Project X - Testing Bullet Penetration
Does SD aid Penetration?
By Chris Bekker

Sectional density has become a hotly debated subject in recent years since some stakeholders of ‘light-for-calibre’ monolithic bullets claim that sectional density is totally irrelevant. The notion is based on the premise that bullet weightloss is negligible and so the monolithic bullet can afford to be lighter in weight. In addition, the lighter bullet can be launched faster and with a flatter trajectory and as a bonus offer less meat bruising than frangible conventional bullets. Whilst this is a virtue as far as long-range shooting is concerned, does it negate the benefit of a high-SD bullet in terminal ballistics? What fuels the controversy is the fact that both low-SD and High-SD bullets can kill but the argument is about which is the most effective. Also remember that some monolithic bullets are just as heavy as their traditional counterparts in soft nosed versions. The question can rightly be asked … Why?

Hunters from the ‘old school’ still prefer a ‘heavy-for-calibre’ bullet (high SD) at moderate velocity where deeper penetration is needed. It seems that the ‘old school’ and the ‘new school’ are still at odds with each other today. Those that opt for non-expanding monolithics focus on extreme penetration. Those that opt for expanding monolithics focus on a compromise between good penetration and a larger hole through the vitals. Those that opt for expanding bullets focus primarily on a larger wound channel but at least with penetration through the vitals. Maximum penetration is not the be all and end all of a bullet’s lethality as the size of the ‘hole’ needs to be considered as well. An effective bullet has to make as wide a wound channel as is possible through the vitals as this facilitates rapid blood loss. Wound channel size is increased by bullet size (calibre), bullet shape and expansion. A round nosed bullet (non-deforming) tends to push flesh aside while a flat faced one crushes what is ahead of it, creating a more open channel. Similarly, a controlled expansion bullet with four even and sharp petals cuts and severs an even more effective wound tract. 

The key to this debate is how light can we go on the bullet (its sectional density) and can velocity make it up? We need to distinguish clearly between monolithic bullets that essentially keep their weight, whilst conventional soft nosed bullets lose considerable weight on impact or even shatter if the impact velocity exceeds the threshold strength of the bullet. Some brands of monolithic bullets are designed to lose the front part of their noses at a certain velocity and then continue in a cylindrical form. Locally we have two brands of monolithic bullets that are ‘light-for-calibre’ – one that lose weight on impact (GS-HV) and another that retains all its weight (Impala) on impact. Unlike the smooth surface of the GS-HV, the Impala Bullet provides a calibre-sized sharp cutting edge, similar as can be found on heavy dual diameter solids. ‘Solids’ of the same weight as ‘softs’ will generally penetrate three times deeper as expansion of diameter is the most inhibiting factor in penetration mechanics. 

Bullet construction also differs across the range of soft-nosed brands that are available today - example, partition bullets (Nosler Partition), bond core bullets (Claw), partitioned and bonded (Swift A-Frame) and solid shank bullets (Rhino). Clearly they will behave differently on impact at varying velocities. I refer to this as the ‘performance band’ of the bullet. The point I am making is that even a frangible bullet can perform well, once it has slowed down sufficiently before impact. On the other hand, some bond core bullets do over-expand at a too high velocity and the bullet’s high sectional density will not come to your aid in ensuring adequate penetration. Bullet construction is another key variable in bullet performance and only through testing can we define performance band limits for various bullet types.

Sectional density is defined as: SD = W/ D x D (Where, the weight (W) is specified in pounds and the diameter (D) in inches.)

Example of sectional density calculation in a .284 calibre bullet:

SD = [ 175 gr / (7000 gr/lb) ] / [ 0.284 in ] 2 = 0.310 lb/in2
SD = [ 142 gr / (7000 gr/lb) ] / [ 0..284 in ] 2 = 0.252 lb/in2
SD = [ 108 gr / (7000 gr/lb) ] / [ 0..284 in ] 2 = 0.191 lb/in2

To summarize: SD is the dimensional property of the bullet or the ratio between its weight and its frontal area. A higher SD factor means that we put more weight behind a given cross sectional area (XSA). With more weight behind a specific frontal area, it stands to reason that penetration will be deeper before I proceed further, I wish to point out the mathematical relationship between SD and XSA. Please note the similarity when one multiplies with SD and divides with XSA. 

