Shoot’n, Fish’n ‘n’ Hunt’n

Bullet penetration is critical, when hunting large and dangerous game, but is sectional density a valid indicator of penetration?

Introduction:

The shooting sports have often been plagued by beliefs and myths that, through being repeated many times, have taken on the illusion of legitimacy. These myths are often repeated by gun writers and shooters, alike, and are simply accepted as true without any thought to determining the validity of the myth. One of the most persistent myths is the claim that bullets with high sectional density values will penetrate deeply into game animals. Conversely, the myth continues, that a low sectional density value will mean that hunting bullet penetration will be poor, even inadequate. If all projectiles, proportionate with their calibre, were constructed to have exactly the same hardness, exactly the same shape and exactly the same impact velocities, then sectional density might be a valid way to compare potential penetrators. However, while man may have been created equal, projectiles most certainly aren’t!

You will find reference to this myth, about sectional density and penetration, in dozens of books and in hundreds of magazine articles about hunting large game. You will often hear it repeated on the rifle range, round the campfire or whenever two or more hunters get together. You will also hear it repeated by some projectile manufacturers who have either been seduced by the myth, themselves, or who are simply seeking more ways to sell their products. No matter how many times you hear it repeated, it is still just a myth and defies logic.

In this article, I am going to discuss some of the issues that affect hunting bullet penetration and, in doing so, hope to demonstrate that the value of a projectile’s SD is not a valid indicator of bullet penetration. I hope to do this succinctly and with as much brevity as possible. However, a comprehensive treatise on hunting bullet penetration would require a very lengthy discussion, more in the form of a book with lots of scientific explanations, so I will be forced to gloss over some points and would urge the reader to follow the links, at the end of this article, if you wish to learn more about SD and bullet penetration. Through this discussion, I hope to demonstrate that the value of the SD, of any projectile, is of little real consequence when discussing penetration.

Where did the myth start?

The myth, that a high SD value will produce deep penetration, is a myth of long standing. Where this myth originated, is not totally clear but it can be traced back, at least, to the 1940’s when John Taylor published his highly respected book ‘African Rifles and Cartridges’. Whether John Taylor originated this myth, or whether he got this notion from another source, is not known. Regardless of where it comes from, Taylor is often quoted as the source of this myth. Taylor’s book is so well regarded, and rightly so, that no one seems to stop and question everything that he said. While there is a plethora of valuable insight and information, in Taylor’s writings, he is still human and, therefore, still fallible. His comments about sectional density and bullet penetration are one of his fallible moments.

John Taylor’s iconic ‘African Rifles and Cartridges’; could this be the source of the myth about sectional density and bullet penetration?

So allow me, if you will, to discuss some of Taylor’s comments in regards to SD, as I think it is important to realise how little Taylor understood this issue and to realise the confusion he has introduced to this topic. To begin with, Taylor contradicts himself when he blames SD for the poor penetration of some projectiles because he also talks about poor or excessive expansion of the very same projectiles. The most blatant of these examples is when Taylor talks about the 10.75×68 (.423 Mauser) cartridge which he writes off as generally a poor choice for large African game. This is what Taylor actually had to say:

“The sectional density of the 10.75’s bullet is poor, hence its lack of penetration and general unreliability.”

Elsewhere, after listing a number of failures with the 10.75×68, he states:

“The reason for all of this is, of course, the very poor sectional density of the bullet. I certainly, and very emphatically, can not recommend this cartridge.”

For the record, the SD of the 10.75mm, 347 grain projectile, is .277 and this is a greater SD than almost all of the projectiles up to, and including, many .375 projectiles. Only the heaviest 6.5, 270, 7mm, 308, 8mm, 358 and 375 calibre projectiles have higher SDs. If SD was the sole determinate, when discussing penetration, then all of our deer and plains game cartridges would also be poor penetrators when loaded with the usual range of projectiles. It is true that our deer and plains game cartridges do not require as much penetration, as big game cartridges do, but the comparison demonstrates that there is more to the issue of bullet penetration than simply the value of the SD.

Taylor also stated:

“But the 10.75mm lacks penetration. It’s better than the 11.2mm; but that’s about all you can say for it. It won’t kill an elephant on a frontal head shot, there’s plenty of evidence on that point; and is incapable of driving even a full-patch bullet thru a buffalo’s shoulder.”

 Then he goes on to describe how a FMJ 10.75mm disintegrated on the boss of a Cape buffalo.

“…I once had a full-patch bullet from one of these rifles blow to bits on the boss of a wounded buffalo’s horns without having the slightest effect on him – I might as well have fired a revolver at him. There was just a leaden block to show where the bullet hit him.”

