
That brings us back to the larger problem with stopping power.

Neurological incapacitation demonstrates perhaps more clearly than any other mechanism why where a projectile goes can matter more than the numerical characteristics printed on the ammunition box.

A projectile carrying tremendous energy that fails to intersect critical anatomy cannot produce neurological incapacitation simply because it is powerful. Conversely, a considerably less energetic projectile that happens to disrupt a critical central nervous system structure may produce an immediate effect.

The cartridge influences whether the projectile can penetrate deeply enough and remain structurally capable of reaching those structures.

The shooter influences where that projectile begins its path.

Anatomy and circumstance determine what it actually encounters along the way.

Neurological incapacitation can therefore be extraordinarily fast.

Producing it intentionally and predictably is another matter entirely.

Mechanical Stops

The human skeleton provides the structural framework that allows the body to function.

Bones support weight, joints permit movement, muscles generate force, and tendons and ligaments transfer and control that force. Together, these structures allow a person to stand, walk, run, maintain balance, carry weight, and manipulate objects.

Damage those structures severely enough and physical capability can be reduced or lost.
This is the mechanical component of stopping an attack.

Unlike a psychological stop, the attacker does not necessarily decide to stop. Unlike neurological incapacitation, the brain may remain fully conscious and capable of sending commands to the body. Instead, the physical structure required to carry out those commands has been damaged.

The effect depends heavily on what is damaged and how severely it is damaged.

A fractured finger is unlikely to prevent someone from running. Damage to a hand or wrist may interfere with the ability to grip or manipulate an object without affecting mobility. A severely damaged knee, hip, femur, or pelvis may substantially impair the ability to stand or move. Significant damage to an arm or shoulder may reduce the ability to use that limb.

This makes mechanical incapacitation highly dependent upon anatomy.

Not all bones perform the same function, and not all fractures have the same consequences. Some skeletal structures bear substantial loads, while others primarily protect organs or provide attachment points for muscles. A fracture can range from a relatively minor crack that leaves considerable function intact to extensive disruption that makes normal movement mechanically impossible.

Just as importantly, striking a bone and mechanically disabling it are not the same thing.

A projectile may glance off a bone, create a relatively minor fracture, penetrate it, fragment it, or cause extensive structural failure. Projectile diameter, construction, velocity, impact angle, and the particular bone involved can all influence the result. The age, health, and anatomy of the individual introduce still more variables.

The pelvic structure is commonly discussed in conversations about mechanical incapacitation because it carries much of the weight of the upper body and transfers that load into the legs.

But the pelvis is not a single bone waiting to shatter when struck.

It is a complex ring of bones and joints. A projectile passing somewhere through the pelvic region does not automatically cause catastrophic structural failure, and even a pelvic fracture does not necessarily mean that a person will immediately collapse or become completely incapable of movement.

Location and severity matter enormously.

Significant disruption of weight-bearing portions of the pelvis, hip joint, or major bones of the legs can make standing, maintaining balance, or moving effectively extremely difficult. In sufficiently severe cases, the person’s nervous system may still be issuing the command to stand or move forward while the damaged skeletal structure is simply no longer capable of supporting the required load.

That is mechanical incapacitation in its clearest form.

But even complete loss of mobility does not necessarily mean the threat has ended.
Someone who can no longer stand may remain conscious. They may retain full use of their arms. They may still possess a weapon. Their mental state and willingness to continue the attack may be completely unchanged.

This is the major limitation of a mechanical stop.

Mechanical incapacitation can remove a capability without necessarily removing intent.

A damaged leg may prevent pursuit without preventing other purposeful action. An injured arm may eliminate the effective use of that limb while leaving the legs and opposite arm functional. Even substantial skeletal injuries may therefore change the nature of the threat rather than immediately eliminate it.

Mechanical damage can also overlap with the other mechanisms discussed in this article.

A projectile that fractures bone may simultaneously damage major blood vessels, contributing to circulatory failure. Bone fragments and projectile fragments may damage surrounding nerves or tissue. Damage near the vertebral column can potentially involve the spinal cord. Pain and the realization that a limb no longer functions may also produce a psychological stop.

Real injuries rarely respect the neat categories we use to explain them.

The categories are useful because they describe different mechanisms, not because every gunshot wound fits cleanly into only one of them.

