If I knew I'd be in a gunfight within the next 20 minutes, I'd be laying my hands on a 12-gauge. But the best I can predict is that, as an officer of the law, I might be in a gunfight sometime in my career. So I carry a handgun.

But the question remains: Outside of a "street howitzer," which gun and which cartridge is best in a gunfight? In other words, which combination yields the most firepower?

Today's commonly accepted definition of "firepower" is a large-capacity magazine and a rapid rate of fire. But this doesn't take into account the effectiveness of the rounds fired. And that is, of course, the most critical concern.

Suppose you allowed me to land a series of 25 mild punches to your solar plexus over a period of a couple of hours. What would be the net physical effect? Now imagine that the kinetic energy of all those blows went into a single hit. The aggregate force would be equal. But is there any doubt how the result would differ?

What I'm saying here is that high-capacity magazines are nice, but police shootings average about 2.8 rounds in 2.5 seconds, with all parties heard from. Therefore, in evaluating defensive handgun choices, the number of rounds available is not the way to determine effectiveness nor define firepower.

Shutting Down a Threat

Let's agree that no rational person wants to get shot with anything. But getting shot in the real world doesn't have the same effect that it does in TV shows and movies. In such entertainments, police wing the bad guy and the bad guy goes down. It can happen that way, but don't bet your life on it.

Shooting a human being is much messier and much less precise in real life than it is on TV. For example, fat can protect organs and plug holes, preventing external bleeding and encouraging the attacker to ignore the wound. As for pain, don't count on that to shut anyone down. Nature has a reason for programming us to lose an arm and feel only a tingle or numbness. And remember, your attacker may be on alcohol, stimulants, PCP, or he or she may be just plain strong and mean enough to take a bullet and keep coming.

If you find yourself in a shooting situation, your goal is involuntary physical shutdown of the person who threatens your life. And to do that you have to hit your attacker with your shots and hit him hard.

Shock Waves

Lots of factors-mass, design, velocity-go into determining how hard a bullet hits. And there are a lot of misconceptions about how they all contribute to stopping power.

For example, much has been written about the hydrostatic shock produced by energy dump when a bullet fails to exit the body. It's been posited that such shock waves open a devastating, temporary wound cavity far larger than the bullet diameter. But at handgun velocities, how damaging can these shock waves be? The answer is not very.

Col. Martin L. Fackler, MD-through his unparalleled opportunities as a U.S. Army surgeon to observe and treat the results of thousands of small arms hits-is probably the world's leading expert on wound ballistics. He argues that the elasticity of most tissue prevents serious trauma from hydrostatic pressure.

A related claim involves sonic wave damage. Fackler disputes this as well. He points out that the device used to sonically break up kidney stones produces over 1,000 sonic waves per treatment at three times the amplitude generated by a handgun round, but it doesn't traumatize the surrounding tissue.

33 Rounds

So what does it take to shoot and stop an attacker? Fackler and other experts say it can only be accomplished two ways: destruction of a vital organ such as the heart or a vital body structure such as a hip joint.

The fact is, even if we shot all the test animals we could afford and even put a few rounds through a few human volunteers, we still wouldn't know precisely what it takes to stop a given attacker in his tracks on a given occasion. The civilian record stands at 33 rounds, and all of that trauma didn't cause instantaneous shutdown.

One of the most carefully dissected incidents in modern gunfighting has been the 1980 Kolowski-Burchette encounter in Illinois. This unscheduled 30-second field test of 115-grain, first-generation Silvertips occurred when two state troopers stopped convicted felon and suspected killer Wayne O'Brien.

In the ensuing gunbattle, the heavily muscled and very drunk O'Brien took 13 rounds. He absorbed two hits in the cardiac sac, with one of them slicing his aorta. In addition, two slugs ripped through his neck, and others punctured his stomach, his lungs, a kidney, and his liver.

Yet with all that damage from police handgun rounds, most of which demonstrated excellent penetration and expansion, he fought on as if merely annoyed. Then he returned to his bike and died. With a fully oxygenated brain and glands pumping, the man had maintained aggression for perhaps 12 to 18 seconds after his heart had been pulverized.

You may be thinking all the Illinois troopers had to do to stop O'Brien was "shut down the computer." But it's not that simple. Examples abound of headshots that got people's attention but failed to discourage them promptly.

And the issue always comes down to two factors: size and power of the bullets.

Imagine that you've been immobilized and a rigid metal frame has been strapped securely to your right chest. Two guide rings, one a half inch and a second 10 inches from your chest, will accommodate a surgical steel metal rod to be driven at high speed by a piston. As quickly as it is driven 12 inches into your body, it will be withdrawn. If you had to choose, would you prefer a rod of .17 inches in diameter or .60 inches?

The lesson from this example and from dozens of real-life gunfights is simple: size matters.

Brain Waves

What are the psychological aspects of stopping power? Does a bullet entering your body throw the mind into shock? The answer is maybe, and maybe not. Even a grievous wound may not convince an attacker to stop.

It's all a matter of why the attacker is  fighting. And to understand this principle, consider the following:

You're walking through the park with your 9-year-young daughter when a man with a .45 approaches and tells you exactly how he intends to sodomize her and then bury her alive. He orders you to turn and walk away. Instead, you lunge as he puts a round through your upper left chest. Do you let the psychological impact of being shot stop you?

Retired police detective Evan Marshall has studied thousands of actual shooting incidents, all different and all reported in such detail as the participants could or would recall. And the one thing they reveal is that if the person being shot has strong motivation to continue his attack, i.e. save his daughter from a fate worse than death, then he will probably keep charging as long as he is physically able.[PAGEBREAK]

Choosing a Compromise

Let's go back for a minute to our discussion of what it takes to shut down an attacker. You have to hit the target and you have to hit it as hard as you can. These two goals, while by no means mutually exclusive, are difficult to achieve. Consider that it's much easier to keep a small caliber handgun on target, but the bullets may not have the mass and the energy to shut down the threat as quickly as you need to.

