The Killing Power of Centerfire Hunting Rifles
By Chuck Hawks
One of the most fascinating and controversial subjects gun writers and other hunters need to address is the question of killing power (or stopping power, knockdown power, etc.). For the purposes of this article I am going to use the term "killing power" to represent all of the above.
As handgun guru Jeff Cooper has pointed out, in a gunfight it is not actually necessary to kill your opponent, but it is absolutely necessary to stop him from killing you. Thus stopping power, not killing power, should be the goal in a defensive pistol. In that context there is an important difference in meaning between the two terms. He illustrated the difference something like this: acute leukemia (or other such dread disease) has almost 100% killing power, but almost no immediate stopping power. Conversely, a clean tackle by a NFL defensive back has nearly 100% stopping power, but almost no killing power.
That is all perfectly true, but as applied to hunting rifles and cartridges, terms such as "killing power," "knockdown power," and "stopping power" are generally used pretty much interchangeably. A game animal must be both killed and prevented from traveling any great distance (stopped, if you will) if it is to become dinner or a trophy.
There seems to be any number of theories about the subject of killing power and little hard, repeatable, test data. Most evidence in anecdotal. The result is a situation where anyone fast with a theory can pose as an expert, regardless of the amount of research they have actually conducted. In many cases terminal ballistics, animal physiology and psychology, the literature on the subject, experience, and even good sense are simply ignored.
One common pitfall is to place too much emphasis on personal experience. Very few shooters, hunters and writers (if any) have killed a sufficiently large sample of animals of any species to draw statistically valid conclusions from their own experience. As far as I know, killing power tests of big game rifle cartridges along the lines of the famous Strasbourg handgun cartridge stopping power experiment, in which hundreds of instrumented goats of standardized size were shot under carefully controlled conditions, have never been performed.
Some of the famous gun writers of the 20th Century, such as Jack O'Connor, Pete Brown, Francis Sell, Elmer Keith, Bob Hagel and Warren Page, lived in a time when game was plentiful, travel relatively easy, and bag limits either liberal or non-existent. Under such conditions it was possible to shoot hundreds of head of North American, African and Asian big game animals. With very rare exceptions, that level of practical experience is beyond the reach of modern gun writers.
The opinions of the masters of that earlier age are worth noting. They constitute an extremely valuable body of practical experience that will never be equaled. But keep in mind that those great hunters and writers of yesteryear were also men, with all the foibles of our species. Their experience was sometimes deep but narrow, and they could be as prejudiced as the next man.
Such personal prejudices must be taken into account. The late Elmer Keith, for example, is well known for his advocacy of hunting rifles of .33 or larger caliber using bullets of 250 grains or heavier. He killed a large number of all sizes of thin-skinned game in North America and other parts of the world with medium bore rifles. What worked for Mr. Keith will work for you and I, too.
But Elmer Keith let his preference for powerful medium (and large) bore rifles become disdain for rifles of smaller caliber. He came to consider all-around rifles such as the .270 Winchester, 7x57, .280 Remington, 7mm Magnum, .308 Winchester and .30-06 marginal even for deer hunting and practically useless for larger game such as elk and moose.
Elmer Keith once wrote disparagingly that the .270 Winchester was an adequate coyote cartridge. Meanwhile Townsend Whelen, Jack and Eleanor O'Connor, Warren Page and many others were regularly and reliably taking elk, moose, and all manner of African plains game with .270, 7mm, and .30 caliber rifles. Clearly, Mr. Keith was prejudiced. He personally disliked Jack O'Connor, a famous advocate of the .270, and this colored his opinions and his reporting.
The truth is that Mr. Keith's advice concerning the capability of medium bore rifles was generally on the money. But his opinions about the smaller caliber rifles were practically worthless. I pay attention when I read Keith's material about medium and large bore rifles, but simply disregard what he wrote about the standard calibers.
Many prominent theories about killing power seem to equate one or more of the following with big game killing power:
Bullet dwell time
One of the theories I would put in the last category above is "dwell," which I believe originated in the UK. The theory here is that the longer a bullet passing through an animal spends in the animal (dwell time), the greater the "shock" effect on the animal's nervous system. This take on killing power favors low velocity. Low bullet impact velocity, of course, necessarily means lowered kinetic energy, linear momentum, and so forth.
This theory is so off the wall that when I first heard of it, the bullet dwell theory momentarily gave me pause. Upon reflection I began to wonder, what if the bullet does not pass clear through the animal? Is it then infinitely deadly? I think not!