Mechanical incapacitation also demonstrates another limitation of using simple ballistic measurements to predict stopping power. Penetration depth and expanded bullet diameter can tell us something about what a projectile is capable of doing, but neither measurement tells us whether the projectile will actually intersect a critical structural component.

A projectile may penetrate deeply without striking a major bone. Another may strike bone relatively early in its path and behave very differently from the same projectile fired into bare ballistic gelatin.

Once again, location matters as much as capability.

The projectile must reach the relevant structure, and it must damage that structure severely enough to meaningfully reduce physical function.

Mechanical stops therefore occupy an important middle ground in the stopping-power discussion. They may not produce the immediate loss of consciousness associated with severe central nervous system disruption, and they do not necessarily require waiting for blood loss to eventually impair brain function.

Instead, they work by damaging the physical machinery required for purposeful movement.
That can profoundly alter the outcome of an encounter.

But it should never be confused with a guarantee that the encounter is over.
A person who has lost mobility has lost an important physical capability. They have not necessarily lost consciousness, intent, or every means of continuing the threat.

Circulatory Stops

The human circulatory system is, in many respects, a biological hydraulic system.

The comparison is imperfect, but useful. The heart functions as the pump, blood vessels provide the distribution network, and blood serves as the transport medium carrying oxygen and nutrients throughout the body. Blood leaves the heart primarily through the arterial system, travels through progressively smaller vessels into the tissues, and eventually returns through the venous system.

For the system to function, the body must maintain sufficient circulating blood volume, blood pressure, and blood flow to critical organs.

Penetrating trauma can compromise that system by damaging blood vessels, the heart, or highly vascular organs.

The consequences depend heavily upon what is damaged.

Injury to relatively small blood vessels may produce limited bleeding that has little immediate effect on the person’s ability to function. Damage to a major artery, large vein, the heart, lungs, liver, or other highly vascular structures can produce much more substantial internal or external blood loss.

This is where shot placement once again becomes enormously important.

Two wound paths separated by only a small distance may produce dramatically different results. One may pass primarily through muscle and smaller vessels, while another intersects a major blood vessel or organ and causes rapid hemorrhage.

As blood is lost, the circulatory system attempts to compensate. The heart can increase its rate, peripheral blood vessels can constrict, and blood flow can be preferentially maintained to critical organs. These mechanisms can temporarily help preserve blood pressure and brain function despite significant injury.

But compensation has limits.

As blood loss becomes sufficiently severe, the circulatory system can no longer maintain adequate perfusion. Blood pressure falls, and the delivery of oxygen to the brain and other critical organs becomes inadequate. Mental function deteriorates, physical performance declines, consciousness can be lost, and eventually normal biological function becomes impossible.

From the standpoint of stopping an attack, however, there is an enormous complication:

All of this takes time.

A wound can be fatal without being immediately incapacitating.

That distinction is critical to understanding terminal ballistics.

A projectile may produce an injury from which the person will not survive without immediate medical intervention, yet that person may retain sufficient blood pressure, oxygenation, consciousness, and muscular function to continue purposeful action for some period afterward. How long that period lasts depends on the structures damaged, the severity and location of the injury, the rate of hemorrhage, the individual’s physiology, and numerous other variables.

Sometimes useful function may deteriorate rapidly. Other times, the interval can be considerably longer than the few seconds in which a violent encounter is decided.

The important limitation of circulatory incapacitation is time.

Unlike sufficiently severe disruption of the central nervous system, blood loss generally requires a physiological process to occur before purposeful action becomes impossible. How quickly that happens depends on the structures damaged, the rate of hemorrhage, and the physiology of the individual.

This is also why modern trauma medicine places such enormous emphasis on hemorrhage control. Programs such as Stop the Bleed teach ordinary people to recognize and control life-threatening bleeding because severe blood loss is a major preventable cause of death following traumatic injury.

From the perspective of trauma medicine, the objective is to preserve circulating blood volume and maintain perfusion until the injury can be treated. From the perspective of terminal-ballistics research, we are examining what happens when penetrating trauma compromises that same system.

Circulatory disruption can be devastating and ultimately fatal. But it does not necessarily provide the immediate “off switch” often imagined in discussions of stopping power.
A person can sustain an ultimately fatal wound and remain capable of purposeful action for some period afterward.

