How Gel-Test Guppy Metrics Can Be Used to Evaluate an Expanding Hunting Bullet’s Likely Field Performance, Part 4: Empirically Predicting Field Penetration Length Using the Guppy-Metric L(T).

By Scott Fletcher

“If the world was perfect, it wouldn’t be.” – Yogi Berra

Part 3 explained how the Guppy-metric V(ST) can be considered a common-denominator empirical indicator of a lower-bound field-wound volume (TBSTV) through a common set of vital organs. Maximizing such wounding requires that a bullet first penetrate hide, muscle tissue, and any protective bone/bone combination in order to completely breach these organs. This article discusses the Guppy-metric L(T) and how its test value, obtained in 20% synthetic gel, can be applied to empirically predict a field penetration length, as indicated by data obtained on the 2023 zebra management hunt.

One of the primary objectives of any bullet test is to obtain a penetration length through the test medium. Regardless of the medium used for testing, there is no known published mathematical or statistical relationship that directly predicts the field penetration length that will be obtained through a big-game animal’s hide, tissue, and bone by an expanding hunting bullet based on its test penetration length. Any field penetration length prediction based on such testing is empirical.

A typical interpretation of test results relies on the simple evaluation judgement of “more test penetration is better” because it likely represents a greater field penetration. In the case of trophy hunting non-dangerous big-game, the penetration lengths obtained from testing various cartridge-bullet combinations are seldom (if ever) evaluated in terms of how much penetration length is empirically “enough” to satisfactorily breach a particular animal’s thoracic cavity (boiler room) from any stipulated shot angle.

A test-penetration length can typically be directly measured regardless of the testing medium that is used. In addition to the test-length measurement, the retained bullet is also typically weighed to determine the total percentage of material spalled away by the medium during penetration. This weight-loss magnitude is inferred to be a “reliance” indicator of the tested bullet’s ability to achieve satisfactory field penetration. At best, “too much weight loss” can be judged as indicating the bullet’s field penetration through bone and tissue could be significantly less than implied by the test because its momentum will be adversely affected. At worst, such weight loss can be judged as indicating the bullet could catastrophically fail on bone, resulting in limited-to-no penetration either into or through any vital organs. As discussed in the article found here, judging a bullet’s field penetration based on its test weight loss is either misleading to totally incorrect.

Part 3 discussed the rationale for using a “performance-standard” cartridge-bullet combination as a basis for comparing Guppy-metric values to select potentially viable alternative cartridge-bullet candidates. This same “performance-standard” approach was used to evaluate the potential field penetration lengths of the 30-caliber bullet candidates identified in Part 3 based on their penetration-length test values in 20% synthetic gel.

As with evaluating V(ST), the cartridge-bullet combination that served as the performance standard was the 375 H&H firing a 300-grain Sierra Game King (300 SGK). This cartridge-bullet combination had been used on a total of 11 plains-game animals weighing from about 150 to 700 pounds (68 t0 318 kg). The 300 SGK had typically exited these animals from broadside, front-quartering, and rear-quartering shot angles at an average shot distance of 135 yards (123 m).

The Guppy-metric L(T) was selected to empirically model effective field penetration from all shot angles other than full frontal. (Refer to the Guppy model and Guppy Tech). Effective field penetration is defined as a bullet’s complete penetration through the animal’s thoracic cavity as indicated by its retention on the farside hide. L(T) is NOT intended to represent a field penetration that exits the animal, as there can be literally no total field penetration length to actually measure for comparison to the test length. As with determination of the Guppy-metric V(ST), testing to determine L(T) was performed at a distance of 135 yards (123 m). Test results are presented on Table 2.

The 300 SGK had an L(T) test length of 24 inches (61 cm). As with the V(ST) test results, the 30-caliber candidate bullets all had L(T) values that were less. The 200 WWC had an L(T) value of 23 inches (58 cm); the 240 TSMK had an L(T) value of 20-1/2 inches (52 cm); and the 220 SPH had an L(T) value of 19 inches (48 cm).

The shorter L(T) values of the 30-caliber candidate bullets compared to the 300 SGK implied that none could consistently exit the farside hide from the stipulated shot angles. However, their L(T) test values were judged to be compatible with producing “enough” penetration to be retained by the farside hide, consistent with effective penetration’s definition. This judgement was made regardless of the retained weight of each bullet obtained during testing, as tabulated on Table 2.

Effective penetration was the basis of the management hunt’s stipulated penetration performance criterion. There was no field-performance requirement that any 30-caliber bullet completely exit a zebra from the stipulated shot angles.

