July 2003

Terminal Sectional Density

A more reliable basis for predicting bullet penetration;
plus an important related discovery

The performance we get from putting high TSD to work is nearly "bullet proof"

By Al Marion

while back Ted Keith sat in my garage as I finished cutting and wrapping a bull elk for the freezer. As we exchanged stories it should be no surprise that our conversation drifted to proper elk rifles and bullets. At some point Ted asked, "What did you take this one with, Al"? I responded, "A 400 grain .45 caliber hard cast flat point launched in a sabot from a .50 caliber Remington muzzleloader."

That raised Teds interest because his Dad (Elmer) used virtually nothing but cast bullets for handgun hunting, and during many years of writing did more than anyone to teach others about their virtues. As the discussion unfolded, I explained to Ted that I used hard cast bullets in my muzzleloaders because muzzleloaders were low energy weapons that did not perform as well with bullets that became burdened with large mushrooms. Instead, they did their best work with bullets having very modest frontal areas like the flat points on hard cast bullets. As Elmer Keith knew, the same could be said for handguns which deliver even less energy than popular .50 caliber muzzleloaders.

Wound cavity produced by the .50/610gr White Super Slug

As I explained the rest of my rationale for bullet choice including the importance of terminal sectional density (TSD), deep penetration and velocity of bullet passage, Ted remarked that he wished his Dad could be there to participate in the discussion. He reinforced that Elmer clearly knew the importance of deep penetration for all big game hunting whether the weapons were handguns or rifles.

"But", Ted went on, "I never heard Dad mention TSD or talk in this kind of interconnected detail about how and why bullets do what they do during creation of a wound. I am sure that he would be having a wonderful time if he was sitting here engaged in this conversation." I confess that no one would have had a better time than I if only Teds wish could have come true.

Ted and I have had more conversations about bullets, big game and his dad. It seems clear that TSD was not part of Elmers vocabulary because the first I know if it being discussed was in a book that appeared after Elmer passed away. (More on that book later.) None-the-less, I have no doubt that Elmer knew something about the principles of TSD but without ever using that label.

The fact that you are interested enough in bullet performance to be reading this means that you probably already know something about sectional density. For those unfamiliar, the calculation is simply bullet weight expressed in pounds divided by its squared diameter expressed in inches. When calculating sectional density, or "SD", we use the weight and diameter of an unfired bullet. That is important to remember because for terminal sectional density, or "TSD", we collect measurements differently. (An example of both calculations appears beneath the muzzleloader chart.)

The long time theory has held that all else being equal, a bullet with higher SD (more weight per diameter) will penetrate deeper. Thus, a 180 grain .308 bullet with a sectional density of .271 should penetrate deeper than a 165 grain .308 bullet with a sectional density of .248. IF all else is really equal, the theory works out pretty well.

The trouble is that almost all of the time all else is NOT equal. Change bullet brands, metallurgy, point style, jacket thickness, internal design, impact velocity, etc., and ordinary sectional density values become less and less reliable as a basis for judging which bullets should penetrate deeper. In contrast, the concept of terminal sectional density can be very helpful to hunters wanting to select a bullet that will deliver not only deep penetration, but the most decisive overall performance.

It should be noted that when I speak of "decisive performance" there is more than just depth of penetration that is of interest. Producing adequate wound width is also very important, and we will get to that soon. Later, in fact, we will make an interesting discovery about a relationship between wound width and high TSD values.

Let me pause here to give credit. I did not discover the concept of TSD. I wish I had, and during experiments more than a decade ago my mind was dancing around the edges of this useful tool. But before the light bulb came on to define and label it I found TSD discussed in a book written by Veral Smith of LBT. The book I refer to is entitled, Jacketed Performance with Cast Bullets. (LBT, HCR 62 Box 145, Moyie Springs, ID 83845.)

Simply stated, if you want to explore the science of bullet casting or understand more about how bullets behave in your gun and in game, your learning will remain incomplete without Verals book. I was enchanted enough with the 2^nd edition that I also sprang for the 3^rd edition when it came out. It was in that third edition where I first found TSD discussed.