That distinction between lethality and incapacitation deserves a closer look.

Lethality is Not the Same as Incapacitation

This distinction deserves emphasis.

A wound can be lethal without producing an immediate stop. Likewise, an encounter can be stopped without producing a lethal wound.

Those are separate outcomes.

If an attacker runs away when confronted with an armed defender, the threat has stopped even though no one was injured. From the standpoint of the defender, the desired outcome has been achieved. The threatening action ended.

At the opposite extreme, an attacker may sustain a wound that will ultimately prove fatal yet remain conscious and physically capable of purposeful action for some period afterward. If that person continues fighting for another thirty seconds, the wound may unquestionably be lethal while still failing to produce the immediate stop the defender needed.

This distinction is at the heart of the stopping-power discussion.

For more than a century, much of the debate has blurred together three different questions:

1. How much damage does the projectile produce?
2. How likely is that damage to eventually prove fatal?
3. How quickly does that damage cause the threatening action to stop?

Those questions are related, but they are not interchangeable.

A projectile that produces more extensive tissue damage may increase the probability of serious injury or death without guaranteeing immediate incapacitation. Likewise, an injury that appears relatively minor when examined afterward may have been sufficient to psychologically convince an attacker to surrender or flee.

This is why mortality statistics alone cannot completely answer questions about stopping power. Knowing that a particular wound was ultimately fatal tells us how the injury ended. It does not necessarily tell us what the wounded person was capable of doing during the seconds or minutes immediately following the injury.

The opposite is also true. Survival does not mean the firearm failed to stop the attack. If the threatening action ended immediately and the attacker subsequently survived their injuries, the defensive objective was still accomplished.

Stopping is an immediate behavioral or physiological outcome. Lethality is a medical outcome that may not be determined until considerably later.

Terminal ballistics can tell us a great deal about what a projectile does after impact. We can measure penetration, expansion, fragmentation, retained weight, velocity, and other characteristics. Those measurements help us understand the projectile’s capability to reach and disrupt important anatomical structures.

What they cannot completely predict is how quickly a particular human being will cease purposeful action.

That requires us to look beyond the projectile.

Think in Systems

Once we understand the mechanisms that actually stop an attack, we can begin looking at stopping power differently.

Instead of treating the cartridge as the entire system, we need to think about the system as a whole.

That system includes:
• The shooter
• The firearm
• The ammunition
• The shooter’s familiarity with the firearm
• Accuracy
• Controllability
• Reliability
• Capacity
• Carryability
• Training and experience
• The circumstances of the encounter
• The physical and psychological condition of the attacker

Every one of these variables can influence the probability of successfully stopping a threatening action.

Some are closely related. Firearm size and weight can influence controllability. Cartridge selection can affect recoil and capacity. Ammunition can influence reliability. Training and familiarity can improve practical accuracy and reduce the likelihood of fumbling with controls under stress. Carryability influences whether the firearm is actually present when it is needed.

Other variables are largely outside the defender’s control.

The defender does not choose the attacker’s determination, anatomy, mental state, physical condition, or response to injury. Nor do they necessarily choose the lighting, distance, available cover, number of attackers, direction of movement, or amount of warning before the encounter begins.

Stopping power therefore starts looking less like a property of ammunition and more like a probability produced by a complex system.

One way of visualizing this is to think about building a character in a tabletop game.

A highly trained shooter might receive advantages from experience, accuracy, recoil control, and familiarity with their equipment. Give that shooter a firearm they know well and another variable moves in their favor. Give them reliable ammunition that demonstrates consistent penetration and terminal performance and the probability improves again.
Now start changing the conditions.

Give the same shooter an unfamiliar firearm with poor sights, an unfamiliar trigger, or controls they have rarely practiced using and some of that advantage disappears.
Reduce the size and weight of the firearm until recoil becomes substantially more difficult to control.

Introduce darkness. Add movement. Introduce surprise, physical injury, confined spaces, awkward positioning, or multiple attackers. Change the attacker’s determination, physiology, or response to injury.

The equation changes with every variable.

Unlike a tabletop game, however, we do not know the values. There is no character sheet telling us that training provides a +4 advantage while poor lighting imposes a -2 penalty. We cannot assign a numerical modifier to the attacker’s determination or calculate exactly how much firearm familiarity compensates for increased recoil.

