Debunking the Infamous Rifle Knockdown Power Myth

The myth goes something like this: Use enough gun and you'll knock down game in its tracks
rifle knockdown power
With the right cartridge you can knock a Cape buffalo clear off its hooves (this is not true at all). Ron Spomer

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For generations of gun writers, an occasional article on knockdown power has been money in the bank, as readers are ever-ready for yet another round, and then another, of this inexhaustible topic.

Personally, I’ve been content to leave the topic to others. Not because I have no opinion on the subject, but because I have dozens. However, most theories and explanations of what happens when a bullet hits an animal always strike me as having too many holes (no pun intended).

The reason I’m now willing to enter the fray is because at last there seems to be documented evidence, based on scientifically controlled research, as to why certain calibers demonstrate miraculous “knockdown power” while others don’t. The answer, I promise, isn’t what you’d expect, and you may not agree, but it’s worth hearing because it’s going to add lots of fuel to the debate. Even the way I came upon the information is a story worth telling, but first let’s make sure we have some understanding of what knockdown power is and how it is generally recognized.

To begin with, the term “knockdown power” is a misnomer, unless you’re referring to a giant cannonball, or perhaps a motor vehicle crashing into a deer on some midnight highway. Knockdown power, as it refers to sporting-rifle bullets, is an animal’s physical reaction to the bullet’s impact and entry. More specifically, if the animal falls immediately as if the earth has been jerked from under it, the bullet is said to have delivered knockdown power and the happy hunter congratulates himself for the wisdom of using a great cartridge.

On the other hand, if the animal doesn’t fall immediately, wanders about or dashes off, even after being hit with a well-placed shot, the cartridge or bullet may be condemned as having poor or nonexistent knockdown power, its maker roasted in campfire effigies and lurid rumors spread.

The fact of the matter, however, is that no reasonably applied big-game caliber is either good or bad all the time–and there’s the rub.

The Lights-Out Method

whitetail deer knockdown power
Decades ago, whitetail deer hunters commonly aimed for the head and neck. But that’s when still hunting was the most popular tactic and shots were at close range. John Hafner

Technically speaking, near-absolute knockdown effect can be achieved with about any caliber cartridge by what we can call the “lights-out” method, which is either a brain shot or a shot that shatters the neck vertebrae and spinal cord. The old-time deer hunters I grew up with were great believers in the neck shot because it minimized meat loss and, more important, usually dropped the animal in its tracks.

READ NEXT: Don’t Aim for a Deer’s Head

Those were the days when calibers such as the .30/30 and .35 Remington ruled the woods, still-hunting was practiced by almost everyone, 50 yards was a long shot and open sights were the norm. Today, with much-changed hunting techniques and equipment, we don’t hear nearly as much about hunters favoring neck or brain shots. This is just as well, because aiming at the smaller parts of an animal’s anatomy is, literally, a hit-or-miss proposition.

Taylor’s Theory

Another approach to knockdown power is simply to overwhelm the animal with bullet size–the “bulldozer effect” I suppose it might be called.

Historically, there have been many proponents of big heavy bullets, and none more so than John Taylor, a gun and ballistic disciple of considerable African experience. Believing that the formula for calculating bullet energy is skewed too heavily in favor of velocity (I tend to agree with him on this, by the way), Taylor concocted a system for calculating what he called the Knock-Out Value of various calibers.

Since he was mainly interested in ivory hunting, Taylor was concerned about the concussion effect of various cartridges on head-shot elephants when the brain itself was missed. According to his tables, a pachyderm would be unconscious for about half an hour when knocked out by a 900-grain slug from a .600 Nitro Express, whereas the beast would remain unconscious only about 20 minutes when hit by the 720-grain bullet from a .577 Nitro–a difference that no doubt has altered the course of history.

While logic and observation make it clear that big calibers can have an overwhelming effect on game–e.g., whitetails shot with a .375 H&H–the strength of the evidence begins to wane when we factor in the unmentionable sin of declining marksmanship. Some (dare I say many?) hunters are not comfortable with hard-kicking rifles and are liable to flinch and jerk when they touch one off, which of course results in poor shot placement.

Weatherby’s Theory

On the other side of the aisle are those who argue that game animals are more likely to be instantly poleaxed by high-velocity bullets that transmit a shockwave through the nervous system. The leading apostle of this gospel was none other than Roy Weatherby, who preached long and hard on the velocity theme and won many converts. The term usually applied to the circuit-breaking effect of high-velocity impact is “hydrostatic shock,” but I think hydrodynamic shock is more apt.

Of course, we can rightly figure that by combining the opposing elements of the debate and firing big, heavy bullets at sizzling velocities we get a double dose of knockdown power. But anyone who fires, say, a .460 Weatherby Magnum (about 85 foot-pounds of recoil) instantly realizes that this sword cuts both ways.