SD = Weight of bullet / D x D  ,,,  XSA = D x D / 1 (inversely related)

High SD vs Low SD: With regard to conventional bullets, the question of sectional density is of particular importance. Proponents of high SD bullets believe the heavier bullet is better placed to lose some weight than a lighter one as it ploughs through an animal. Its overall retained weight will thus be higher. This condition is also aided by the lower velocity, at which heavier bullets are shot at. Lighter bullets driven at the highest velocity stand a good chance to over expand or shatter on impact with resultant shallow penetration. Losing weight means the bullet is also losing its momentum (driving force). So it stands to reason that a heavier bullet (higher SD) will generally perform much better on game. That is why the 7 mm Mauser performs better with heavy-for-calibre bullets than light-for-calibre bullets. Lung shots at close range when the velocity is still high, may cause dramatic deaths as the bullet explodes, but is risky at angled shots with the vitals being further away and to crush heavy bone that is likely to obscure the vitals. For varminting purposes, where we need immediate bullet blow-up, these bullets come to their right, but they are no longer reliable hunting bullets for game at present day velocities.

The time has come to test the new theory of those pundits that say SD does not matter. They claim that SD is actually irrelevant and illogical. As bullet mass is inherently part of the SD parameter, this is quite a serious indictment to make. Lets see what my experiment reveals. To minimize the effect of frangible bullets that lose their weight, I decided to use strongly constructed expanding monolithic bullets which essentially retain all their weight. I opted for the Barnes-X bullet that expands to a very even and beautiful mushroom across a velocity range of 2,000 fps to 2,600 fps in wet packs. Around 2,700 fps it will lose petals and penetration dynamics will then change - so I decided to avoid being too close to this threshold limit. I used 175-gr 7 mm bullets (.284”), and cut 6 mm off the base each time so I could end up with a lot of 175 grainers, 142 grainers and 108 grainers. I actually got a toolmaker, Hubert Saayman, to do this for me on his lathe. This represents nice even jumps in lowering the SD. I was guided by Somchem’s advice and loaded the 175-gr bullet with 38 grains of S365. For each lower weight I increased the charge with 2.5 grains. For each lower weight I seated the bullet 2 mm deeper so that I could still achieve sufficient purchase on the shortest bullet. This was done to progressively increase the velocity to make up for the loss of bullet weight or SD.

I chronographed my loads and was happy with the fact that I was not losing any petals on the lightest and fastest bullet. I suppose I could have used solid non expanding bullets, but I had none and I would have needed a bigger wet pack. Also I could have increased velocities further on the two lighter bullets by changing to a faster burning propellant, but I considered that a moot exercise as expansion/fragmentation dynamics would have been different and that is precisely what I was trying to avoid. Nevertheless, the end result was almost an even jump of 100 fps and 120 fps. I would have liked to have had a fourth load, the 108 grainer at 3,000 fps, just as a benchmark.

As SD is a static ratio of a bullet’s weight to its frontal area, it is not the full story as the bullet still needs to be put into motion by a force before it could exhibit penetration characteristics. Motion of an object has two elements, kinetic energy and momentum. I prefer momentum as it gives equal importance to weight and velocity, whereas the energy formula uses velocity to the power of two and in so doing over emphasizes the importance of velocity. Visualize momentum as being a runaway freight train coming at you. Various ballistic experts accept SD as a key driver of penetration mechanics. Even if the lightest bullet in our experiment could be driven at 3,000 fps, it would still have fallen short on momentum value, which is an indication of a bullet’s bone crushing ability to me. As mentioned before, at an impact velocity of 3,000 fps, the Barnes-X bullet would have shed all its petals in the first few inches, losing weight and thus momentum in the process; clearly an undesirable condition.

Penetration mechanics are very complex and quite a few mathematical models exist - all of which are basically too complex for those without a degree in mathematics. Another complicating factor is the fact that penetration dynamics vary in different media, for example steel, concrete, wood, body armour, water, flesh and bone. I do not profess that a wet pack is the perfect medium, but it does provide a relative comparison in that medium. To watch water cans explode when shot with high velocity bullets also prove nothing. Whilst the body consists largely of water, the cell structure of living tissue is compartmentalized and more elastic (elasticity differ for various body parts and organs) than a homogenous medium such as water. Dr. Norbert Hansen proved to us that the design of a bullet’s nose can also have a marked effect on penetration in water. He provides his bullet design with a sharp edged flat steel insert at the tip, about 60% of bore size, to lessen the resistance.

In artillery warfare, a common formula in use (as a basic reference) is to look at the ‘Momentum Density’ parameter - this is used in the context of armour piercing bullets. Momentum density indicates the force (momentum) over the area (XSA) that is to be penetrated. The logic being that when a greater force is applied to the same area (XSA), it will penetrate deeper. I calculated the ‘Momentum Density’ figures and expressed them in index form, so I can readily judge correlation with my actual penetration results. As XSA is related to SD, as shown previously, and the momentum value is now engaged in the momentum density formula, the indicative value is enhanced. Let us now look at the actual results that were obtained by shooting the various bullets into a wet pack at 25 yards. Impact velocity will naturally be slightly lower than chronographed velocities at 5 yards, which has been stated here as being muzzle velocities (close enough).

