Bullet Penetration, Part 1: Can an Expanding Hunting Bullet’s Typical Test Metrics Be Used to Accurately Predict Its Field-Penetration Length?

By Scott Fletcher

“Measure what is important, don’t make important what you can measure”.  -  Robert McNamara

Bullet penetration. Hunters in online reviews and media authors in their content indicate all can be obsessed about an expanding hunting bullet’s ability to penetrate into and through a big-game animal’s vital organs. Assured field penetration can be the primary criterion for selecting an expanding hunting bullet, with its ability to create a large wound volume while preserving edible meat sometimes given little-to-no consideration. These reviews and content indicate a penetration obsession can sometimes justifiably morph into concerns approaching paranoia if the bullet will be used to take heavy dangerous game.

This two-part article discusses pertinent issues about an expanding hunting bullet’s penetration capabilities using published test data, personal gel-test data, and field data obtained on the 2023 zebra management hunt. Part 1 discusses if an expanding hunting bullet’s ballistic and test-media data that are commonly available can predict its field-penetration length through a big-game animal. Part 2 presents a field-penetration goal through any big-game animal that can be numerically defined, and discusses a Guppy-metric value, obtained from testing in 20% synthetic gel, that can empirically predict the field-penetration length of an expanding hunting bullet. Part 2 also presents a testing strategy to determine if any cartridge-bullet combination likely has “enough” penetration to meet the field-penetration goal.

All manufacturers use qualitative adjectives like “reliable” and “deep” to describe the penetration capability of their expanding hunting bullets. Some have slo-mo videos of these bullets penetrating completely through gel blocks of typically unspecified composition and dimensions, causing it to do the “shimmy-shake-and-quake”. The implication is that such a visual depiction, including a pass-through penetration, is representative of what will occur when the same bullet is used on any big-game animal. However, manufacturers typically do not identify the properties of the media used for any testing, nor the penetration lengths obtained by their expanding hunting bullets.

If a manufacturer-tested bullet has been retained in media, its mushroom diameter and retained weight data can be shown on a photo, sometimes referenced to impact velocity or impact energy. A photo sequence can sometimes show how the mushroom diameter and retained weigh vary with impact velocity, but there is typically no indication that the corresponding test-penetration length also varied. The implication of these photos is the furnished impact velocity (or energy), mushroom diameter, and retained-weight data are representative of likely field values and are potentially sufficient for predicting a field-penetration length.

“Wildcat” penetration testing done by hunters is testimony to both the lack of manufacturer-furnished penetration-length data and no explanation of how (or if) a field-penetration length for their expanding hunting bullets can be predicted based on their test metrics. Common test media used by “the independents” have included paper (e.g. newsprint or catalogues, either wet or dry), lumber, sand, water-filled plastic jugs, and gels of various compositions. The selected test distance may or may not be the typical or the average shot distance expected on a hunt. In addition to the actual penetration lengths obtained, typical published supporting test data also mimic the manufacturer-furnished data of bullet impact energy or impact velocity, the retained mushroom diameter, and either retained weight or weight loss.  

The primary unspoken assumption about the applicability of these wildcat- test results appears to be the selected test media offer the same or better resistance to the penetration that will be encountered by the bullet as it passes though hide, tissue, and bone on its way through the animal’s thoracic cavity (rib cage). Although the primary objective of the test is to determine a penetration length, there is typically no discussion of how (or if) the test-bullet’s mushroom diameter, weight loss, and impact energy affected its penetration value. Furthermore, there is typically no test-based guidance offered by the independent testers for how much penetration is “enough” to completely pass through vital organs in any big-game animal’s rib cage.

In the engineering world, a test is considered relevant only if its resulting value either confirms or provides a parameter necessary for correctly predicting a specified design goal. Can an expanding hunting bullet’s test metrics of mushroom diameter, weight loss, and impact energy typically furnished by a manufacturer or a wildcat tester accurately predict its field-penetration length? The following real-world example provides the basis for answering this question.

Photo 1 presents retained bullet configurations and test data for both a 30-caliber, 200-grain Woodleigh Weldcore (200 WWC) and a 30-caliber, 220-grain Sierra Pro Hunter (220 SPH). Both were fired from a 300 Winchester magnum (300 Win) into 20% synthetic gel blocks placed at 135 yards. Animal hide about 0.05 inches thick was draped over the front face of the first gel block. No actual bone or material that simulates bone was embedded in the gel.

The photo presents test data arranged in an evaluation-priority sequence likely used by most hunters to interpret and judge the tested bullets’ performance:

-Impact Energy: “Energy” in any guise is typically a mandatory test value, with “more” being universally recognized as “way better”.

- Impact Velocity: Impact velocity is necessary to calculate a bullet’s impact energy. When used in conjunction with a bullet’s ballistic coefficient, it is an input parameter necessary for commonly available software to calculate a bullet’s drop and wind drift, with a greater velocity reducing both. As with impact energy, “more” impact velocity is “way better”.

- Penetration: The bullets tested have to penetrate “a lot”. When testing multiple bullets fired from the same chambering, the one with the most penetration is typically assessed as the “best” in the inevitable bullet-performance contest between (or among) the tested bullets.

- Weight Loss: Bullets that lose appreciable weight can have test-penetration lengths less than bullets that don’t. The presumed reason for the inferior penetration is because the bullet’s momentum was reduced by its weight loss. A bullet assessed to have lost “too much weight” can be judged as having too much risk for field use because of a potentially poor field-penetration length, regardless of the test-penetration length it achieved.

