Bullet Penetration, Part 2: How to Empirically Predict an Expanding Hunting Bullet’s Field-Penetration Length Through a Big-Game Animal.

By Scott Fletcher

“You can’t always get what you want. But if you try, sometimes, well, you might find you get what you need”.  -  Lyric to the song “You Can’t Always Get What You Want” by Michael Jagger, Mick Jagger, and Keith Richards.

Part 1 presented data that show an expanding hunting bullet’s test metrics of weight loss, mushroom diameter, and impact energy likely cannot be used to predict its field-penetration length through a big-game animal. This article presents a testing-based empirical method for numerically predicting the likely field-penetration length of an expanding hunting bullet, as justified by field results obtained on the 2023 zebra management hunt. This field penetration length prediction can be used in conjunction with a numerically defined penetration-length goal to identify if any tested bullet will likely have “enough” field penetration to breach any animal’s rib cage.

Neglecting the brain, an animal’s vital organs are contained within its thoracic cavity (rib cage). An expanding hunting bullet’s primary job is to penetrate into and through the animal’s rib cage to catastrophically debilitate/destroy these organs to produce a quick death.      

Photo 1 shows the limits of a zebra’s rib cage after its organs have been removed. The dowel shows the bullet’s left-to-right penetration path from a slight front-quartering shot. The holes show that the rib cage was completely breached by the bullet. Photo 2 shows the bullet hole through the zebra’s heart, indicating both the nearside and farside lung had also been completely penetrated by the bullet.

An expanding hunting bullet passing through vital organs and exiting the rib cage will either be retained by the farside hide or completely exit the animal. Photo 1 and Photo 2 indicate the bullet would have satisfactorily done its primary job of destroying vital organs even if it had only been retained by the farside hide. Such penetration, as indicated by a bullet being retained by the farside hide, is considered effective because the intended vital organs were completely breached.

The broadside rib-cage width of most big-game animals can be determined with online AI assistance. This rib-cage width can serve as the hunt’s field-penetration length goal that must be achieved by the bullet. The intent of any pre-hunt bullet testing should be to determine if the test-penetration length at least matches or preferably exceeds the broadside rib-cage width of the intended big-game animal at a typical or average shot distance. For non-dangerous game, such testing will determine if any candidate cartridge-bullet combination likely has “enough” penetration to satisfactorily breach the rib-cage and destroy the intended vital organs.

As an example, online AI research determined the thoracic cavity width of a Burchell’s zebra ranges from about 14 to 18 inches, indicating that a field-penetration length goal of 18 inches would be reasonable. On five prior hunts in South Africa, shots on plains game had averaged about 135 yards. Testing in 20% synthetic gel at this distance indicated cartridge-bullet combinations considered for use on the hunt resulted in penetration lengths ranging from 19 to 23 inches, all greater than the 18-inch field-penetration goal.

Gel-test and field data presented in the 2023 zebra management hunt report indicate the Guppy-metric value of L(T), as identified on the Guppy model and defined in Guppy-Tech, can empirically predict an expanding hunting bullet’s effective field-penetration length. Effective field penetration is defined as a bullet’s complete penetration through vital organs within the animal’s rib cage, as typically demonstrated by its retention on the farside hide. L(T) does NOT predict a penetration length that exits the animal because there is literally no measurable total field-penetration length to compare to the test length.

The empirical relationship between L(T) and an effective field-penetration length appears to be valid provided:

1) The testing is performed only in 20% synthetic gel. This gel has greater resistance to penetration compared to either FBI ordinance gel or 10% synthetic gel. This resistance to penetration is judged to be greater than the resistance of animal tissue, and appears to reasonably and conservatively model (simulate) the combined penetration through hide, tissue, and limited combinations of bone. (See Note 5, below.) Testing in either FBI ordinance gel or 10% synthetic gel would likely result in L(T) test-penetration lengths greater than those obtained in 20% synthetic gel, indicating effective field-penetration lengths far greater than those that will actually be obtained.

2) The empirical equation found here must demonstrate the selected cartridge-bullet combination is satisfactory for taking the weight of the animal to be hunted. The zebra management hunt data indicate the bullet weight determined by this equation likely has an adequate reserve of sacrificial weight that enables the bullet to maintain satisfactory momentum, typically making bullet “weight-loss” during penetration essentially a non-issue.

3) The bullet’s test impact velocity is representative of its likely field impact velocity, and all expected field impact velocities fall within the selected bullet’s sweet spot impact-velocity range, as indicated by its generic design or specified by its manufacturer. (Refer to report sections 9.0 on p. 21, 11.2 on p. 27, 12.1 on p. 44, the articles found here and here, and eBook Chapter 13, Chapter 14, Chapter 15, Chapter 16, and Chapter 17.)

4) Only broadside and quartering shots are taken. However, the field-penetration lengths obtained on the 2023 zebra management hunt indicate an expanding hunting bullet with a satisfactory effective penetration for a broadside shot likely has sufficient penetration to reach and breach vital organs on a full-frontal shot.

5) Only one nearside and one farside rib are breached. Although effective penetration was achieved on the 2023 zebra management hunt after the nearside scapula and multiple rib bones were breached, a progressively increasing number of bones breached should be expected to produce a progressively decreasing field-penetration length. Breaching the nearside shoulder joint or the spine should be expected to produce a dramatic decrease in field penetration, to the extent that the bullet may not penetrate the entire rib cage.

The previous notes underscore that the interpreted relationship between L(T) and any resulting field-penetration length is not absolute, but is empirical and therefore approximate. However, management hunt data indicate adherence to the caveats in these notes will likely result in an L(T) test value that appears to be a reasonably conservative predictor of a field- penetration length, sometimes accommodating bone breaching more significant than just a nearside and farside rib.

For example, Photo 3 indicates a 200 WWC fired from a 300 Win penetrated two nearside ribs, then penetrated through the spine by physically removing about a 2-1/2-inch wide by ½-inch deep segment, then penetrated through one farside rib before being retained by the farside hide. In Photo 4, a 220 SPH fired from a 300 Win also penetrated two nearside ribs, then penetrated through the spine by physically removing about a 3/8-inch wide by 3/8-inch deep segment, then penetrated through one farside rib before being retained by the farside hide. Furthermore, compilation of penetration data for all bullets used on the management hunt indicates the bullets used completely exited the animals on six of eleven shots (55%), including front and rear quartering.

Evaluation of data contained on Table 5, Table 6, and Table 7 of the 2023 management hunt report indicates the empirical relationship between L(T) and the bullet’s field-penetration length does not depend on either the test-bullet’s or the field-bullet’s retained mushroom diameter, weight loss, and impact energy. As a consequence, L(T) values of any cartridge-bullet combination obtained from testing in 20% synthetic gel at the expected hunt distance can be directly compared to identify which one will likely produce “enough” penetration to achieve the designated effective-penetration length goal.

As discussed in this article, testing an expanding hunting bullet in 20% synthetic gel allows empirically predicting an expanding hunting bullet’s field-penetration length based on its Guppy-metric value of L(T). The added benefit of this testing is that an empirical prediction of field wound volume can also be obtained by determining the Guppy-metric V(ST), as discussed here, and an empirical prediction of the degree of meat damage can also be obtained by determining the Guppy-metric I(V), as discussed here. These metric values allow any cartridge-bullet combination, tested in 20% synthetic gel    at the same distance, to be numerically evaluated to identify which combination of test-metric values will likely produce the total field performance that meets each hunter’s specific hunt objectives.