There are more killing power theories in the "Other" category than I could possibly address in this article. Like dwell, most of them contain obvious logical flaws and are fairly easy to eliminate from any serious discussion of killing power. I would advise extreme caution concerning any theory that flies in the face of widely accepted data.
This is the opposite of the bullet dwell theory, although both postulate "shock" as a primary component of killing power. The high velocity advocates, among whom the late Roy Weatherby is perhaps the best known, generally believe in something they describe as "hydrostatic shock."
This theory, as I understand it, says that the impact of a high velocity bullet transmits a massive hydraulic pulse through the veins and arteries of the stricken animal. This sudden pulse causes such a shock to the cardiovascular and nervous systems that the animal simply dies in its tracks, regardless of whether the bullet hit in a vital spot. The usual analogy is to the hydraulic braking system of an automobile.
Roy Weatherby, on an African safari, intentionally gut shot dozens of plains animals with a .257 Wby. Mag. rifle in an attempt to demonstrate this theory. Most (perhaps all) were one shot kills. I understand that he even killed a Cape buffalo with his .257 Magnum.
I accept Mr. Weatherby's account of these experiments (that is, that the animals died quickly after being shot), but I seriously question the hydrostatic shock explanation for those kills. Any doctor or zoologist can tell you that the veins and arteries of all mammals are quite flexible. They have a lot of potential to expand or "give." Because of this, hydrostatic shock is very unlikely. To return to the automobile brake analogy, hydraulic brakes simply won't work if the brake lines give when the driver hits the brake pedal. The hydraulic pulse is not transmitted, but rather absorbed in the brake lines--or, in the case of an animal, the veins and arteries.
I am a fan of Weatherby rifles and the .257 Weatherby Magnum cartridge, but my guess is that those African animals simply succumbed to the tissue damage and blood loss caused by the impact of those high velocity .257 bullets. I don't think that hydrostatic shock had much to do with it.
None of the foregoing should be interpreted as an argument against high velocity. High velocity is clearly advantageous to killing power. A 150 grain .30 caliber bullet is deadlier at an impact velocity of 2800 fps than at 2400 fps. It carries more kinetic energy and (if it matters) more momentum, too. It also shoots flatter and is therefore easier to hit with, and bullet placement is very important to killing power, as we shall see.
Many "experts" promoting killing power theories dependent on some sort of shock are pretty vague about just what constitutes this shock. As it applies to some of the less empirical killing power theories, "shock" appears to refer to a sudden cessation of an animal's vital bodily functions due to being hit by a bullet. But, and here is the kicker, not necessarily due to blood loss or actual physical damage to any particular vital organ or organs.
The odd thing is that this occasionally seems to happen. Reliable reports of animals hit in a non-vital spot (or even merely grazed) that simply fell over dead are rare, but not unknown. Many hunters, including me, have witnessed something along those lines. Some sort of shock is usually credited for such kills, perhaps correctly.
Unfortunately, this sort of shock effect cannot be relied upon. It may infrequently occur, but game animals are most reliably killed by placing the bullet into a vital area, disrupting the function of an organ immediately necessary to continued life.
A lot of gun writers believe that bullet diameter pretty much determines killing power. These folks argue that the .45-70 with its fat bullet is a better killer than high intensity cartridges such as the .270 Winchester and .30-06. Likewise, they believe that the .44 Remington Magnum is a better deer killer than the .30-30 Winchester. I have written an entire article on the latter subject, "Compared: The .30-30 and .44 Rem. Mag.," which can be found on the Rifle Cartridge Page. The .44 Mag. is a fine deer cartridge, but it turns out that by every measure of killing power (except bullet frontal area), the .30-30 is the superior cartridge.
However, frontal area (the cross-sectional area of the bullet) does play a part in killing power. The bigger the hole punched in an animal, the more damage is done to whatever tissue it passes through (assuming all other factors to be equal). The performance of solid (non-expanding) bullets is easiest to understand. A .45 caliber solid creates a larger wound channel, given equal penetration, than a .30 caliber solid. And a .30 caliber solid creates a larger wound channel than a .22 caliber solid. Expanding bullets tend to confuse the picture, because they introduce additional variables, but do not alter the basic facts.
Bullet weight may have no direct effect on killing power; after all, it is the destruction of vital tissues that matters most, however it is achieved. But bullet weight does have ancillary benefits. For any given caliber, a heavier bullet has a higher sectional density (SD) than a lighter bullet. Sectional density is one of the major factors in penetration. Just as a larger caliber bullet creates a larger diameter wound, a bullet with greater SD creates a deeper wound, all other factors being equal.