Regardless of which side of the debate you cheer for, there remain many examples of game well-hit with any caliber and cartridge you wish to name that wasn’t knocked off its feet in an instant.

A Better Theory

A couple of years back I was passing a pleasant afternoon in a bird-watching blind in the wilds of Namibia. A previous guest had obligingly left a few copies of a South African outdoor magazine and as I idly leafed through the pages my attention was arrested by an article on knockdown effect. It was not the same tired old stuff about ballistics and penetration, but the result of a controlled study carried out by professional veterinarians engaged in a buffalo culling operation.

Whereas virtually all of our opinions about knockdown power are based on isolated examples, the data gathered during the culling operation was taken from a number of animals. Even more important, the animals were then examined and dissected in a scientific manner by professionals.

Predictably, some of the buffalo dropped where they were shot and some didn’t, even though all received near-identical hits in the vital heart-lung area. When the brains of all the buffalo were removed, the researchers discovered that those that had been knocked down instantly had suffered massive rupturing of blood vessels in the brain. The brains of animals that hadn’t fallen instantly showed no such damage. So what is the connection?

Their conclusion was that the bullets that killed instantly had struck just at the moment of the animal’s heartbeat! The arteries to the brain, already carrying a full surge of blood pressure, received a mega-dose of additional pressure from the bullet’s impact, thus creating a blood pressure overload and rupturing the vessels.

If this is the key to the “knockdown” mystery, it has answered a lot of previously unanswered questions. It’s certainly the best explanation of knockdown I’ve heard yet, but it also poses a new quandary. How do we time a shot to hit on the beat? 

Anecdotes from the Field

bull moose
Hunters often form opinions on "knockdown power" after only one or two experiences in the field. John Hafner

I like the “heartbeat theory” because I’ve spent a lot of time in the hunting fields and seen some strange examples of knockdown power and lack thereof. Moose have a reputation for being tough critters, and it’s not uncommon for them to walk off after being hit. I planned to use this to my advantage one miserable day years ago when I came across a nice bull standing brisket-deep in the freezing cold water of a high valley bog. My gun was a .44 Magnum revolver and the way I had it planned was to shoot him where he stood, which I assumed would put him on the move. Then, when he came out of the water, I’d finish him off with another shot or two. Very clever, I thought.

Now, I’ve shot several moose with a variety of calibers, including heavy hitters like the .338 Win. Mag. and 8mm Rem. Mag., but I have never had one drop as fast as that big boy did. (Next time you’re feeling like a rugged sportsman, try gutting a moose while sloshing about waist-deep in freezing water during a snowstorm.) But that’s only half the story.

READ NEXT: The Top Moose Cartridges and Bullets

Later on in the hunt an elk I shot with the same revolver and ammo was only mildly impressed. I had to track that bull down and finish the job with a rifle. The distance at which I shot the elk was about the same as the moose, and both were side-on chest shots. If either of these two instances had occurred without the other I would have arrived at two entirely different conclusions about the knockdown power of my .44 Mag. and handloads.

And then there was the time when late in the day my Blackfoot guide Leo and I were riding back to our elk camp in the high country south of Yellowstone Park. I was slumped in the saddle with my thoughts focused on the toddy awaiting me at camp when Leo wheeled his horse and came charging back down the trail, pointing back over his shoulder. The cause of Leo’s excitement was a truly grand mule deer taking his ease in a meadow that sloped up from the trail. The distance wasn’t all that great, and after I’d crawled behind a tree that provided cover and a solid rest for my rifle, I took my time and aimed carefully.

Judging by the way the big deer acted when I fired, I thought I’d missed. He tossed his head back, as if reacting only to the sound of my shot, jigged a bit like he was deciding which way to run, then settled down peacefully. He was turned to the opposite side when my second shot hit, and for another long, unbelieving moment I thought I’d missed again. Then he gently went down. Both bullets had hit dead on target, and when Leo and I opened him up the entire chest cavity was mush. Yet he hadn’t been “knocked down” the way we like to see.

mule deer
Deer hunters commonly misunderstand terminal ballistics. John Hafner

It is often claimed that when an animal is spooked and running adrenalin, he’s harder to knock down. I’ve seen this happen enough to be convinced it’s true, but the mule deer in the episode just described didn’t seem to have a care in the world. My rifle, by the way, was a .338 Win. Mag. loaded with 250-grain Nosler Partition handloads. But in that instance it didn’t pass the “knockdown” test.

The Cape Buffalo Test

Several years later I was chasing giant eland in Africa and using the identical .338 Win. Mag. load I’d bagged the above deer with. We’d been hunting on foot all day, it was hot and desert-dry and my PH and I were hunkered down under some shade taking a rest before the long trek back to camp. As we sat there, gurgling water down our chins from a hemp water bag, what should appear but two big Cape buffalo bulls. And when I say big, I mean one was enormous, a once-in-a-lifetime record-book keeper.

