Is "Knockdown Power" Really a Myth?

I’m not here to disagree with that, and I’ve said some version of it myself. If you have to choose between a medium-power cartridge that you can shoot accurately and The Piledriver Magnum 3000 that can’t hit the broad side of a barn, go with the former. Shot placement will always be king.

At the same time, I think there’s some nuance that gets lost in the righteous cause of mythbusting. I also think the graybeards who get criticized for believing in knockdown power are sometimes straw-manned. Their actual position isn’t fairly represented, and so it is much easier to, well, knock down.

What Is “Knockdown Power”?

Rifle hunters have long observed a curious phenomenon when a big game animal is shot with a bullet in the vital region of the lungs and heart. Sometimes, it runs for 30, 40, or 50 yards before tipping over and expiring. Other times, the animal drops where it stands.

This is what most hunters mean when they talk about a cartridge having knockdown power. They don’t mean that the bullet actually knocks an animal off its feet and throws it back, Hollywood style. They simply mean that it is more likely to cause the phenomenon of an animal dropping in place.

The problem is, unless the bullet strikes the nervous system directly, the cause of that phenomenon isn’t entirely clear. Hunters report using the same cartridge and the same bullet and placing that bullet in the same heart-lung area, only to see those two very different results.

What’s more, the knockdown effect has been observed with cartridges across the bullet energy spectrum. It isn’t just the magnums that make an animal tip over right away, and the shot doesn’t have to be particularly close. MeatEater’s Garrett Long has shared a video of his friend shooting a mule deer with a 6.5 Creedmoor from over 400 yards away. The Creed isn’t a super powerful cartridge, and by 400 yards it’s only imparting about 1,200 ft.-lbs. of energy. But that deer crumples like it was crushed by a ton of bricks.

Despite the best efforts of science-minded gun writers, hunters have continued asking about knockdown power. Why? Just watch that video again. If you’re a hunter, it will probably be the most satisfying thing you see today. That’s a job well done. The animal was killed instantly, and the hunter didn’t have to cope with a shock of adrenaline-fueled anxiety as the quarry ran away over the next ridge.

That’s why, even if we admit that lots of bullet energy doesn’t guarantee the knockdown phenomenon, I have sympathy with those who pursue it.

Ok, But What Causes It?

One theory is that the knockdown phenomenon is caused by something called hydrostatic shock. Hydrostatic shock is imparted from a bullet as it dumps its energy into an animal’s body. This energy causes remote damage to the brain and nervous system, and sometimes it’s enough to push the animal’s off button. There have been scientific studies pointing to this fact, which you can read more about here.

Another, related theory appeared in this article from Outdoor Life. Reporting on a separate article from a South African outdoor magazine, gun writer Jim Carmichael theorizes that the knockdown effect is caused by the bullet entering the animal at the moment its heart beats. If the bullet strikes when the arteries in the brain are filled with blood, the energy transfer is enough to burst those arteries and pull the rug from under the animal’s hoofs. Carmichael doesn’t say which magazine he was reading, and I haven’t been able to find it, but it strikes me as plausible.

The through-line in both of these theories is that bullet energy is somehow responsible for the knockdown effect. If that’s true, you might assume that the more bullet energy you dump into the animal’s body, the better the odds that you’ll have a short tracking job. Which, it should be said, sounds a lot like the graybeards who claim that bigger, more powerful cartridges have “knockdown power.”

No Guarantees

Of course, the seemingly random nature of the knockdown effect throws some cold water on those claims. If that second theory is true, and you need to hit the animal as its blood pressure spikes, chasing big-time bullet energy is a fool’s errand. Blood loss and destroying vital organs are the only things that actually end an animal’s life. Knocking it down is nice, but ultimately not necessary.

The truth of this was brought home to me on a recent nilgai hunt in South Texas. Nilgai are famous for absorbing bullets and running away, which is why guides tell you to reload immediately and be ready to put another round in the animal if it thinks about getting up.

The guys I was hunting with told me a story about a nilgai that was shot high in the back and dropped to the ground. It lay still for, according to them, several minutes before hopping up and running away. They never found any blood, and they never saw that big bull again. In that case, the knockdown phenomenon was in full effect but did zero good.

On the other hand, I nearly had a panic attack when I shot a bull and it ran away like nothing happened. But since I had hit it in both lungs, it tipped over and died within a few minutes. Further inspection revealed that the bullet had not passed through the body, though we did find a fragment lodged in the opposite rib cage. The magnum 7mm bullet dumped all its energy into the animal and even though it was a fatal shot, failed to produce the knockdown effect.

The contrast between those outcomes illustrates why many warn against prioritizing knockdown power. It may be satisfying to drop an animal in its tracks, but it’s ultimately not what fills your freezer.

And Yet…

The good news is that we don’t need to choose between knockdown power (as far as it exists) and accurate vital shots. That’s because increasing the amount of energy imparted to an animal isn’t just about picking the biggest, fastest, shoulder-bruising cartridge. It’s also about picking the right kind of bullet.

The South Carolina Department of Natural Resources conducted a study at a hunting club that documented the details of 493 whitetail deer harvests. One of those details was the effect of bullet construction on the knockdown effect. They found that deer shot with softer, more rapidly expanding bullets dropped dead 58% of the time, while deer shot with harder, controlled expansion bullets dropped 49% of the time. What’s more, deer shot with those softer bullets only ran an average of 27 yards while deer shot with the harder pills ran 43 yards.

“This study indicates that rapidly expanding bullets lead to deer running less often and less distance, and when they run, they leave better sign,” they concluded.

What explains this finding? It could be that rapidly expanding bullets destroy more tissue and lead to more blood loss. But blood loss takes time to have an effect, and so it doesn’t explain why deer shot with softer bullets more often drop in their tracks.

Another explanation is that rapidly expanding bullets impart more energy into the animal and are therefore more likely to produce hydrostatic shock. In the case of this study, we would be more certain of this hypothesis if we were able to cross-reference caliber used with bullet type. If deer dropped more often with high-powered cartridges using soft bullets, we could fairly assume that kinetic bullet energy combined with a rapidly expanding bullet truly does have “knockdown power.”

Unfortunately, that information isn’t available. I reached out to the author of the study, Charles Ruth, who told me the sample size was too small to do that analysis. He did not find any “apparent relationship” between caliber used and the distance deer ran, but he can’t say for sure whether soft bullets going fast are the secret to knocking an animal off its feet.

It could still be that more powerful cartridges using rapidly expanding bullets will knock an animal down most frequently. Based on what we know about hydrostatic shock, I’m inclined to believe this is the case. Still, what we do know suggests that knockdown power isn’t a myth, but its source is bullet construction just as much as bullet energy. If you want to maximize your odds of that oh-so-satisfying lights-off shot, think just as carefully about how your bullet is constructed as about how fast it’s moving.