Most importantly, we do not get to see the die roll before the encounter begins.
Chance remains part of the equation.

A completely inexperienced person with a .22 LR pistol could fire one poorly aimed shot and, through sheer luck, produce an immediate stop. That outcome would not demonstrate that the .22 LR possesses extraordinary stopping power or that training is unnecessary. It would demonstrate that improbable outcomes sometimes occur.

At the other extreme, a highly trained shooter using a reliable handgun and capable defensive ammunition could do many things correctly and still encounter circumstances in which the attacker does not immediately stop.

Neither example disproves the importance of equipment or training.
They demonstrate the limits of certainty.

The Problem With Anecdotal Evidence

Firearms discussions are filled with stories about stopping power. Talk to enough police officers, soldiers, hunters, or concealed carriers and eventually you will hear stories about a particular cartridge performing either incredibly well or incredibly poorly.
Two stories shared with me while reviewing this article illustrate the problem particularly well.

Now consider a very different story.

Taken at face value, these stories appear to support one of the oldest arguments in handgun history:

9mm required numerous hits. .45 ACP required one. Therefore, .45 ACP has greater stopping power.

Except we cannot actually draw that conclusion from these stories.

Both accounts can be completely true while telling us very little about the relative effectiveness of the two cartridges.

We do not have controlled conditions. We do not know enough about the exact wound paths, anatomical structures damaged, ammunition used in the 9mm incident, sequence of impacts, or physiological and psychological condition of the people who were shot. In the Vietnam account, we know even less about the wound beyond the veteran’s recollection that one round was fired and the opposing soldier immediately went down.

What we have are two observations:

One person reportedly remained capable of fighting after receiving numerous 9mm wounds. Another reportedly stopped immediately after receiving a single .45 ACP wound.

What we cannot establish is why.

The .45 ACP projectile may have intersected anatomy that produced rapid incapacitation. The 9mm projectiles may not have done so until later in the encounter. Psychological, neurological, mechanical, or circulatory factors may have contributed to either outcome.

We simply do not know.

That is the danger of using individual shootings to establish universal rules about stopping power.

An anecdote can accurately describe what happened without establishing why it happened.

Neither story proves that 9mm is ineffective. Neither proves that .45 ACP is superior. Just as importantly, neither proves that the cartridges are equivalent.

They are individual outcomes from two extraordinarily different events.

This also helps explain how experienced and credible people can develop strongly held, and sometimes completely contradictory, opinions about handgun cartridges based on what they have personally witnessed.

Their experiences may be entirely genuine. Their conclusions may even be reasonable given those experiences.

But experience is not the same thing as a controlled experiment.

Individual shootings are snapshots of extraordinarily complicated events involving different shooters, firearms, ammunition, wound paths, anatomy, mental states, environments, and circumstances. When enough variables are involved, unusual outcomes should not surprise us.

Two truthful stories can therefore appear to contradict one another without either one telling us the whole story.

If anything, the contrast between these accounts reinforces the central argument of this article:

The cartridge is only one variable in a much larger system.

That is the real value of thinking in systems.

It forces us to stop asking whether one cartridge, one bullet design, or one numerical measurement contains the answer by itself. Instead, we recognize that the outcome emerges from the interaction of many variables, some controllable and some not.
We are not searching for a magic cartridge that eliminates uncertainty.

We are trying to understand the system well enough to identify which variables matter, which ones we can influence, and which ones we simply have to accept.

Luck will always exist. Sometimes it will work in our favor. Sometimes it will not.

Luck is a terrible strategy. Preparation is how we improve the odds

What Does the Cartridge Actually Contribute?

None of this means caliber or ammunition selection does not matter. It absolutely does. The mistake is assuming that any single measurement, whether kinetic energy, momentum, velocity, bullet diameter, or expansion, can independently predict how effective a cartridge will be.

A projectile carries both kinetic energy and momentum into the target, and both describe real physical properties of that projectile. However, neither value tells us exactly what will happen after the bullet enters tissue. Two projectiles can carry similar amounts of energy and produce very different results because they behave differently during penetration.
Penetration is influenced by projectile mass, velocity, diameter, shape, construction, expansion, yaw, fragmentation, and the resistance encountered along the wound path. Bullet design determines how those variables interact. A projectile designed to expand rapidly may behave very differently from a non-expanding projectile of the same caliber, even when both leave the muzzle at similar velocities.