Usually the preferred calibers for dangerous African game like Cape buffalo begin with at least a .375 H&H, or better yet, something on the order of the .458 Win. Mag. or its even huskier brethren. But the .338 was all I had, and I wasn’t about to let a trophy that good pass, even if I’d been armed with a peashooter.

Normally, the plan of attack when hunting buffalo with a heavy-caliber rifle is to hit them forward in the shoulder so as to bust some bones and break the animal down so he can’t get at you. Even if it doesn’t prove to be a killing shot, it’s safer, since a wounded buffalo on the ground is rather more reasonable to deal with than one on his feet. But with my relatively puny .338 and expanding nose bullets, this didn’t seem like such a good idea. A shot in the forward superstructure might fail. Instead, I decided to go behind the shoulder and slip the expanding Nosler into the heart-lung area. This probably wouldn’t kill the bull instantly, so I resolved to pump in more bullets as fast as I could work the bolt.

cape buffalo knockdown power
Jim Carmichel after a successful Cape buffalo hunt. Outdoor Life

As it turned out, the bull was dead and down before I could get the crosshairs back on target–one of the most astonishing demonstrations of knockdown effect I’ve ever seen.

A year or two later my longtime hunting pal, the legendary outfitter Jack Atcheson Sr., and I were messing around in northern Zimbabwe with another legendary professional hunter, Mike Rowbotham. We were hunting nothing in particular when we came across a herd of about 200 buffalo. Apparently they had had a lot of hunting pressure and were plenty wild. When they caught on to what we were up to they broke out in a bellowing stampede. Jack and I were both carrying .338s, mine loaded with my favorite .250-grain Nosler Partition handload. One pretty good bull got separated from the main body of the herd and came galloping by at a reasonable distance so I swung the crosshairs ahead of him, hit the trigger and followed through rather prettily. The bull went down on his nose and was dead before he stopped sliding.

After those two experiences I was fairly well convinced that the .338 was about the perfect medicine for Cape buffalo, but now I’ve changed my thinking. I believe that what happened on those two instances, and on the moose, was a physiological phenomenon (the “heartbeat theory”) often observed by other hunters but never–until now–understood. —Jim Carmichel

Understanding Terminal Ballistics

knockdown power cartridges
The energy advantage of a .375 H&H over a .308 doesn’t translate to “knockdown” power.  Yamil Sued

Bullet mass, velocity, and kinetic energy don’t hammer game like a meteor from heaven. They kill by breaking down vital organs that supply blood to the brain. This can take from several seconds to several minutes, during which time game can, and often does, run.

Bullet energy really matters only when it’s insufficient to penetrate enough to reach those vital organs. As many a bowhunter will testify, that energy level doesn’t need to be high. Plenty of deer, elk, and even moose have been poached with .22 long rifles.

Many hunters think that bullet energy will flatten game if the bullet stays inside the animal and dumps all of its energy there. Sorry—that doesn’t always work either, as I’ve learned firsthand many times. Two years ago, I put such a bullet quartering from behind into a running whitetail’s chest at 30 yards. It kept running. I put another shot in at 50 yards. The buck ran over a hill. I followed, spotted the animal lying in short grass, and punched one more round through its chest. A few seconds later, it expired. None of these bullets knocked the deer over. None exited. All were 150-grain bullets launched from a .300 Win. Mag. at 3,100 fps.

Does this mean a .300 Winchester Magnum is too puny for whitetails? Consider a 300-grain slug from a .375 H&H Magnum. My wife used one to shoot an African common reedbuck about the size of a Carolina whitetail doe. That buck took the hit and dashed forward a good 80 yards before wobbling and falling over.

At the other extreme, I once fired a 60-grain bullet via a .22/250 Rem. to a whitetail buck standing 100 yards away. It passed through the deer and expended its remaining energy in the dirt. That buck dropped as if unplugged.

The truth is, there is no sure-fire guarantee, no silver bullet, no magic cartridge that can anchor a deer or even a coyote every time. Hit the central nervous system and bullet energy hardly matters. Down it goes. Punch through the lungs, heart, or liver, and sometimes an animal collapses instantly. But more often it stands, walks, or runs for as many as 10 seconds before falling blood pressure makes it faint. This is what you see when a heart-shot buck runs, starts wobbling, falls, tries to rise, falls again, and expires.

Shot Placement and Bullet Selection are Critical

What the best hunters can do, in addition to parking their bullets in the right place, is use bullets that maximize vital tissue destruction. Sometimes these are frangible projectiles that break up inside, spread out, and cause massive hemorrhaging. Sometimes these are lead bullets that expand from 1.5 to 2 times their original diameter and lodge against the hide on the far side. Sometimes they are monolithic bullets with petals that open wide to tear tissue and exit the off side, leaving a hole for blood trailing. —Ron Spomer