If TSD has been discussed anywhere other than in Verals book, I am unaware. It deserves more attention, so here it is along with some related observations that build on the importance of TSD.

Wound cavity produced by the .44/250gr Nosler Partition

A word about my testing is in order. All muzzleloading test results were gathered using a standard charge of 100 grains of Goex FFg fired at 50 yards. Modern rifle data was collected with loads adjusted to duplicate impact velocities at 150 yards when fired from a .300 Winchester and a 7mm Remington Magnum, and .44 Magnum data was collected as described beneath that chart.

The ballistic media I use does not have the elastic "memory" of living tissue. So the media retains maximum wound diameters that existed while the bullet was doing its work, i.e. the cavity does not spring back partially closing the temporary wound. Because of that I miss the opportunity to evaluate what would otherwise be a somewhat smaller permanent wound opening. On the other hand, I have the advantage of examining a wound that shows a more accurate picture of the total trauma delivered. It presents a vivid reflection of where and how "wisely" the bullet spent its energy.

Having compared "same bullet" wounds in media and in game, it is apparent that penetration in game is somewhat deeper than in my media. Bear in mind, however, that knowing the exact width and depth of wounds is not nearly as important as understanding the comparisons made between them. The comparisons tell us in relative terms which bullets excel in performance characteristics that are important to us.

The calculation for TSD is the same as noted earlier for SD except we use recovered bullet weight and final frontal or mushroom diameter. These measurements more closely represent important bullet characteristics as they existed during creation of the wound, not characteristics that were present earlier in the chamber or in flight. Unfortunately, except for "solids" that essentially retain weight and shape while creating a wound, we need to recover and measure bullets in order to make the TSD calculation. But since bullets delivering desirable penetration in game commonly exit, access to data for TSD assessment can be a challenge.

With that in mind, the data in accompanying charts along with photographs of bullets and some of the wounds they created in media should put you well on your way toward understanding the concept of TSD. Once TSD is understood, photographs of recovered bullets that you see in hunting literature will suddenly become more meaningful.

L-R: Unfired and recovered .45/300gr Hornady XTP (SD .210), .50/405gr Black Belt (SD .231) and .45/335gr hard cast flat point (SD .235). TSD’s were .078, .076 and .347 respectively. In all muzzleloader hunting circumstances imaginable, especially those involving heavy game, hard cast flat point bullets are dramatically more capable. TSD predicts it, scientific testing supports it and repeated results on game confirm it.

You may not always have the necessary measurements to calculate TSD but you will easily distinguish the profile of recovered bullets that are more capable from the profile of those that are more likely to fail. A useful rule of thumb is to select bullets having a reputation for finishing their work with a profile roughly as long or longer than the bullets frontal diameter. In contrast, bullets that end up resembling a "pancake" are to be shunned.

As you study the accompanying charts be aware that even TSD calculations do not include all details of bullet behavior that affect wound results. When we measure final weight and frontal diameter we miss intermediate measurements that were changing as the bullet was doing its work. Weight and shape changes do not happen instantaneously. They take place over time and distance within the wound, and even the shot-to-shot pattern of those changes is never identical. Consequently, TSD values will never be perfect. As you will see, however, TSD is far more reliable for predicting penetration than ordinary SD.

In the muzzleloader chart for example, based on ordinary sectional density values the .50 caliber 610gr White Super Slug should penetrate much deeper than the .44 caliber 250gr. Jacketed Nosler Partition. After all, sectional density is a whopping .349 for the White and only .194 for the Nosler. However, test results show these SD values to be unreliable predictors of penetration. The relationship between outcomes was actually reversed with the low SD Nolser penetrating more than 25% deeper than the high SD White Super Slug!

Terminal sectional density values on the other hand were .085 for the White and .146 for the Nosler. Whereas depending on ordinary SD proved unwise, the relationship between TSD values was consistent with observed penetration.