Again, let's consider solid bullets; in this example .30 caliber round nose bullets of identical form weighing 150 and 180 grains. SD is basically the ratio of a bullet's weight (in pounds) to its diameter squared (in inches). The SD of a 150 grain .30 caliber bullet is .226; the SD of a 180 grain .30 caliber bullet is .271. If fired at the same velocity into a test medium, the 180 grain bullet will penetrate deeper than the 150 grain bullet. In an animal it will create a deeper wound channel, destroying more tissue, unless both bullets exit the far side of the animal. Heavier bullets also retain velocity and energy better than lighter bullets of the same shape.
Energy, the ability to do work (or damage in the case of a bullet fired from a rifle) is an important component of killing power. It should be obvious to practically anyone that a bullet carrying more energy when it hits the target has the potential to do more damage than a bullet carrying less energy. Energy is what powers such important functions as penetration, bullet expansion, and tissue destruction. In the U.S. it is measured in foot pounds (ft. lbs.).
Kinetic energy is the most commonly used measure of a rifle's "power." It is the figure(s) listed, along with velocity, in practically all ballistics tables. It can be computed quite easily and is essentially the product of a bullet's mass times its velocity squared. If you want to calculate a bullet's energy at home, multiply the square of its velocity (in feet per second) by the bullet's weight (in grains) and divide by 450,400.
Energy is a pretty good rough estimate of killing power as long as you are comparing two reasonably similar rifle calibers and bullets that are not too dissimilar in sectional density. Compare a 200 grain bullet fired from a .35 Remington rifle to the same bullet fired from a .350 Remington Magnum rifle and you will find that the .350 Magnum caliber rifle is more powerful--its bullet carries more energy to the target. This squares quite nicely with reality, as the .350 Rem. Mag. has proven to have greater killing power.
Compare a 130 grain bullet from a .270 rifle with a 150 grain bullet from a .30-06 rifle, using standard factory loads, and you will find that at 100 yards the .270 bullet is carrying 2225 ft. lbs. of energy and the .30-06 bullet is carrying 2281 ft. lbs. (Remington figures for Core-Lokt Pointed Soft Point bullets). The Remington Core-Lokt bullets for the two calibers are very similar in performance, and those are very similar energy figures, so you would expect the two rifles to be essentially equal in killing power. Decades of use on big game have proven that the two calibers and loads are indeed just about equal in killing power.
Kinetic energy figures can be misleading, however, if dissimilar calibers and bullets are compared. The same Remington ballistics table that provided the energy figures for the .270 and .30-06 loads above also shows that the .30-30 factory load using a 150 grain Core-Lokt bullet carries 1296 ft. lbs. of kinetic energy at 100 yards. It also shows that the Remington .22-250 factory load using a 55 grain Power-Lokt varmint bullet carries 1257 ft. lbs. of energy at 100 yards. Does that mean that the .22-250 is approximately equal to the .30-30 as a deer and general CXP2 class big game cartridge?
Absolutely not! While the energy figures are comparable, the sectional density, frontal area, penetration, bullet performance, and consequently the killing power are completely different. Experience has proven that the .30-30 with the 150 grain Core-Lokt bullet is an excellent CXP2 class game cartridge, and the .22-250 with the 55 grain Power-Lokt bullet is woefully inadequate. The .30-30 and .22-250 are too dissimilar to compare on the basis of kinetic energy.
Energy is an important, but not the only, indicator of killing power. Cartridges that do not develop adequate energy are unlikely to place very high on any rational killing power list.
Momentum is calculated differently from energy in that it is the product of mass and velocity--not the square of velocity. This slants the result in favor of heavy bullets, and makes momentum the darling of the big bore crowd. Momentum is what tends to keep a mass in motion moving, in accordance with one of Newton's physical laws. It is unclear how (or if) this relates to killing power.
Momentum is not widely referred to in the world of ballistics (terminal or otherwise). The term "momentum" is not even included in the Glossary of Terms in the back of the Hodgdon, Hornady, Lyman, Nosler, or Speer reloading manuals that happen to be piled on my desk as I write these words. (I checked!) I am inclined to conclude that momentum is a sort of red herring, favored by gun writers from the big bore school because it gives credibility to their existing prejudices in favor of big bore rifles and heavy bullets.
The term "pounds feet" invented, I believe, by Elmer Keith, is another favorite comparative tool of the big bore school of gun writers. Pounds feet is merely momentum divided by 7000 (the number of grains in a pound). The comparative result is the same as an ordinary calculation of momentum, but using the term pounds feet gives the result a sort of pseudo scientific ring, as the lay reader may easily confuse "pounds feet" with the legitimate scientific measurement "foot pounds."
