Is the Sectional Density of an Expanding Hunting Bullet a More Important Metric than Its Weight When Evaluating Potential Terminal Performance?

By Scott Fletcher

“Any metric is just a factoid unless it contributes to understanding and solving a problem”. – Scott Fletcher

The answer to the question posed by the article’s title is “no”.

Sectional density is an empirical terminal performance metric that is intended to indicate a bullet’s relative ability to penetrate based on its weight compared to other bullets of the same generic design and diameter (caliber) when fired from the same chambering into the same media (animal tissue, wet newspapers, gel, etc.). A greater sectional density value indicates a potentially greater penetration. The inferred basis for this penetration is the bullet’s momentum, which is simply a bullet’s mass multiplied by its impact velocity.

Photo 1 shows 375-caliber bullets of various generic designs and weights that have been used in my 375 H&H (case shown). Left to right: a 235-grain Barnes TSX, a 270-grain Speer jacketed soft point, a 300-grain Sierra Game King, a 350-grain Woodleigh Weldcore protected point, and a 350-grain Sierra Match King with a hand-installed poly tip. These bullets will be used to determine if a bullet’s sectional density is more important than its weight when evaluating potential terminal performance. The evaluation presented in this article is applicable for any expanding hunting bullet caliber (diameter) fired from any chambering.

The actual sectional density value is determined by taking a bullet’s weight, in pounds, and dividing it by its diameter, in inches, squared. For example, the sectional density of the 235 grainer is SD = (235/7000) / (.375)2 = .239. The “235/7000” term converts grains to pounds. Likewise, the sectional density of the 270 grainer is .274; the 300 grainer is .305; and both 350 grainers are .356. Based on these sectional density values, both 350 grainers are inferred to penetrate farther than any of the other bullets.

The basis for the sectional density calculation is empirical rather than scientific. The bullet’s weight, rather than its mass, is used. (Weight is not the same as mass when the unit used is pounds…..don’t get me started!) Furthermore, the bullet’s cross-sectional area is assumed to be square rather than round. The resulting sectional density number represents a weight within a fictitious area with no height. Since there is no height, there is no basis for a volume, and the resulting number does not represent a true density.

The “lack of science” associated with any empirically-calculated metric value, including sectional density, is not a fatal-flaw issue. The only requirement is that the calculated value should consistently/reliably predict an outcome that can be verified or measured. If all the bullets in Photo 1 were the same generic design and fabricated with materials of the same metallurgy, testing in the same media would likely indicate a progressively greater penetration length with progressively increasing weight. In that circumstance, the bullet’s sectional density would likely show a measurable outcome consistent with its value, with bullets having a progressively increasing sectional density likely producing a progressively increasing penetration.

Prior to 1948, all expanding hunting bullets were essentially the same generic design and metallurgy, configured primarily as a spitzer or flat-base, semi-point or roundnose. As a consequence, using sectional density as an evaluation metric had reasonable merit to empirically assess an expanding hunting bullet’s qualitative penetration length.

However, each bullet in Photo 1 is significantly different even though they are the same diameter. There are three generic designs represented by the five bullets. All have different tip configurations which affect how rapidly each bullet will expand. The rate of expansion significantly affects penetration, with bullets exhibiting rapid initial expansion typically penetrating significantly less. All have components with different metallurgy, with bullets comprised of harder, stiffer material expanding slower with resulting increased penetration. Because of these differences, confirming the intended relationship between sectional density and penetration length would be impossible, even if these bullets were shot into the same medium with the same chambering.

Discussing each bullet’s generic design, specific design features, and metallurgy with their likely effect on its penetration and other terminal performance characteristics is well beyond the scope of this article. However, both the 350-grain Woodleigh and the 350-grain Sierra shown in Photo 1 have the highest sectional density of .356, potentially indicating similar as well as superior penetration lengths compared to the other bullets. If these two bullets were gel tested, the Sierra match bullet would likely penetrate way less because of the “soft” metallurgy of both its jacket and core, its “thin”, uniform-thickness jacket, the plane of weakness between its jacket and core, and its tip configuration that is intended to initiate rapid initial expansion. The field consequence of these combined design and material features is the Sierra can be expected to have marginal-to-poor penetration through the shoulder of a heavy dangerous-game animal, such as a Cape buffalo, on a broadside shot. The result could potentially be only partial penetration through the thoracic cavity, with limited/poor wound cavity volume in the animal’s lungs and heart.

An in-depth evaluation of the varying generic designs, component metallurgy, and tip configurations of the bullets shown in Photo 1 would indicate that the 235-grain Barnes TSX, the bullet with the lowest sectional density, would likely out-penetrate all the other bullets by a significant margin regardless of any test or field circumstances. This likely performance reality demonstrates that qualitatively evaluating a bullet’s likely penetration based simply on its sectional density magnitude can result in misleading-to-wrong expectations of actual performance given today’s multiple generic bullet designs and materials available for any given caliber (diameter). Bottom line: an expanding hunting bullet’s sectional density is a poor-to-ineffective metric for evaluating the likely penetration performance of bullets with different generic designs in the same caliber.

An expanding hunting bullet’s actual weight is a more important metric for evaluating its effectiveness and potential terminal performance, regardless of its generic design. As discussed in the three-part article [SF1] on hunting cartridge selection, the game weight of a non-dangerous animal the bullet is capable of expeditiously harvesting can be estimated with a simple empirical equation using its weight, independent of its generic design. Once a bullet with a weight that is compatible with the weight of the animal to be hunted is determined, its generic design can then be selected to best accommodate specific hunt circumstances.

Using bullets with greater weight will result in a greater estimated game weight for any selected chambering and caliber (diameter), provided the impact velocity at the expected shot distance is within the “sweet-spot” velocity range of the selected bullet’s generic design.  Furthermore, both gel testing and skinning-shed autopsy data in the 2023 management hunt report indicate that a bullet’s weight can be thought of as a shrapnel reservoir. More bullet weight provides a greater volume of potential shrapnel that not only increases the wound cavity volume, but enhances rapid bleed-out due to the wound tributaries it creates.

As explained in this article, qualitatively evaluating an expanding hunting bullet’s relative penetration potential based on its sectional density without regard for its generic design can result in unrealistic/erroneous expectations of its actual performance. Furthermore, such an evaluation obscures the fundamentally more important concept that “more bullet weight is likely better”, independent of its diameter and generic design. “Heavy” bullets are the basis for Africa “tried-and-true” field terminal performance regardless of chambering, caliber (diameter), and species, even when using currently-denigrated cup-and-core bullets.

The field-performance reality of “tried and true” using “heavy” bullets                   is still relevant today with modern magnum chamberings and multiple generic bullet designs, as demonstrated with data in the 2023 management hunt report. As presented in sections 8.0 and 9.0 of this report, the bullet’s weight, rather than its sectional density, is a necessary ingredient in the terminal performance metric mix needed to evaluate its field merit in achieving a recovered animal. 

 [SF1]