Also from the muzzleloader chart there is an interesting 3-way comparison. The .45/300gr. Hornady XTP has a sectional density of .210, the .50/405gr. Black Belt has a higher SD of .231 and the .45/335gr. hard cast flat nose has a sectional density virtually the same as the Black Belt at .235. Based on sectional densities, we would expect the Black Belt and the hard cast flat point to penetrate somewhat better than the XTP and about the same as each other. In truth, the Black Belt penetrated a bit less than the XTP, and the hard cast bullet with virtually the same sectional density as the Black Belt penetrated more than three times as far! What gives?

Well, if we had been basing expectations on a working knowledge of TSD instead of SD values we would not have been lead astray. TSD for the Hornady XTP worked out to be .078, two points higher than the Black Belts .076. While that difference is far too small to predict or explain any difference in penetration, the TSD of .347 for the hard cast bullet is powerful evidence that it will outperform the others by a huge margin and do it EVERY SINGLE TIME! Later we will return to this same three-way bullet comparison for a different lesson.

Examine the TSD values listed in the charts. Correlation with total penetration is not perfect for the reasons noted above, but an indisputable pattern emerges. In general, total penetration corresponds with TSD values, and where TSD values are notably higher, total penetration is consistently deeper. We cannot say the same for SD values.

The chart presenting data for .30 caliber 180 gr. bullets offers a lot of learning. All bullets were fired from the same .300 Winchester using the same load except for the change in 180 grain bullets. So, all bullets began their wounding work with essentially equal energy and importantly, with the identical sectional density.

Simply put, with NO differences in sectional density and nearly a 100% difference in penetration, we have more compelling evidence that SD values are unreliable as a basis for predicting which bullets will penetrate deeper. As you can gather, TSD is much more reliable.

Now, I can already hear some of you mumbling, "Yeah, but those high TSD numbers and deeper penetration are bought with smaller mushrooms and everybody knows those wimpy mushrooms just dont cut it when it comes to taking real game. After all, they just zip through game without doing much damage, right?" Well, not exactly.

It is true that pointed bullets failing to expand at the target do indeed zip through game without destroying a lot of tissue. But those are bullets with a virtual absence of frontal area. In contrast, the higher TSD bullets featured here had modest frontal working areas either from effective controlled expansion on impact or they were made with a "pre-expanded" frontal area in the case of the flat points on hard cast bullets. Importantly, while each of them was at work creating a wound they had enough frontal "contact" area to create effective wound width.

In a moment we will discover that "enough" (read that "preferred") mushroom size is smaller than most hunters think. Bullets working with conservative frontal areas (higher TSD) do more than just penetrate deeper. Overall, they actually deliver more desirable wound profiles and are more effective game getters.

The thought process we are about to pursue will make more sense if you understand more about the key factors at work in creating a wound. Consider the wound created by the .30/180gr. Fail Safe. The wound features an open cavity for some distance after entry that is much wider than the mushroom on the bullet that created the wound. It is helpful to understand how that happens.

In a word, the answer is "splash". Did you ever stand on a boat dock and drop a flat rock on still water? If you did you couldnt miss the splash but you may not have paid attention to the direction of the splash at the moment of contact. It fanned out nearly flat across the waters surface, or perpendicular to the direction the rock was traveling.

Now visualize propelling the reasonably flat frontal surface on a bullet through flesh at a velocity that is sufficient to create "splash". (Remember that we are dealing with far more velocity and energy here than with the dropped rock.) Tissue in the path of the bullet is splashed aside with enough violence to destroy and displace adjacent tissue. Higher velocity makes a more violent splash and a broader wound. And at "splash velocity" and above, a larger mushroom also increases splash. As the bullet proceeds, an open wound much wider than the bullet itself is created, and the continuation of that wider-than-bullet wound goes on so long as the bullets velocity of passage remains high enough to generate destructive splash.

With expanding bullets, a rather pronounced maximum wound width commonly occurs between 3 and 5 inches of penetration. This "bulge" in the wound profile results from the momentary combined effect of the mushroom having opened to maximum size while the bullet is still moving at nearly its entry velocity. Then, because every bullet decelerates continuously, every wound reflects that by tapering narrower beyond the entry bulge. Bullets with smaller mushrooms typically produce a less pronounced bulge and a more gradual taper in their wound. If velocity drops below the level required for destructive splash, wound diameter closes to a size just large enough to permit passage of the now lethargic projectile. In live flesh this splashless "puncture" wound springs virtually shut immediately after bullet passage.