Hunt penetration data for the 200 WWC are compiled on Table 5; hunt penetration data for the 240 TSMK are compiled on Table 6; and hunt penetration data for the 220 SPH are compiled on Table 7. The combined data from these tables show each bullet breached the nearside shoulder scapula, up to three nearside ribs, either the spine or the neck vertebrae, and one farside rib. Bullets completely exited the animals on six of eleven shots (55%).

Effective penetration occurred on 10 of 11 shots (91%) from a variety of shot angles. The only time where ineffective penetration occurred was the second shot from a 200 WWC into Zebra Z-6, identified in Table 5. The unusual and unrepresentative circumstances associated with this second shot are discussed in section 12.3 on page 45 of the referenced report. The second shot by this bullet produced a drop-to-the-shot reaction attributed to hydrodynamic shock even though it failed to penetrate through to the far-side hide. Excluding the second shot into Z-6, the 200 WWC exhibited effective penetration from a variety of shot angles, completely exiting zebras in two of the six (33%) total shots taken.

Study of Table 5, Table 6, and Table 7 indicates bullets breaching bone experienced up to 60% weight loss (Z-7 on Table 5, 40% weight retained) and a jacket-core separation (BWB on Table 6). In spite of such weight loss and the presumed catastrophic failure of a cup-and-core separation, no bullet failed to breach any bone or failed to achieve effective penetration.  A 200 WWC breached a zebra’s spine (Z-1 on Table 5), as did a 220 SPH (Z-2 on Table 7). A 240 TSMK breached a zebra’s neck vertebrae (Z-4 on Table 6). In the case of the jacket-core separation by a 240 TSMK on the black wildebeest, both a nearside and a farside rib were breached. Although the 240 TSMK’s jacket was retained on the black wildebeest’s farside hide, its lead core completely exited the animal.  

Further study of Table 5, Table 6, and Table 7 indicates multiple and complex combinations of bone breaches occurred in the 11 shots. The simplest “logical” bone breaching combination of one nearside rib and one farside rib only occurred once (Table 6, BWB), or 9% of the time. The only bone breaching combination that occurred more than once is the highly improbable sequence of two nearside ribs, the spine, and one farside rib on Z-1 (Table 5) and Z-2 (Table 7). Further adding to the improbability of this penetration combination is the aim point for both shots was the shoulder, with both bullets being deflected upward into the spine by the nearside ribs, then deflected downward by the spine into the farside rib.

Photo 17 is of the thoracic cavity of Z-1, with the bullet travel alignment indicated by the dowel. The bullet traveled from right to left. Although not clearly indicated by the photo, the notch in the spine made by the 200 WWC is about 2-1/2 inches (6.4 cm) wide and about ½ inch (1.3 cm) deep.

The bone-breaching variables just discussed indicate there can be no reasonable/simple relationships for directly calculating or scaling likely field penetration lengths based on the L(T) test values. These highly variable bone breaching combinations further justify no bone placement in the test gel. In addition to such bone placement producing random test results, no single bone placement configuration/combination in the gel could simulate the real-world bone breaching combinations presented on the referenced tables.

As discussed in Part 3, no bone placement in the gel allows the test value of V(ST) to be considered as a “common denominator” for empirically evaluating a lower-bound field wound volume. Similarly, the hunt data indicate the test value of L(T) can be interpreted as a “common denominator” for empirically predicting the effective field-penetration length of any cartridge-bullet combination.

The hunt data indicate L(T) can be conservatively judged as an empirical indicator of a bullet’s effective field-penetration length through a big-game animal 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 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 penetration lengths far greater than those that will actually be obtained in the field.

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 hunt data indicate that the bullet weight determined by this equation likely has an adequate reserve of sacrificial weight that makes 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, and 12.1 on p. 44.)

4) Only broadside and quartering shots are taken. Note the definition of effective penetration is the bullet be at least retained by the farside hide on broadside and quartering shots; the only farside hide in play with a full-frontal shot is on the hind quarters. (The “Texas heart shot” is not considered.) Such an extended penetration length is unreasonable for a bullet that would otherwise exhibit satisfactory penetration with the majority of shot angles.

5) Only one nearside and one farside rib are breached. A progressively increasing number of bones breached should be expected to produce a progressively decreasing field-penetration length. Breaching either the spine or neck vertebrae should be expected to produce a dramatic decrease in field penetration, to the extent that the bullet may not breach the entire thoracic cavity.

The L(T) value obtained from testing should be compared to the expected thoracic cavity width of the animal that is to be hunted. For broadside and quartering shot angles, L(T) should be greater than the animal’s expected thoracic cavity width.

Effective penetration through an animal’s thoracic cavity will produce some degree of meat damage regardless of shot placement. Modeling the relative degree of such meat damage with the Guppy-metric I(V) is discussed in Part 5 of this article, found here.