Clearly, if the FMJ projectiles, that he was using, would blow up on the boss of a buffalo, they certainly will not kill elephants with frontal brain shots nor penetrate, broadside, a Cape buffalo. Yet, other hunters have reported good performance, with the 10.75×68, on dangerous game so there must be something else at work, and not simply the value of SD. Taylor also describes how some 10.75×68 projectiles would punch completely through eland which, considering the size of an eland, hardly seems to support the case that it has insufficient penetration due to poor SD.

“…I found the various types of expanding bullets very unreliable; usually they lacked penetration on all but the lighter varieties of game, and yet I had expanding bullets whip clean thru (sic) eland without setting up at all on body shots – the exit hole was no larger than the entrance wound.”

So, in reality, Taylor was describing poorly designed and constructed bullets that either failed to penetrate adequately or over-penetrated. It is known that many German projectiles (1.), of the time, were not the best for large game, but that the British projectiles, while not perfect, were more suited^1.. The Germans recognised the shortcomings of their projectiles and, in time, corrected the problem but not before many of their cartridges had earned poor reputations. Regrettably, Taylor doesn’t identify the brands of ammunition and projectiles than he is referring to, which is crucial information for any meaningful evaluation, and, instead, simply condemns the calibre, in all of its brands and loadings.

Taylor’s illogical conclusions, in regards to poor penetration and the alleged link to SD, have been, inexplicably, blindly accepted by one gun writer after another. Further, the belief that low SD values will result in poor penetration, has since been transferred, by these same writers, to any projectile with a low SD. What is further curious is that some of these writers have written articles, and extolled the virtues, about their own favourite controlled expansion, homogeneous or FMJ projectiles. Yet, very few writers seem to have noted the inconsistency between the myth of SD and the penetration and effectiveness of these new generation, hunting projectiles.

Why this myth is a problem:

Now, to demonstrate how this myth can cause problems, let me relate a short tale about a hunter, that I know, who had been misled by the SD myth. This gentleman is an intelligent man but he had come up with a theory that, if carried out, could have been very dangerous and would certainly have been illegal and unethical. He had compared the SD of the projectiles of his favourite 6.5×55 rifle to the SD of .375 H&H projectiles and, because the 6.5 was higher, he had come to the conclusion that the 6.5 would out penetrate the .375 and, therefore, this would make the 6.5×55 an effective buffalo rifle.

Fortunately, he had not put his theory to the test, but this story demonstrates how the SD myth can give hunters a totally misleading view of bullet penetration. Now there is no doubt that a shot with the right 6.5 projectile, on an un-alerted animal and with perfect bullet placement, you could kill a Cape buffalo. However, if he doesn’t go down quickly, then things could get very ‘ugly’. A tiny 6.5 calibre hole, in a buffalo’s heart, would most likely allow the beast, who is now less than happy with the hunter’s performance, plenty of time to stomp and gore the hunter, and anyone else he comes across, into a gooey pool of strawberry jam, before the animal finally expires.

While the Sectional Density of the heaviest 6.5mm projectiles might exceed those of most .375s, that does not make the 6.5, or any other deer/plains game calibre, an adequate buffalo rifle. 

Now, it is true that many deep penetrating hunting bullets do have high sectional densities, but to state that it is because of their sectional density, that these bullets achieve deep penetration, is a false statement. It would be like saying that the aesthetics of sports cars gives them the look of being fast and so, because they are fast, it must be their looks that make them go fast. Clearly such a statement is false and so is the view that sectional density determines penetration. The truth is that there are projectiles, designed for deep penetration, that have relatively low SDs and some projectiles, with high SDs, that are poor penetrators. A high SD does imply the potential for substantial momentum of the projectile, which would aid deep penetration, but fails to take into consideration the infinitely more important considerations of velocity, shape and construction.

So what is Sectional Density (SD)?

‘Sectional Density is the ratio of a bullet’s weight, in pounds, to the square of its diameter in inches. Bullets of the same shape but with more weight, in relation to their diameter, retain their velocity and energy better.’ (From Hornady 8^th Edition)

As this definition states, SD is the ratio of the projectiles weight to the square of its diameter and that projectiles with a high SD will retain velocity and energy better than those with a low SD. In other words, they will have greater momentum. However, what this definition doesn’t make clear is that SD is ONLY relevant when we are looking at the bullets passage through air. In other words, SD is a term relating to the external ballistics of your rifle/cartridge combination and, as we will see, is of little consequence when discussing terminal ballistics; which is where the bullet impacts and penetrates the target.