Bullet diameter matters for the same reason. A projectile that expands to a larger frontal diameter has the potential to crush and disrupt a larger volume of tissue as it penetrates. But expansion comes at a cost. Increasing frontal area also increases resistance, which can reduce penetration. A bullet that expands dramatically but stops before reaching critical anatomy may ultimately be less effective than a smaller projectile that penetrates deeply enough to reach it.

This is why ammunition designers are constantly balancing competing objectives. They are trying to achieve sufficient penetration while producing useful expansion or other terminal effects, and they must accomplish that across a range of impact velocities and intermediate barriers. There is no free lunch hidden somewhere inside a cartridge case. Improving one characteristic often requires accepting a compromise somewhere else.

The same trade-offs exist when comparing cartridges.

A more powerful cartridge may provide greater velocity, heavier bullets, larger diameter, or greater penetration potential. Those advantages are real. But obtaining them may require greater chamber pressure, more propellant, a larger cartridge, or some combination of the three. At the firearm level, that can translate into greater recoil, increased ammunition weight, larger firearm dimensions, reduced magazine capacity, or reduced controllability.
Those relationships are not universal, however, because the cartridge is only one component of the system.

Firearm design matters. A cartridge that is unpleasant to shoot from a lightweight compact pistol may be entirely manageable from a larger and heavier handgun. Grip geometry, bore axis, firearm mass, operating system, barrel length, and even the shape and texture of the grip can influence how effectively a shooter controls the firearm.

Shooter physiology matters as well. Hand size, grip strength, body mechanics, experience, and tolerance for recoil vary considerably between individuals. One shooter may handle a larger and more powerful cartridge extremely well, while another may achieve substantially better accuracy and faster follow-up shots with something smaller.

Training matters because none of the terminal performance discussed in this article exists until the projectile actually reaches something important.

That creates another set of compromises. A smaller firearm may be easier to conceal and therefore easier to carry consistently, but its reduced mass and smaller gripping surface can make it harder to control. A larger firearm may offer better sights, greater mass, a longer sight radius, more gripping area, and greater practical shootability, but it may also be more difficult to conceal and less convenient to carry.

Magazine capacity introduces yet another trade-off. Increasing cartridge diameter generally means fewer cartridges can fit within a magazine of a given size. Whether the additional performance of the larger cartridge is worth giving up ammunition capacity depends on the firearm, the cartridge, the shooter, and the circumstances in which the firearm is expected to be used.

This is why discussions about stopping power become so difficult when they are reduced to caliber alone. Comparing cartridges on a ballistic table can tell us something about their capabilities, but it cannot tell us the probability that a particular shooter will successfully use a particular firearm during a defensive encounter.

The question is not simply:

Which cartridge is more powerful?

That question can often be answered with measurements.

The more useful question is:

Which combination of firearm, cartridge, ammunition, capacity, controllability, and shooter proficiency gives this shooter the highest probability of placing effective rounds where they need to go under realistic conditions?

That answer will not necessarily be the cartridge with the greatest muzzle energy, the largest bullet, the highest capacity, or the least recoil. It is the combination that provides adequate terminal performance while allowing the shooter to reliably deliver that performance.

Stopping power, therefore, is not a characteristic contained inside a cartridge.
It is the result of the entire system working together.
 

Ballistics Gel Testing is Important but Over Emphasized

Ballistic gelatin, especially when combined with high-speed video, has increasingly become a marketing tool. That does not make gelatin testing useless. Quite the opposite: properly calibrated ballistic gelatin provides a standardized and repeatable medium for comparing bullet performance. The problem begins when performance in gelatin is treated as though it perfectly predicts performance in the human body.

A spectacular slow-motion video of a hollow-point expanding in bare gelatin demonstrates what that bullet did under those specific test conditions. It does not tell us exactly what will happen if the bullet first passes through a heavy winter coat, strikes an arm before entering the torso, impacts bone, passes through glass or another intermediate barrier, or strikes at a substantially different velocity.

