Addressing the Foot-Pound Fallacy
July 28, 2016
Decades have passed since Col. Townsend Whelen theorized we should have 1,000 ft.-lbs. of energy at the animal to cleanly take deer-size game. This isn't a bad rule of thumb because kinetic energy expressed in foot-pounds is a proven scientific formula: The amount of force required to move one pound one foot.
I have always agreed with Whelen, but not for the obvious reasons. The 1,000 ft.-lbs. standard is not guaranteed to flatten a deer, but provided other factors (like adequate bullet construction) are present, this level of kinetic energy is required to get the bullet into the vitals. And, ultimately, adequate penetration into life-essential organs is the only way to kill game.
Whelen's rule was for deer-size game. Others, me included, have gone further and suggested 2,000 ft.-lbs. at the animal provides a sound minimum for elk, and for 100 years we have established something between 4,000 and 5,000 ft.-lbs. as a sensible minimum for the world's largest game.
Several alternative methods have been promulgated. Some of them are useful for comparison, but none is quite as scientific as kinetic energy expressed in foot-pounds. Elmer Keith worked the opposite, "pounds-feet" or momentum, which may have some value.
John "Pondoro" Taylor's theory of "knock-out value" included little science but did take into account bullet frontal area. Still touted occasionally, his KO value actually has utility in comparing one cartridge to another, but its failure for today's hunter is that it was intended to compare solid, non-expanding bullets and not the bullet designs commonly used now.
How then are we supposed to view energy? Think about it this way. Which is likely to affect you the most: taking a hammer blow just below the sternum or being stabbed with a knitting needle in the same place? The knitting needle is more likely to be fatal, given some time, but the hammer is going to have a much more physical impact on you initially. This is because the hammer develops more kinetic energy, all of which is transferred upon impact.
To my knowledge, we have not found a way to properly evaluate, let alone measure, the transfer of kinetic energy from a projectile to a living target. We do know a surface wound is messy and painful, but, absent infection, not necessarily fatal. We also know perforation of the heart or both lungs is generally fatal, but all experienced hunters have seen heart- or lung-shot animals go for some distance while others drop in their tracks to similar shots.
The whole thing is complicated by the fact that no two living creatures react in exactly the same way upon receipt of a bullet. But while sheer kinetic energy as a number probably isn't as important as we make it, I do think we're missing something vital in not really understanding how much and how quickly energy is transferred.
There are two basic schools of bullet performance: Those who prefer complete penetration and those who want all of the bullet's energy expended in the animal. The latter group is probably also divided into those who like to find their beautifully mushroomed bullets against the skin on the far side and those who are perfectly happy if the bullet goes to pieces as long as it first penetrates into the vitals.
There is a sound rationale for an energy level offering complete penetration in that the resulting exit wound offers better blood trails, and if we're faced with problematic shot angles we can probably count on sufficient penetration. I'm not aware of any testing designed to measure velocities of bullets exiting from targets that simulate game animals, but you have to assume retained energy for any exiting bullet — energy not expended in the animal.
The "stay in the animal" school of thought is an extremely valid argument. Whatever kinetic energy the bullet had was expended inside the animal, regardless of whether the bullet fragmented in the vitals or lodged or against the hide on the far side.
But here is where it gets tricky. If energy transfer from bullet to animal tissue is important — and I believe it is — is there any way to measure it?
Let's look at four scenarios. Bullets A, B, C and D hit the shoulder of a deer with 1,000 ft.-lbs. of remaining energy. Bullet A expands prematurely. It blows up, creating a nasty surface wound only an inch deep. Bullet B also blows up, but it makes it into the chest cavity, with penetration of about nine inches. Bullet C is found against the hide on the far side. (This is a mid-size deer, so penetration is about 15 inches.) Bullet D exits, center punches a sapling on the far side and exits even that.
Bullet A just plain failed, but it transferred all of its energy on impact. The deer may well have been knocked flat by the sheer impact but also may have gotten up and run off. Bullet B didn't exactly fail (depending on what you wanted), but aside from fragmentation, it expended all of its energy during nine inches of penetration. Bullet C held together and fully expended its energy during 15 inches of penetration. It came to rest on the far side because it lacked the energy to penetrate the skin a second time.
You see what I'm getting at? Bullet D did what a lot of folks want their bullets to do, but after exiting the animal it still had enough velocity and energy to penetrate a tree, which does the hunter no good at all. It penetrated well, but clearly expended only a portion of its energy within the animal. If you are of the school desiring through-and-through penetration, you have to accept that bullet energy will be wasted.
Is this bad? Not necessarily. You must decide which bullet performance pleases you and gives you the most confidence.
For sure, overpenetration is better than lack of penetration. In my example, Bullet A is pure trouble, but Bullet D is fine if it's what you want — and I did want that for many years. Today, however, I'm okay with Bullet B on deer-size game, and I'll take Bullet C across the board. Exit wounds do leak a bit more and expedite tracking, but over the years I have become more convinced I do less tracking when the bullet stays in the animal and expends all of its energy.
We know bullet shape is critical to ballistic coefficient, which is essentially a comparative measure of a bullet's ability to retain velocity. It is not so widely known that bullet shape also affects energy transfer, and, unfortunately, we can't have it both ways. Sharply pointed bullets are aerodynamic and have higher BCs, but they're the knitting needle from my earlier comparison. Blunt-nose bullets lose velocity more quickly and have a more arcing trajectory, but they are the hammer.