Once a projectile leaves the atmosphere and enters another medium, such as water or flesh (which is mostly water anyhow) then the parameters that determined the exterior ballistics, including SD, are no longer applicable. In water, or flesh, which is around 800 times denser than air, there are other parameters that determine penetration. For a simple demonstration, you only have to consider how ballistically efficient projectiles will travel through many thousands of metres of air yet will only penetrate one or two metres of water. Clearly, after impact, the importance of the projectile’s SD, and other external ballistic parameters, has been supplanted by other considerations.

From the definition, for SD, you can also see that the shape and construction of the projectile has no effect on its’ SD. A .30 calibre, 180 grain bullet has a SD of 0.271 regardless of whether it is a boat-tail or flat base, whether it is a spitzer, spire point, round nose or even if it is a wadcutter. Similarly, it does not matter whether it is a cast bullet, a lightly constructed varmint-style bullet, a long range target bullet,a full metal jacket or homogeneous bullet; a .30 calibre, 180 grain bullet has the same SD regardless of all of these other variables and this is true for all combinations of calibre and bullet weight. If SD was the prime indicator of penetration, as some would imply, wouldn’t all of these different variations of 180 grain, .30 calibre projectiles exhibit the same amount of penetration? Yet, experience tells us, that they will not exhibit even similar penetration.

These five .30 calibre, 180 grain projectiles all have the same sectional density; irrespective of shape and construction; but they certainly will not achieve the same depth of penetration on game.

Bullet spin rate and penetration:

Rifle barrels incorporate rifling, at predetermined twist rates, to impart spin on the projectile to give it stability during its flight through the air. Projectiles must be matched to the barrel’s twist rate if we are to achieve the required accuracy. However, as previously stated, the tissue of game animals is at least 800 times denser than air. The speed of rotation, which our rifled barrels impart on the projectile, may be sufficient for its flight through air but it will be woefully inadequate for its passage through tissue.

The required spin rate, to provide stability of a projectile travelling through flesh and organs, would have to be ridiculously fast, perhaps as fast as 30 or 40 times the spin rate required for accurate flight through air. This would mean that our barrel would have to have a twist rate faster than 1 turn in 1 inch! Clearly, this is not practical. This means that on impact, and in every case, our projectile is going to be very unstable as it penetrates our target animal. This instability, during penetration, means that it would only take the slightest impulse to cause our bullet to tumble, resulting in poor penetration and bending, deformation or breakup of the projectile. Tumbling is, therefore, going to be a common occurrence and will severally impair penetration and, therefore, lethality.

Long streamlined projectiles, which are ideal for shooting accurately over longer ranges, are the most likely to tumble and, once they do, their path through the target animal becomes very unpredictable. Parallel sided round nose projectiles are less inclined to tumble and this is why traditional big game projectiles have usually been of this shape, although they are nowhere near perfect, either. However, regardless of the design of our bullet, the inherent instability, when penetrating living tissue, means that a large percentage of our projectiles will tumble, deform and deviate away from the target organs located deep inside the animal.

This issue is not too critical when hunting small to medium game, as the rate of tumbling, deformation and deviation, is generally not sufficient to miss the vitals, altogether. However, when hunting large and dangerous game, the results of this instability might be catastrophic and result in a wounded, and very angry, animal whose vitals are still functioning.

Bullet deformation and penetration:

When discussing the penetration of expanding projectiles, whatever their shape, it is important to realise that as the projectile expands and deforms, then the drag generated by the mushroom, or whatever shape it takes on, is going to be the main factor that affects penetration. The larger the deformation, or expansion, then the greater the drag and the less penetration the projectiles will achieve and the less importance SD has on this matter. Speed of expansion is also critical and, if the projectile expands too quickly, then penetration depth may be extremely poor and even totally inadequate. The speed of expansion is going to be a function of velocity and bullet construction. A fragile, or poorly constructed bullet, pushed too fast will simply blow up on impact and cause a horrible wound that is unlikely to be fatal, which is a situation that we clearly want to prevent. A hard bullet, pushed slowly or at long range, may just punched straight through the target with no expansion and reduced lethality.

For many of my early years of hunting,  I had this mental image of expanding projectiles penetrating in relatively straight lines and expanding as they pass through the vitals of the target animal. This image was always based on the premise that the projectile continues to penetrate, nose first. I mention this because I am sure that many hunters have this image in their thoughts when they squeeze the trigger on their intended target. However, in reality, this is just wishful thinking and it is more probable that the expanded projectile, being inherently unstable to start with, will become even more unstable as it expands and will tumble as it goes. The drag caused by the expanded nose may even result, following tumbling, in the projectile travelling base-first and the expanded portion acting like the fins or parachute of a high drag tail on an aircraft-delivered bomb. Clearly, if this happens, then the planned expansion of the projectile will no longer occur and penetration will be severally impaired.