There is rarely a disclaimer beneath the high-speed video saying, “If the bullet hits bone, your results may vary.” Likewise, the dramatic expansion demonstrated in bare gelatin does not guarantee that the bullet will expand identically after passing through clothing or other intermediate materials.

This does not mean gelatin testing is misleading. It means the conclusions drawn from it sometimes are.

The Federal Bureau of Investigation (FBI) recognized these limitations when developing its standardized ammunition testing protocol. Rather than relying exclusively on bare gelatin, the FBI evaluated ammunition through clothing and common intermediate barriers such as steel, wallboard, plywood, and automotive glass. These tests provide a much broader picture of projectile performance and are better suited to identifying ammunition that performs consistently across a range of conditions.

Even these tests cannot perfectly predict the outcome of an actual shooting. Human anatomy, shot placement, impact angle, intermediate tissue, bone, clothing, barriers, and individual physiology introduce variables that no block of gelatin can completely reproduce.
Ballistic gelatin is therefore best understood as a measurement tool, not a prediction machine. It allows us to compare penetration, expansion, fragmentation, and projectile behavior under controlled conditions. That information is valuable, but it remains only one piece of the much larger terminal-ballistics puzzle.

Playing the Odds

Once we accept that stopping power is the product of a complex system, the practical question becomes much simpler:

What can we actually control?

We cannot choose the attacker, their mental state, their physiology, or their willingness to continue. We cannot know the lighting, distance, movement, barriers, number of attackers, or amount of warning we may have.

Those variables are part of the uncertainty.

Preparation is how we influence the variables that are not.

We can select a reliable firearm.

We can choose ammunition that demonstrates adequate penetration and useful terminal performance across a reasonable range of conditions.

We can choose a firearm and cartridge combination that we can actually control rather than simply selecting the most powerful cartridge we are willing to tolerate.

Most importantly, we can train with it.

We can become familiar with the sights, trigger, recoil, controls, and handling characteristics of the firearm. We can learn how quickly we can recover from recoil and place another accurate shot. We can practice from realistic positions and distances. We can learn where our abilities begin to deteriorate and work to improve them.

That matters because defensive handgun effectiveness is not determined by any single characteristic.

The differences between .380 Auto, 9x19mm, .40 S&W, .45 ACP, 10mm Auto, and other common defensive cartridges are real. Differences in bullet diameter, mass, velocity, momentum, penetration, expansion, recoil, and capacity should not be dismissed with the claim that “shot placement is everything.” But those differences exist inside a much larger system.

A more powerful cartridge cannot compensate for a miss. A theoretically superior bullet provides little advantage if the firearm is so unpleasant to shoot that its owner rarely practices with it. Greater terminal performance may not represent a practical improvement if the additional recoil significantly reduces the shooter’s ability to make accurate follow-up shots.

A high-capacity pistol offers little benefit if the shooter cannot reliably place those rounds where they need to go.

The opposite is equally true.

Excellent training does not make ammunition performance irrelevant. Accuracy cannot compensate for a projectile that consistently fails to penetrate deeply enough to reach important anatomical structures. Controllability does not automatically make the smallest and least powerful cartridge the best choice. Capacity matters, but adding cartridges to the magazine does not improve what each individual projectile is capable of accomplishing once it reaches the target.

This is why there is no universally “best” defensive handgun cartridge.

There are only trade-offs.

A larger or more powerful cartridge may offer greater bullet mass, diameter, velocity, or penetration potential, but it may also produce more recoil or reduce capacity.

A smaller firearm may be easier to conceal and more likely to be carried consistently, but it may also be harder to shoot well.

A larger handgun may be easier to control and more comfortable to practice with, but it may be less convenient to carry every day.

More capacity may provide more opportunities to solve the problem, but capacity alone does not make each individual hit more effective.

Every advantage comes with some combination of cost, compromise, or diminishing return.
That is why the goal should not be to maximize any single characteristic.

The goal should be to find the best balance of reliability, terminal performance, controllability, capacity, concealability, and shooter proficiency for the individual carrying the firearm.

We want a firearm that is reliable enough to function when needed, capable enough to deliver useful terminal performance, controllable enough to allow accurate shooting, practical enough to carry consistently, and comfortable enough to train with regularly.
We want ammunition that performs consistently rather than spectacularly under one idealized test condition.