That is my simplified version of what goes on during creation of a wound. Certainly, the process is more complicated when one considers the host of real life variables that interact to modify results. In his book, Veral Smith describes the process similarly but includes more detail. The book is worth studying for those wanting to refine their thinking on this subject. None-the-less, I have examined literally hundreds of wounds in game and in media and have yet to find a meaningful feature in a wound profile that was not adequately explained by the dynamics of the simple "splash" theory presented above. I trust that it will be helpful to you as well.

As you have noted in the charts, differences in wound dimensions are not limited to depth of penetration but also include differences in wound diameters. Notice that at penetration depths of 6 inches and beyond, bullets working with smaller mushrooms created wounds that were roughly equivalent or even broader than wounds created by larger mushrooms! Why? All else being equal, bullets with smaller mushrooms retain a higher velocity of passage. And conservative mushrooms at higher velocity easily contribute as much splash violence in a wound as larger mushrooms do at significantly lower velocity. Velocity of passage is the key.

In fact in the overall dynamics of wound development, assuming a bullet has an adequate frontal working area, I am convinced that prolonging velocity of bullet passage above the level needed for destructive splash contributes more to decisive wounding than any other single factor. And if what I believe is true, then TSD suddenly becomes directly linked to overall wound performance because the two bullet features that help preserve velocity of passage are the same two that elevate TSD values. Of the two features (smaller frontal area and greater weight) a smaller frontal area will typically contribute more to decisive wounding.

You see, before reaching six inches or so of penetration, bullets burdened with a large frontal area lose a large portion of their energy and velocity. The effect of opening a large mushroom in game is not unlike the effect of opening a parachute in free fall. In both cases, theres a lot of energy transfer and velocity loss in a very short time and distance.

Thus, the opportunity for a bullet with large mushroom to open a decisive wound is impaired with ultimate success depending heavily on vital tissue residing within a short distance after entry. Beyond six inches or so of penetration, compared to bullets with more modest frontal working areas, larger mushrooms are a net liability. Worst of all, they always cause more rapid deceleration and less penetration. Further, they often give up more weight due to weaker construction and exposure to more violent forces near entry.

On the other hand, bullets with a modest frontal working area sacrifice little or nothing of significance in early wound width. And because they spend their payload of energy at a strategically slower pace, they prolong delivery of splash velocity so that with virtually any sane body angle these bullets commonly open a decisive wound all the way through big game.

Do bullets carrying large mushrooms work at all? Of course they do. They have worked for a long time because most hunters have been taught to hold behind the shoulder. And for hunters using bullets that produce large mushrooms, holding behind the shoulder is good advice where game presents a broadside shot. When hunters succeed in placing such shots, and many do, bullets with large mushrooms deliver excellent results because they have hit a target area where vital tissue is close to bullet entry.

But where shots are anything other than broadside through the rib cage, the bullet path that lines up with the center of the chest involves entry somewhere other than just behind the shoulder. Then it is virtually unavoidable that bullets carrying large mushrooms will spend too much energy on non-vital tissue before reaching the chest cavity. Especially on larger animals, these are the circumstances where hunting with more capable bullets can deliver the total difference between success and disaster.

To be fair, assuming the use of adequate weapons in the first place, game the size of deer does not demand the highest TSD in bullet choices. But for the sportsman who uses the same weapon for deer, elk, moose, etc., a wise decision is to select one bullet and load for all game. It saves on re-sighting, helps avoid ammunition mix-ups and requires that only one trajectory be remembered. Of course, in such a plan the ammunition choice must be governed by the most challenging hunting assignment anticipated.

In case you wondered, bullets with high TSD also work exceedingly well when placed behind the shoulder in a classic broadside shot. Yup, thats even true when you use your moose load on deer. And whether your target is moose, deer or something else, the quick humane kill will not result from a ghastly entry wound caused by a "devastating" mushroom. It will result from a modest entry wound and a superior wound profile deep inside.