Put simply, this means that all defensive handgun choices are a trade-off between what you can control and what it takes to do the job. Let's look at the pluses and minuses of some possible choices.

"Dirty Harry" fantasies aside, the .44 magnum may be eliminated from consideration as a defensive handgun because the hardware is almost invariably too big and too heavy to carry or conceal. Moreover, with recoil more than twice that of its nearest competitor, recovery and a follow-up shot are improbable. Does it offer stopping power? You bet. But this bear killer's penetration potential raises serious liability questions in an urban setting.

Should you or a friend want to max out your stopping power short of an unwieldy .44 mag but stay with a .44 revolver, consider Dirty Harry's cartridge of choice. He fired .44 specials in that big Model 29.

The .357 magnum is a proven stopper. But here again there is more recoil than many shooters find comfortable, and the .357 revolver is famous for high muzzle flash, which ruins night vision-a serious consideration since most self-defense shootings take place in low light or no light.

Many officers carry 9mm autopistols. And this weapon does have its advantages. The size of the round means that your magazines can carry more "bumblebees," and the slight recoil of the "nine" makes it a good choice for accuracy, but it may not hit hard enough to do the job. A 9mm round barely reaches the energy of a hot 38 special.

Let's discount the 10mm. It never quite caught on. The hardware is hard to find, and the ammo sources have all but dried up. Its offspring is, however, highly popular.

The 10 Lite, aka .40 Short-or as Smith & Wesson likes to call it, the .40 S&W-has become one of the leading law enforcement rounds in just a decade. This trend can be traced to a bloody 1986 incident involving FBI agents and bank robbers. Two agents were killed and five badly injured in a 140-round shootout in Miami in which the criminals were struck many times by pistol rounds and shotgun pellets yet managed to stay alive and deadly for minutes after they were mortally wounded.

In the aftermath of the Miami shootout, the Bureau was not about to admit to faulty tactics. Blame for the unfortunate debacle had to be laid off on the lack of the right equipment. The conclusion was that the good guys didn't have the right guns and ammunition.

In their quest for the magic weapon, the FBI conducted a series of five-round tests, which would have been an embarrassment at a junior high science fair. With that, they dismissed the .45 ACP as "already fully developed," whatever that meant. The 10mm was judged too hot to control and possibly overly penetrative. But by down-stepping the power on the 10mm, the .40 was born. The FBI had its magic bullet. Its father was organizational ego, its mother posterior covering.

But is the .40 S&W all it's cracked up to be? As the JEDI chart reflects, it is a more powerful round than the .38 or 9mm. However, Whit Collins, the man who conceived the .40, warned that while it was designed to be a superior replacement for the 9mm, it would never equal, much less exceed, the .45 ACP.

So what compromise will most efficiently produce the ultimate in behavior modification and accuracy?

Within the range of .38 specials through .45 ACPs, hardware abounds to satisfy the requirements of small enough, light enough, concealable, manageable enough, and socially acceptable. Picking from both ends of the mid-spectrum, lay a snub .38 atop a .45 ACP Colt Officers Lightweight. They are of the same length and height. And at 24 ounces, the 8-round autopistol is just five ounces heavier than the five-round snub revolver. But when it comes to stopping power there is no comparison.

About six years ago, a leading gun magazine asked 10 nationally known shootists/gunsmiths to name their preferred defensive handgun cartridge. All 10 opted for the .45 ACP.

Your pick will depend on your comfort level. Most of us have heard people say they never could hit with a particular caliber or that they can't seem to miss with another. Whatever feels best in your hand and you are able to carry with confidence should be your choice. Just remember to partner your weapon of choice with a cartridge that has adequate stopping power.

May the Force Be with You

Typically, muzzle energy measured in foot-pounds is used as a quick-and easy-measure of stopping power. Most of us scan a chart published by a major ammo manufacturer and see, for example, that a certain 9mm round shows 362, while a .45 ACP comes in at 356. Equal? Not that simple.

Although kinetic energy figures are handy keys, they are not a valid reflection of stopping power. Because the formula for calculating muzzle energy requires that bullet speed (by far the largest number in the mix) be squared, the power factor is distorted in favor of a smaller, lighter, and faster projectile.

M. L. Josserand, the long-departed French physicist and mathematician, developed an energy delivery formula that offers a much better gauge of stopping power. The Josserand formula factors in the projectile size by multiplying the standardized kinetic energy at the muzzle by the cross section (area) of the bullet.

To determine the standardized kinetic energy of a cartridge, first multiply the bullet weight in grains by the velocity in feet per second squared. Then divide by 7,000 (the number of grains per pound). You must then divide that result by 32.16 (the constant of gravity at sea level) to reach the muzzle energy in foot-pounds. Or you could just look this figure up on a chart.

Calculate the bullet cross section, or area, by squaring the radius and multiplying that number by pi (3.1416). Multiplying the area by the muzzle energy gives us the Josserand Energy Delivery Index, or JEDI.

As the accompanying JEDI chart reflects, any ammo below .38 special yields very limited stopping power. Yes, the peewees may be better than no gun, but not by much.

At the other end of the spectrum are the long guns, herein presented for comparative purposes, and the .44 magnum.

Sgt. Roland C. Eyears is an Ohio Police Firearms Instructor and an 11-year veteran of the St. Louisville (Ohio) Police Department. This is his first article for POLICE.