And we want enough proficiency with the entire system that using the firearm does not become another problem to solve during an already chaotic event.

That is what it means to play the odds intelligently.

The question is not simply:

Which cartridge is more powerful?

The better question is:

Which combination gives this shooter the highest probability of placing effective rounds where they need to go under realistic conditions?

That is ultimately what defensive handgun selection is about. We cannot eliminate the uncertainty of a violent encounter. We can only stack as many controllable variables in our favor as possible before one ever occurs.

Civilian Defense Is Not Military or Police Work

This discussion also changes considerably depending on context.

A civilian defensive shooting is not the same problem as a military engagement or a law-enforcement operation. The immediate objective during a violent confrontation may broadly be similar, to make the other person stop what they are doing, but the mission, circumstances, equipment, training, and resources available can be dramatically different.

Military personnel may operate as part of a team with rifles, machine guns, body armor, communications equipment, medical support, and other resources. Their weapons and ammunition may also be selected around requirements that have little relevance to a private citizen carrying a concealed handgun. Engagement distances, intermediate barriers, ammunition logistics, compatibility with existing weapons, and the ability to engage threats under very different circumstances can all influence military requirements.

Law enforcement presents another set of circumstances. An officer may begin an encounter with a handgun, but may also have body armor, communications equipment, partners, a patrol rifle or shotgun, and additional officers responding to the scene. Depending on the operation, officers may know they are entering a potentially violent situation before the encounter begins and can select equipment accordingly.

A private citizen usually has none of those advantages.

A defensive encounter may begin unexpectedly during otherwise ordinary life. The firearm available is likely whatever the person happened to be carrying at that moment, which for many people means a compact or subcompact handgun selected partly because it can be carried and concealed consistently.

That distinction matters.

The military can select equipment around a mission. Law enforcement can sometimes select equipment around an anticipated threat. The armed citizen generally has to select equipment before knowing what the problem will be.

The expectations are different. The equipment is different. The legal environment is different. The available support is different. Most importantly for this discussion, the circumstances surrounding the use of the firearm can be very different.

This becomes particularly important when military or law-enforcement shooting statistics are used to make broad claims about civilian defensive handgun performance.

Such data can absolutely be useful. It can tell us something about projectile performance, hit location, incapacitation, ammunition failures, intermediate barriers, and the unpredictable nature of people after they have been wounded. Large collections of real-world shootings can reveal patterns that cannot be reproduced completely in a laboratory.

But the limitations of the data have to be understood.

A statistic gathered from police shootings is not automatically representative of civilian defensive shootings. Likewise, observations made during military combat cannot automatically be applied to a concealed handgun encounter. Different weapons may be involved. Different ammunition may be used. Engagement distances can differ. The number of shots fired can differ. The presence of multiple armed participants can differ. Body armor, intermediate barriers, medical response, and even the criteria used to define whether someone was successfully “stopped” may differ.

There is also a basic problem with the word stop itself.

Does a stop mean the attacker immediately collapsed? Does it mean they remained conscious but stopped advancing? Does it mean they fled? Does it mean they were physically incapable of continuing? Does surrender count? What if someone sustained a fatal wound but continued fighting for another thirty seconds?

Those outcomes are very different physiologically, yet depending on how a study or database is constructed, several of them might be grouped together as successful stops.
This does not make military, law-enforcement, or civilian shooting data worthless. Quite the opposite. Real-world data provides an important check against laboratory testing and theoretical arguments. But it needs to be interpreted within the environment from which it came.

The data may be useful. The conclusions drawn from it need to be appropriately limited.

When discussing stopping power, comparing a military rifle engagement, a police shooting, and a civilian defensive handgun encounter as though they were interchangeable experiments introduces variables that have nothing to do with caliber or bullet performance.

They are different problems involving different systems.

And if stopping power is going to be understood as the performance of the entire system, rather than simply a number attached to a cartridge, context has to be part of that system as well.

So What Is Stopping Power?

Stopping power is not something contained inside a cartridge case.

And it certainly is not a number printed on an ammunition box. All of those things can contribute to the outcome, but none of them independently defines it.

At its most basic level, stopping power is exactly what the name implies: the ability to stop an attacker from continuing a threatening action.

The outcome itself is remarkably simple.