What also needs to be realised is that, as the projectile opens up, its sectional density rapidly begins to decrease. This is because, as it expands, the diameter increases and as SD is the ratio of bullet weight to the square of the diameter, you can see that SD can drop very rapidly and exponentially. If, as it expands, any part of the expanding projectile breaks off, and reduces the overall weight of the projectile, then SD diminishes even quicker.

Consider a soft point, 180 grain .30 calibre projectile. The SD, of the projectile, prior to impact, is 0.271. If we assume that it opens up to .40 calibre, after achieving some penetration of the target, then the SD has dropped down to 0.160. If it expands to .50 calibre, by the end of its penetration, then the SD will be down to 0.102. If it also loses 20% of its weight, as petals break off or it starts to shed its’ core, then the SD will be down to 0.0823.

Consider, also, that the shape and construction of the projectile has no effect on its’ SD. Our .30 calibre, 180 grain bullet has a SD of 0.271 regardless of whether it is a boat-tail or flat base, whether it is a spitzer, spire point, round nose or even if it is a wadcutter. Similarly, it does not matter whether it is a cast bullet, a lightly constructed varmint-style bullet, a long range target bullet,a full metal jacket or homogeneous bullet; a .30 calibre, 180 grain bullet has the same SD regardless of all of these other variables and this is true for all combinations of calibre and bullet weight. If SD was the prime indicator of penetration, wouldn’t all of these different variations of 180 grain, .30 calibre projectiles have the same, or approximate, penetration? Yet, clearly, they will not exhibit even similar penetration.

Another example, that shows how SD is not a determinate of penetration, can be seen with modern, homogeneous projectiles. Many of these projectiles are made from brass or copper and, as a result, they are much lighter, for calibre, than their jacketed/lead core cousins. Accordingly, these homogeneous projectiles, by definition, must have lower SDs than convention bullets, yet they will generally penetrate much deeper than conventional bullets. The difference in penetration is due to the design and construction, of these bullets, and not because of their SDs.

Penetration with Solids or FMJ Projectiles:

When you are talking about solid or FMJ projectiles, then a totally different set of parameters will determine the depth of penetration. If a solid, or FMJ, projectile has been correctly built, then expansion is not a factor in determining penetration. Indeed, these projectiles are designed not to expand. What is now critical is whether these projectiles will penetrate, nose first and in a straight line towards their intended target (heart/lungs or brain), or whether they will tumble and deviate from the ideal path. There is little value in a solid or FMJ if, on penetration, it deviates from the line of sight and misses the vital organs completely. Any FMJ or solid that tumbles, and then deforms or bends, will not penetrate as far as it should nor in the direction that it should.

So how do we ensure that our FMJ/solid projectiles will penetrate deeply, nose first, and in a straight line? We could re-barrel our big game rifles with faster twist rifled barrels. This might reduce the instability of our projectiles, after impact, but it cannot fully compensate and correct all of the instability. It is simply not possible to use a twist rate that is fast enough to eliminate all of the instability of our projectiles when penetrating living tissue. By using faster twist barrels, we can only hope to slightly reduce the instability of our FMJ projectiles.

As short projectiles require slower twist rates, in both air and water, we could also opt to use shorter FMJ projectiles in our faster twist barrels. The use of shorter projectiles would further reduce instability. However, even if used with faster twist barrels, these two measures would not eliminate terminal instability and we could only hope to reduce it slightly.  Note that the use of shorter projectiles, most likely with lower SDs, is totally contrary to the myth that says that only high SDs will generate the best penetration!

The other option, to reduce terminal instability and this is the most effective and practical, is to optimise the design of the projectile for penetration and not for ballistic performance. Considering that deep penetration is only really important when hunting the largest game, and that most large game hunting is conducted at relatively short ranges, this trade off is acceptable.  There are a number of designs for improved solid or FMJ projectiles, such as the ‘Super-penetrator’ or Woodleigh ‘Hydrostatic’ designs. These styles of projectile have some common features, such as parallel sides and a flat base. The main differences are in the shape of the nose which can be flat, semi-wadcutter like with a large meplat, in the case of the super penetrator, or concave in shape as with the Hydrostatics. Such projectiles should also be adequately constructed to ensure the minimum amount of deformation and, today, that means that they are most likely to be homogenous solids.