As we have already seen, large diameter bullets tend to create a wide wound channel, and bullets that are heavy for their caliber have good sectional density and tend to create deep wound channels other factors being equal. These are clear advantages of heavy, large bore bullets from the standpoint of killing power. However, I doubt that calculating bullet momentum per se contributes anything to our understanding of the subject. Ditto for pounds feet.
Another of Sir Isaac Newton's physical laws states that for every action there is an equal and opposite reaction, calculated in terms of momentum. This would imply that, in terms of momentum, the recoiling rifle has more killing power than the bullet it fires, since its momentum is in fact the same as the total ejecta from the barrel (the bullet plus the powder gasses). It is true that big bore rifles kick like the devil, but I doubt that even the most fervent true believer wishes to defend momentum as an indication of killing power in that light, for if it were true they should all be dead.
Killing power formulas
From the old "Relative Stopping Power" formula invented, I believe, by General Julian Hatcher to the relatively recent "Optimum Game Weight" formula devised by Edward A. Matunas, writers and experimenters have tried to develop formulas for comparing the relative killing power of firearms. Most of these "Knock Out" values have proven to have little or no correlation with reality. (Although they usually have a very high correlation with their author's prejudices.)
Relative Stopping Power, for example, totally disregards the effects of kinetic energy, sectional density, and bullet expansion on killing power. RSP was developed to compare the stopping power of early 20th Century handgun cartridges. Jeff Cooper referred to it a lot in his earlier articles. It was later applied to rifle cartridges, where I understand it has demonstrated a negative correlation to reality.
For instance, using the RSP formula (and dropping the extra zeroes as Mr. Cooper recommends) I get a factor of 832 for a 400 grain .45-70 bullet at a muzzle velocity (MV) of 1300 fps. For a 200 grain .350 Rem. Mag. bullet at a MV of 2700 fps I get an RSP factor of 540. No offence, but I own and shoot rifles for both cartridges and if the standard .45-70-400 load is really over 50% more powerful than a .350 Magnum I'll eat an entire Remington ammo catalog page by page!
Most other arbitrary killing power formulas have not fared much better. The most useful of the bunch, in my opinion, is the Optimum Game Weight (OGW) formula. This resulted from Edward A. Matunas' attempts to balance a bullet's weight, kinetic energy, momentum, sectional density, diameter, nose configuration, and impact velocity in an attempt to usefully quantify killing power in terms of animal weight.
The result favors neither the big bore nor the ultra-high velocity schools of killing power. OGW estimates the weight of game and the range for which a given load is suitable. This seems to me to be a very practical way to compare the killing power of rifle cartridges.
According to Matunas, some of these factors proved to be self-canceling. Ultimately, he was able to reduce his formula to the factors of bullet weight, impact velocity and certain mathematical constants. Note that a bullet of suitable construction and terminal performance for the task at hand is assumed, and different mathematical constants are used for big game and varmint bullets. You can read all about OGW in the 47th Edition of the Lyman Reloading Handbook.
As skeptical as I am about killing power formulas in general, I like the OGW system as a predictor of rifle/load killing power because it shows a high degree of correlation with the observations of experienced hunters. It does not tend to overthrow previous notions of the relative killing power of the various rifle cartridges, gleaned from years of field use, but rather to confirm them. Like any predictor of killing power, it seems to work best when comparing similar cartridges. I used the OGW formula to create the "Optimum Game Weight Table," which can be found on the Tables, Charts and Lists Page.
The inclusion of the size of the quarry and the range to bullet impact is crucial to a meaningful comparison of hunting rifle killing power. As any zoologist can confirm, it takes a certain combination of bone and muscle to support and move (walk, run, jump, etc.) a given amount of body weight. Therefore animals of similar size and type, be they North American deer, African plains game or Asian jungle game, will be approximately equally vulnerable to the destructive effects of rifle bullets. Larger animals will generally be harder to kill than smaller animals. Note that dangerous animals, like lion, tiger, and grizzly bear sometimes must be stopped in their tracks, and thus require more power than their size alone indicates.
Bullet terminal performance
A factor that has become more and more prominent in discussions of killing power is the terminal performance of hunting bullets. Assuming a rifle of reasonably adequate caliber, selecting the right bullet for the job can play a big role in killing power. Big game should be hunted only with bullets designed for the purpose. Frangible varmint type bullets and FMJ military type bullets are unsuitable for any type of big game hunting, and are illegal in most jurisdictions.