Today most of us shoot spitzer bullets as a matter of course, but if you have any experience with roundnose bullets, you probably know they seem to deal a noticeably heavier initial blow than a spitzer at equal velocity. And in my experience, flat-point bullets hit even harder. I believe we are seeing energy transfer in action.
Obviously, penetration to (or through) the vitals remains essential, but I believe initial energy transfer of traditional blunt-nose bullets is why the good old .30-30 kills deer the way it does despite unimpressive ballistics. And big woods hunters who want to drop deer in their tracks often rely on archaic "brush-busters" such as the .35 Rem., .444 Marlin and .45-70. None of these produce as much kinetic energy as, say, a .270, but they flatten game with authority. Part of it is bullet frontal area, and another part is the blunt-nosed bullet — both of which contribute to a rapid initial energy transfer.
Within the large spectrum of expanding bullets, we simply must take into account bullet design: how much and how quickly they expand. The amount of expansion ultimately determines penetration. The more the bullet expands, the more resistance it meets. Velocity is also a factor because it enables the bullet to overcome resistance, but velocity is also a great enemy to consistent bullet performance. For instance, most .30 caliber bullets perform well at .308 and .30-06 velocities, but some become unreliable bombs in faster .30 caliber magnums.
Regardless of velocity, however, hunters who desire through-and-through penetration gravitate to tougher bullets that hold together well and don't expand a huge amount. Good examples range from the great Nosler Partition to today's homogeneous-alloy expanding bullets.
Hunters who like to find beautifully mushroomed bullets "against the hide on the far side" are likely to choose bonded-core bullets. Expansion tends to be radical, as much as twice original diameter, but core bonding keeps the bullet together, and weight enhances penetration.
Hunters who want maximum damage to the vitals without undue concern about what the recovered bullet looks like — or exactly where it comes to rest provided it gets to the vitals and dispatches the animal quickly — probably have the widest spectrum of bullets to choose from. Traditional cup-and-core bullets still perform well. Lead-core, polymer-tipped, non-bonded bullets are volatile but often work like lightning striking on deer-size game.
Now we're getting into how quickly the bullet expands. I think this has a lot to do with energy transfer. Regardless of how much they ultimately expand — or how deeply they penetrate — round-nose and flat-point bullets tend to deal the heavy initial blow I spoke of. Polymer-tipped bullets tend to expand quickly because, upon impact, the polymer tip is driven down into the bullet to initiate expansion.
Hollowpoint bullets also tend to expand quickly, and I believe rapid expansion also applies to the homogeneous-alloy bullets. Years ago, when the Barnes X was new, I did a couple of cull hunts in Australia. Although expansion is limited in this type of bullet (which today includes Hornady GMX, Barnes TSX and TSSX, Federal Trophy Copper, Nosler E-Tip and more) and penetration is extreme, it seemed then — and still appears — this style delivers a noticeably heavy initial blow out of proportion to the actual expansion.
Whether tipped or not, these bullets are all essentially hollowpoints, with a nose cavity that limits the amount of expansion. My theory is these hollowpoints accomplish their expansion quickly due to their nose cavity and then behave essentially as solids, holding together and continuing to penetrate. These bullets are not everyone's cup of tea because expansion is not radical, and few will be recovered. But they do deliver a heavy initial blow, and on solid tissue, such as a shoulder, you can often hear this in the solid crack of the bullet hitting. I believe this is energy transfer.
It is easy to obtain comparative measures of penetration, and recovered bullets can be measured for expansion and weighed for weight retention. It is difficult, perhaps impossible, to scientifically evaluate this business of energy transfer. I do believe it's real, and we can certainly see it in action by shooting water bottles and melons. Measuring it? I don't have a clue, but last year, at the Bisley range in England, I saw a demonstration that got me thinking.
Traditional bullet testing is done in ballistic gelatin or packed wet newspapers. These media show penetration depth but not how quickly a bullet expands or what kind of wound cavity it produces — factors I believe to be key elements in energy transfer. The test I witnessed in England used ballistic soap, which retains an intact wound channel. It was conducted by a Hornady representative and compared only the company's InterBond, InterLock, GMX and SST (all 150 grains in .308 Win.).
No wider comparison is suggested, and it is accepted this was just a simple demonstration, not a definitive test. But it was interesting nonetheless.
Predictably, the fast-opening SST showed the most distortion, the least recovered weight and the largest cavity diameter: an explosive 4.8 inches. Also predictably, the recovered homogeneous-alloy GMX expanded less than its brethren, retained the most weight (99 percent) and achieved the deepest penetration.
Because the GMX expanded less, its cavity was the smallest at 3.88 inches in diameter, but the distance from entry to maximum cavity diameter was the shortest — just four inches. The difference was not dramatic, but it does seem to confirm my theory that this type of homogeneous-alloy bullet, though more limited in expansion than most lead-core bullets, does its expansion more quickly, perhaps transferring more energy in a shorter distance. Depending on what you want, this does not make it the most perfect bullet. Total penetration was 19.4 inches, meaning it would most likely exit deer-size game.
Second in penetration among these bullets was the InterBond at 16.4 inches, meaning it might exit a deer, depending on shot angle and placement and the size of the deer. The InterLock and SST were equal in penetration at 14.8 inches, which suggests these particular bullets could have been "against the hide on the far side."
The obvious problem with this and most testing media is their uniformity. They can't accurately replicate bones and tissue layers of varying densities, all which change with shot angle and animal size. So, to me, this business of how and when a bullet's kinetic energy is transferred into game animals — and what effect it has — remains unknown. There may not be any definitive way to solve the mystery, but I'm convinced there is much more to it than the sheer number of foot-pounds.