Pieter Olivier witnessed me doing the shooting and helped me to measure the penetration. Two shots of each weight were fired to prove the repeatability of the test. As expected, the lighter and faster bullets, penetrated progressively shallower - roughly 10 cm each time. The increased velocity was not able to make up the lost ground of SD. Bear in mind that all bullets retained 100% of their original weight and expansion is almost identical. The retrieved bullets are testimony to its sterling performance. The only parameters that varied were SD and momentum values. To assess or rank calibres, or different bullet/load combinations, I use the product of these two factors to give me a basic indication of where I stand. Bekker’s KOV = Mo x SD. By multiplying with SD a similar result is achieved as by dividing by XSA, save that SD has a weight component in it. For clarity sake, I will just state the algebraic notation:

Anyway, let us stack the prime indicators against the actual results and judge the correlation.

My conclusion is that sectional density generally assists with deeper penetration, all else being equal. There are other important considerations as well such as the construction of the bullet, the materials that it is made from, the shape of the bullet’s nose and the medium it is being fired into. One does not have to be a statistician to see the highly positive correlation between the three parameters. When the SD correlation is so high, it stands to reason that all the all other variables mentioned must play a much smaller role. SD is a vital parameter, just as we have come to experience over the last hundred years, and in soft nosed bullets it is even more important for the reasons mentioned earlier.

Use SD wisely and not stupidly. SD comparisons should preferably be confined to the same type of bullet and calibre as it is a ratio and thus across the range comparisons of calibres should be made with caution as bullet weights can differ drastically. Furthermore, SD must be coupled with the bullet’s momentum for a more complete picture. Bear in mind that my tests were done in a wet pack and it may differ in flesh and bone, and in addition, shots on game are seldom alike. However, the ultimate test remains in game as that is our application. The parameters according to Lutz Möller are - pull strength for flesh is 40 kp/cm² and 1700 kp/cm² for bone, whilst density for flesh is1 g/cm³ and 2 g/cm³ for bone.

Let us now look at a cross-calibre comparison. A friend of mine, Brand van Deventer, told me that his father shot lion and buffalo over a long hunting career, and was adamant that a 220 grain FMJ bullet fired by a 30-06 Spr penetrated better than a 300 grain FMJ bullet fired by a 375 H&H. He used both calibres on buffalo, and while the 30-06 would penetrate a buff from north to south, he never witnessed this with the 375. Considering that the 30-06 is slower by a 100 fps from the word go, most will argue or find it hard to believe. Brand’s old man had to shoot many buffalo in a follow up after being wounded by 375-hunters and his famous saying was that if you want to kill it, use a 30-06. Well let’s get mechanical about it and see if the wise old man’s experience is born out by science:

The table shows that the momentum density of the high-SD bullet (.331) out-penetrates the low-SD bullet (.305) of the 375 H&H. I just remembered that a certain Mr. R.T. Halverson conducted extensive tests to demonstrate the value of higher sectional densities in penetration tests, in particular the fact that a 220-gr bullet (SD=.331) in his 30.06 Spr out-penetrated the much more powerful 375 H&H with 300-gr bullets (SD=.305). Little wonder that so many elephants fell to head shots by Parks Board wardens all over Africa. In fact, Mr. Bell did it with the minimalist 7 mm Mauser and 704 lb/ft-sec per square inch. It should be considered a bare minimum - definitely too low for my comfort as I would prefer a bigger margin of safety.

We should recognize that low momentum bullets will get stopped far sooner the moment we use expanding bullets spreading the force over a bigger area of resistance. Also, non-expanding bullets need less momentum to be driven to the same depth as an expanding bullet. We need to balance the driving force (momentum) with the expanded area (XSA) to achieve adequate penetration in relation to our quarry - be it impala, kudu, eland or buffalo. For elephant hunting, where extreme penetration is needed, expanding bullets are simply not an option and is the specialized terrain for the bigger bores.

To perpetuate the notion that SD is totally irrelevant appears to me as being totally off the wall, as for SD to be totally irrelevant, the correlation co-efficient aught to be way less than fifty percent. The fact that low-SD bullets have killed many a buck (soft-skinned antelope) may show the adequacy of the bullets used, but certainly not its superiority over high-SD bullets. For an opinion to have any real value it must be based on some empirical evidence and in the case of hunting it refers to testing on live animals, albeit subjective. Once you have shot enough of a particular animal with different bullets the facts will stare you in the face.

In part 2 of this article I will be sharing with you what happens at around 3,000 fps impact velocity. I received a few bullets from Lutz Möller in Germany for testing. His design is a close copy of the GS-HV bullet and the nose section of the bullet also breaks off to a square cylinder in just a few centimetres, changing the penetration dynamics as resistance of a full mushroom falls away and continues its journey as a solid. The comparison with the Impala Bullet should also be an interesting one.