Now, while your mind is still trying to sort out the apparent incongruity of high TSD, modest mushroom bullets sacrificing nothing of significance in wound width, remember the 3-way comparison of bullet penetration we made earlier in the muzzleloading chart? Go back now and compare the wound diameters created by those same three bullets at all penetration depths. If I had listed wound diameters at 3 inches of penetration, the XTP and the Black Belt would have shown somewhat larger wounds. For that matter, so would the hard cast bullet. But by the time they get to 6 inches and beyond where wound performance counts most, the bullet with the smaller mushroom wins there and all of the way through.

Notice that the "mushroom" on the non-expanding hard cast flat point bullet was a tad less than 3/8". Yes, the most effective "preferred" mushroom size is a lot smaller than most hunters think! And because higher TSD bullets go deeper, it is guaranteed that they have more opportunities to deliver decisive damage to more skeletal, plumbing and nervous networks. The wound is long and "splashed open" all the way through. Get the picture?

In the chart for 7mm bullets, note that the 140gr. Barnes "X" and 160gr. Partition Gold performed neck and neck. So did the 140gr. Fail Safe and the 175gr. Nosler Partition. A lighter bullet in a given caliber will always have a lower sectional density, but as we have discovered that may be meaningless. If you choose wisely, a lighter bullet can have an equal or higher TSD than a heavier bullet you may currently use. That can work to your advantage because the lighter bullet offers potentially higher velocity and a flatter trajectory without sacrificing terminal performance.

Or, you can choose to hunt with heavier bullets that also happen to be built with high TSD characteristics. They, of course, will deliver the ultimate in terminal performance. Check out the .284/160gr Fail Safe. Among the 7mm bullets tested it produced the best overall wound profile.

For reader information, I have included a chart with wound data gathered from three cast bullets fired from a .44 Magnum. By now, considering the exposure you have had to TSD and wound cavities, the chart should speak well for itself without further commentary.

When considering the implications of TSD, especially when making comparisons, it is important to compartmentalize your thinking within a class of weapons delivering similar energy. TSD conceptual thinking readily applies to different classes of weapons, however, you may be lead astray if comparisons and resulting expectations mingle mushroom measurements or TSD values taken from muzzleloader or handgun slugs with bullets used, for example, in a .30/06.

Finally, I must not miss this opportunity to strangle a common misunderstanding. We have all heard that bullets should stop inside an animal so that all energy has been delivered there instead of being wasted on the landscape beyond? By now you should know the truth about that misguided mindset, but just in case I have fallen short as an educator it is far less important how much energy is delivered in an animal than it is to deliver enough energy where it matters.

Bullets that routinely stop inside do so because exit is pre-empted by a large mushroom or bullet blow-up that causes a pre-mature energy dump. Thus, to prevent "wasting energy on the landscape", bullets must behave in a way that also happens to increase the likelihood of failure when encountering bone, heavy muscle or paunch contents on their way to the boiler room.

Over time, colorful terms have been used by many sources to describe the mushrooming behavior hunters should demand in the bullets they buy for big game. Youve heard some of them, such as "devastating", "explosive", "impressive knock-down", etc., etc. These bullets may deliver all of their energy inside an animal, but too frequently most of the energy is spent before the bullet reaches where it really matters. Yes, I know these bullets often work well, but "often" is painfully less than "virtually always".

We never know when we buy or build ammunition exactly how much will be required of each bullet we send down the barrel toward big game. Thus, it is foolish to not use bullets with design characteristics that are up to the toughest assignments. The same design characteristics that restrict expansion also protect against major component separations. So, because toughness goes with the territory in high TSD bullets, they routinely survive violent impact with bone and continue on with their assigned task. With regularity, they deliver enough energy where it matters. And gratefully, in all but the most extreme circumstances, any bullet with that capability is certain to continue helping us by opening a leak on the far side.

You might say that the performance we get from putting high TSD to work is nearly "bullet proof".

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