I won’t go into the science here, as it requires quite a bit of explanation and you can find excellent explanations of super-penetrators and hydrostatics at: The Perfect Big Game Solid. However, suffice to say, that the nose shape, and parallel sides, creates a cavitation bubble around the projectile so that only a portion of the nose of the projectile is in contact with the tissue and the rest of the projectile is shielded by an air bubble, which is the cavitation bubble, from the tissue through which it is penetrating. The effect is that drag is substantially decreased and the projectile will penetrate deeper than conventional designs and usually in a much straighter line.

Another benefit, of the creation of this cavitation bubble, is that the projectile will generate a much larger temporary wound channel and corresponding permanent organ damage than would be generated by the projectile alone. Clearly, this enhances the lethality of these projectiles.

As previously stated, after impact, any projectile, regardless of its design, is still going to be very unstable because the spin rate is way too low for passage through the denser medium of flesh. As most of the projectile is now travelling inside the cavitation bubble, the required spin rate is not as high as required by a conventional solid, but it is still way higher than that required for the passage through air. Accordingly, and because of this instability, the projectile will still tend to yaw as it penetrates. However, the pressure of the animal tissue, on the large flat nose, will normally reduce this yawing and, in most cases, keep the projectile travelling nose first.

It must be noted that this yawing, during penetration, can still cause the bullet to tumble if another impulse acts on the bullet, and this could be clipping a bone or striking other hard tissue.  Indeed, a long penetrator is more likely to experience more drag, and a greater risk of tumbling, than a shorter projectile because the longer projectile will require a faster spin rate. Therefore, the amount of yawing is quite likely to be greater with the longer, less stable projectile. This increased yawing of the projectile must also increase the drag on the nose of the projectile and so reduce penetration depth. Of particular relevance, to this article, is the fact that a long penetrator with a high SD may penetrate less, and be more likely to tumble, than a shorter one with a lower SD. So much for the myth that high SDs produces deeper penetration with FMJ or solid projectiles!

Conclusion:

The penetration, achieved by a hunting bullet, is determined by a number of parameters and most notably bullet spin, shape, construction and impact velocity. The myth that high SD values indicate deep penetration needs to be realised for being just that; a myth. A high SD value would suggest that the projectile will have, and retain, lots of momentum which should aid penetration, but the degree of penetration will be quickly eroded by bullet breakup, expansion, deformation or tumbling.  A high SD value is only valid, for deep penetration, if the projectile is of the correct shape and construction and only if it impacts within a given range of velocities. You cannot simply look at the SD value and conclude how well a given projectile will, or will not, penetrate the game that we hunt. To do so, can give the hunter a totally false indication of what game, and under what conditions, he can hunt with a given projectile.

So the next time your read a book, or a magazine article, that condemns a given cartridge for having poor penetration, due to a low SD value, take that comment with a grain of salt and look further for the real reasons that it has poor penetration capabilities. I will bet ‘pounds to peanuts’ that the real reason is design, construction and/or impact velocity of the projectile in question and not the SD.

Further, when you next go shopping for a new projectile, to give maximum penetration with your big game hunting rifle, there are a number of specifications that you need to consider. Of greatest importance is the design and construction of the projectile. Next you need to consider the manufacturer’s recommendations on impact velocities and whether your chosen cartridge will deliver the projectile within those recommendations. Don’t forget to consider your rifle’s twist rate because you can buy the best penetrator available but it won’t be worth a damn if your barrel won’t stabilize it! However, the one specification that is of the least value, to your selection of the best penetrating projectile for your hunting, is the sectional density (SD) of the projectile.

Remember, SD is a parameter of external ballistics and NOT terminal ballistics.

List of references and links for further reading:

• The Perfect Big Game Solid,
• More Stable Penetration with a New Solid Bullets,
• Shooting Holes in Wounding Theories,
• GS Custom FN Solids – The Ultimate Solid, and
• African Dangerous Game Cartridges by Pierre van der Walt – Understanding Sectional Density on Page 77.

Notes:

1.    To be fair to the Germans, it should also be noted that not all British projectiles were ideal, although some were quite good. In addition, American projectiles of the time were not well suited for hunting the largest game, either. As an example, you only have to read ‘African Game Trails’, by Theodore Roosevelt, to get a feel for the shortcomings of American .405 Winchester projectiles on larger game. Similar comments can be found in the writings of Elmer Keith when discussing American .405 projectiles on American game. Yet, when you read about the performance of British .405 Winchester projectiles, a totally different picture emerges. Incidentally, John Taylor was quite negative about the .405 Winchester, in much the same way as he was with the 10.75×68, and he displayed the same bias with the American cartridge as he did with many German cartridges. Coincidence or a case of NIH (Not Invented Here) thinking?