- Mushroom Diameter: The retained mushroom has to be “big” and symmetrically round to indicate the bullet is likely capable of producing good field wounding.  A “small” mushroom diameter on the bullet with the longest penetration can be given a pass if it is symmetrically round and “pretty”.

As indicated on Photo 1, both the impact energy and impact velocity of the 200 WWC are appreciably more than the 220 SPH’s. The 220 SPH lost 40% of its weight, a percentage 40 times greater than that of the 200 WWC’s. The penetration of the 200 WWC is about 19% greater than the 220 SPH’s, a result likely considered “logical” by most hunters based on the values of impact energy, impact velocity, and weight loss obtained from testing. The 200 WWC’s 19% larger mushroom diameter would simply be “icing” on the 200 WWC’s test-results “cake”. In the inevitable contest between these bullets, the 200 WWC would undoubtedly be declared the “winner” and a “way better” bullet. Furthermore, the test data create the expectation that any field values of penetration length, weight loss, and mushroom diameter for a 200 WWC will be both comparable to its test data and superior to any field values of a 220 SPH.

Photo 2 depicts the configuration, selected field data, and retained bullet data for both a 200 WWC and a 220 SPH recovered from zebras shot on the 2023 management hunt. Both bullets were shot from a 300 Win, produced a “drop-to-the-shot” reaction, and were retained by the zebras’ farside hide.

Both the 200 WWC’s field impact energy and impact velocity were (amazingly) identical to its test values. However, it only penetrated 14-1/2 inches on a quartering shot, about 37% less than its test penetration length. Even though its field impact energy is about 32% more than the 220 SPH’s, it failed to out-penetrate it, and did not exit the zebra’s farside hide. The field-recovered 200 WWC lost 52% of its weight, completely unexpected based on its miniscule 1% test-weight loss at the same impact energy. The pretty and round 0.671-diameter mushroom of the test bullet, as shown in Photo 1, has been replaced by a field-recovered bullet’s mushroom that is puny, oval, and ugly. Its approximate “diameter” is 0.397 inches, about 41% less than the test-bullet’s mushroom’s diameter.

The 220 SPH’s field impact energy is about 21% less than its test impact energy. However, its field penetration of 17-1/2 inches on a quartering shot is only about 8% less than its test penetration length. Even though its field impact energy is significantly less than its test impact energy, the field-recovered 220 SPH lost 2% more weight than the test bullet, and the field-recovered mushroom diameter is about 25% more than its test value.

Photo 1 and Photo 2 indicate the field values of bullet weight loss and mushroom diameter are not representative of their test values based on impact energy. The difference between the field and the test values identified in this limited comparative example indicates that reliably predicting field values of weight loss and mushroom diameter based on test values is highly unlikely.  This unlikelihood indicates using these metrics as a basis for predicting field penetration is also highly unlikely. These conclusions are corroborated by similar data obtained from all bullets shot on the 2023 management hunt.

What about using test impact energy to directly predict field penetration length? Impact energy is the fallback, go-to basis for explaining virtually all aspects of a bullet’s terminal performance. If a bullet’s test impact energy predicted its field-penetration length, a reasonable expectation would be that a field impact energy less than its test impact energy should result in a field- penetration length less than its test-penetration length. Conversely, a bullet that produces a field impact energy greater than its test impact energy should have a field-penetration length greater than its test-penetration length.

The limited data shown on Photo 1 and Photo 2 indicate the logical relationship just discussed between an expanding hunting bullet’s test impact energy and its field penetration length does not exist. In the case of the 200 WWC, both its test and field impact energy are the same. If an expanding hunting bullet’s test impact energy was a relevant predictive metric of its field-penetration length, the 200 WWC’s field-penetration length should be the same or at least reasonably comparable to the test-penetration length. However, the 200 WWC’s field-penetration length was only 14-1/2 inches compared to its test-penetration length of 23 inches, about a 37% reduction. A difference of 37% in penetration lengths cannot be considered “reasonably comparable”.

In the case of the 220 SPH, is field impact energy was about 21% less than its test impact energy, potentially indicating its field-penetration length should be on the order of 21% less than its test penetration length of 19 inches. “Doing the math” indicates the predicted field-penetration length should be about 15 inches. The actual field penetration was 17-1/2 inches, within “spittin’ distance” of 15 inches, yet 2-1/2 inches more.

Although the 220 SPH’s field-penetration length was “kinda close” to its test- penetration length, the 200 WWC’s was not. Simply based on the limited data presented, there is no apparent reasonable, mathematically-predictable link between an expanding hunting bullet’s test impact energy and its field-penetration length. If such a mathematical relationship existed, the data in this example indicate the variability in the test-impact energy versus field-penetration lengths would be so extreme that the relationship would likely be of no practical value.

Photo 3 shows the rib cage of the zebra penetrated by the 200 WWC, and Photo 4 shows the rib cage penetrated by the 220 SPH. These photos identify that both bullets breached multiple rib bones as well as the spine. An interpretive discussion of each bullet’s penetration journey, as related to Photo 3 and Photo 4, is well beyond the scope of this article. Suffice to say that a bullet breaching bone causes a variety of penetration variables, and these variables are the primary reason for why there is likely no direct, mathematical relationship between an expanding hunting bullet’s test-bullet properties or its test-impact energy and a measured field-penetration length.

Field penetration data obtained on the 2023 management hunt indicate a Guppy metric obtained from testing in 20% synthetic gel can empirically predict an expanding hunting bullet’s field-penetration length. 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. This metric and the definition of the field-penetration length goal are discussed in Part 2 of this article, found here.