The attacker either stops or does not.

How we arrive at that outcome is anything but simple.

As discussed throughout this article, an attack can end through several different mechanisms. The attacker may stop psychologically, deciding that continuing the confrontation is no longer worth the risk. They may be stopped mechanically because damage to skeletal structures limits their ability to continue moving. Damage to critical portions of the central nervous system may produce rapid neurological incapacitation. Or sufficient damage to the circulatory system may eventually reduce blood pressure and oxygen delivery to the brain to the point that continued purposeful action becomes impossible.

More than one of these mechanisms may be occurring at the same time, and the person defending themselves has very little control over which one ultimately ends the encounter.
What they can influence are the variables they bring into it.

The cartridge contributes by providing the projectile with the physical characteristics necessary to penetrate and disrupt tissue. Bullet construction, mass, diameter, velocity, expansion, and penetration all influence what happens after the projectile reaches the target. Those differences are real, measurable, and worth considering.

But the cartridge cannot determine where the projectile goes.

The firearm contributes through reliability, practical accuracy, capacity, controllability, ergonomics, sights, trigger characteristics, and handling. A firearm that allows one shooter to perform extremely well may be a poor choice for another. The best firearm on paper provides little benefit if it is difficult for its owner to carry, operate, or shoot effectively.
The shooter contributes through skill, judgment, experience, familiarity, and the ability to accurately place rounds under stress. Training cannot guarantee success, but it can increase the probability that the shooter will be able to use the equipment effectively when circumstances are considerably less cooperative than they are on a square range.

Then there is the attacker.

They contribute perhaps the greatest unknown in the entire equation.

Their physical condition, anatomy, mental state, determination, emotional state, intoxication, injuries, expectations, and reaction to being confronted or wounded can dramatically influence the outcome. One attacker may flee at the sight of resistance without a shot ever being fired. Another may continue purposeful action after sustaining injuries that will ultimately prove fatal.

The ammunition did not suddenly become more or less powerful between those two encounters.

The circumstances changed.

That distinction is at the heart of why stopping power has resisted more than a century of attempts to reduce it to a simple number.

We can measure muzzle velocity. We can calculate kinetic energy and momentum. We can measure penetration and expansion in calibrated ballistic gelatin. We can record recoil, capacity, and accuracy. We can study shootings and identify patterns in real-world outcomes.

All of those measurements provide useful information.

None of them, individually, measures stopping power.

Stopping power is better understood as an outcome produced by a system and the probability that the system will produce that outcome when required.

That system includes the ammunition, firearm, shooter, attacker, and circumstances surrounding the encounter. Change one component and the probability of a successful outcome may change with it.

Viewed this way, the century-old search for the handgun cartridge possessing the greatest stopping power starts to look like the wrong question.

That does not mean cartridge selection is irrelevant. It means cartridge selection is one engineering decision within a much larger system.

The goal should not be to find the cartridge with the highest imaginary stopping-power score. Nor should it simply be to carry the largest caliber, the greatest muzzle energy, or the highest magazine capacity available.

The goal is to stack the variables that we can control in our favor.

Select a reliable firearm. Choose ammunition capable of adequate penetration and useful terminal performance. Choose a combination you can control. Carry it consistently. Become familiar with it. Train with it. Understand its limitations and, perhaps more importantly, understand your own.

Then recognize that even after doing everything correctly, uncertainty remains.

There is no cartridge that guarantees an immediate stop. There is no particular bullet diameter that overrides anatomy. There is no amount of muzzle energy that can compensate for a projectile that fails to reach something important. And there is no ballistic advantage that can compensate for a shot that misses entirely.

That is perhaps the most important lesson buried beneath more than a century of arguments about stopping power.

The cartridge matters. The bullet matters. The firearm matters. The shooter matters. The circumstances matter.

They matter because they all contribute to the probability of achieving the only result that ultimately defines stopping power:

The threatening action stops.

Sometimes luck will play a role. An inexperienced shooter with a marginal cartridge may succeed under terrible circumstances, while a highly trained shooter with excellent equipment may encounter circumstances where every advantage seems to work against them.

That is the uncomfortable reality of defensive shootings. There are no guarantees, only probabilities.

And while luck may occasionally save someone who was completely unprepared, luck is not a strategy.

-Jay-