The perfect game bullet should not expand so fast that it fails to reach the vitals of the game being hunted. Nor should it expand so slowly that it essentially drills straight through the animal, expending the majority of its energy on the landscape beyond the target. An ideal bullet should reach the animal's vitals and use the majority of its remaining energy destroying organs and tissue. Often such bullets will be found just beneath the hide on the off side of the carcass. Larger animals generally require tougher bullets than smaller animals. For more detailed information about hunting bullets, see my articles "Bullets for Big Game Hunting" and "The Killing Power of Big Game Bullets" on the Rifle Information Page.
In my opinion, and that of most experienced shooters and hunters, bullet placement is by far the most important factor in killing power. If you put a properly constructed bullet from a rifle of even marginally adequate power into a vital spot, a quick and reliable kill is the usual result. See the excellent article "The Importance of Bullet Placement" by Ryan Kay on the Rifle Information Page for more on this subject.
Unfortunately, recognizing that bullet placement is the most important factor in killing power does not help us decide what caliber of rifle to use for hunting a particular type of game. Also, like most theories about killing power, the school of bullet placement can be carried to extremes.
For example, a 55 grain FMJ bullet through the heart from a .223 Remington rifle may indeed kill a charging lion. Unfortunately, it may not prevent him from first disemboweling the hunter. In that situation I think that even I (a strong bullet placement advocate) would rather trust my life to a solid hit with a .375 Magnum bullet, even if it narrowly missed the lion's heart. So while bullet placement is the single most important factor in killing power, it is not the only factor.
I think that most reasonable authorities would argue the case for an adequate bullet precisely placed. And, at least to an extent, the better the bullet placement, the less powerful the rifle needs to be. This is why, in the right hands, a medium power rifle like a .30-30 serves so effectively for taking large animals like elk and moose. But, in the hands of an indifferent marksman, the .30-30 can be a terrible wounder.
While great power may be called for in some circumstances, such as the lion example above, it will not make up for a lousy shot. If you shoot a deer in the paunch with a .375 Magnum a quick kill is not guaranteed. If fact, it is probably less likely than if you had shot the deer with one of Weatherby's .257 Magnums, because the .375 bullet may well go straight through without expanding. In any case, a gut shot deer is a poorly shot deer and likely to be a problem no matter what rifle was used.
I would also suggest that while, with proper bullet placement, a certain caliber might be adequate, something else may be more adequate. For example, while a 100 grain .243 Core-Lokt bullet in the lungs may well bring down an elk, there is no doubt that a 200 grain Core-Lokt bullet from a .350 Rem. Mag. rifle in the same spot is better in the sense that it does more extensive tissue damage, causing the lungs to fill with blood faster and the animal to die sooner. The .350 bullet is also more adequate in the sense that it is more likely to successfully smash through a rib or penetrate heavy muscle tissue to reach the lungs should that be required.
Summary and conclusion
So what have we learned? First of all, diligent and careful research greatly expands the body of data that can be analyzed. The result of such research is a broader and better understanding of the subject of killing power. Kills described by the top gun writers from yesteryear up to the present constitute a valuable body of evidence, but the veracity of the source must always be considered.
Factors like momentum and dwell contribute little or nothing to our understanding of the mechanics of killing power. Most killing power formulas should be taken with a large dose of salt, as they tend to reflect the prejudices of their creators. An exception might be the OGW formula, which seems to be about the best of the bunch and a useful predictive tool when comparing generally similar calibers.
It seems that velocity is a positive factor in killing power primarily because increases energy at bullet impact. Using a heavy bullet for a particular caliber is good because it increases SD and thus penetration. There is a trade off here, as the heavier the bullet in any given caliber, the lower the velocity. Attempts to combine high velocity with heavy bullets almost inevitably results in heavy recoil, which degrades the shooter's ability to achieve accurate bullet placement (the most important factor of all). So a balance must be struck between bullet weight, velocity and, ultimately, recoil.
Bullet diameter contributes to a larger diameter wound channel and therefore increases killing power, other factors being equal. Bullet construction and performance can be very important factors in killing power.
Kinetic energy powers important bullet functions like penetration and expansion and is therefore very important. It is a useful comparative tool as long as it is used to compare calibers used for similar purposes and bullets of similar performance and SD. Energy information is widely available in ballistics tables.
The most important factor of all is bullet placement, but even it does not exist in a vacuum. Perfect placement of a totally inadequate bullet will not necessarily result in a quick kill. But good placement of an adequate bullet within its useful range for the size of the game sought is a prescription for success. That is a good thought with which to close!
Copyright 2004, 2006 by Chuck Hawks. All rights reserved.