How Gel-Test Guppy Metrics Can Be Used to Evaluate an Expanding Hunting Bullet’s Likely Field Performance, Part 3: Empirically Predicting Field Wounding Using the Guppy- Metric V(ST).

By Scott Fletcher

“Speak English! I don’t know the meaning of half those long words, and what’s more, I don’t believe you do either!” - The Eaglet from Alice in Wonderland by Lewis Carroll

However, Part 2  described how cracks produced in gel by passage of an expanding hunting bullet simulate a wound comprised of bloodshot tissue surrounding a bullet hole. The article also indicated Guppy-metric values based on gel-crack measurements can be applied to empirically evaluate the tested bullet’s likely field performance. This article discusses the Guppy metric V(ST) and how its common-denominator gel-test value can be applied to competitively compare the lethality of any cartridge-bullet combination and empirically predict a likely maximum travel distance based on a lower-bound magnitude of field wounding, as indicated by data obtained on the 2023 zebra management hunt.

Colonel Townsend Whelen of the U S Army Ordinance Corps asserted a bullet’s lethality is only dependent on the wound-volume magnitude it creates, i.e. the greater the wound volume, the more lethal the cartridge. Such lethality can be assessed by a simple measurement: time to death. Progressively more lethal cartridges produce progressively shorter times to death from wounding to vital organs other than the brain. This assertion was supported by Colonel Martin Fackler, a physician in the U S Army Medical Corps. Colonel Fackler did gel-testing research on small-arms ammunition in which he concluded bullets that produce large wound cavities are likely to produce quick times to death.

Application of Colonel Whelen’s assertion and Colonel Fackler’s research conclusion indicates the time to death of an animal can be directly related to its travel distance after the bullet has breached common vital organs. Unless an animal drops to the shot, its typical response is to sprint to safety. This sprint speed is considered as essentially the same for most common-weight, big-game species. As a consequence, “doing the math” results in the travel distance being directly related to the animal’s time to death, as well as being directly related to the wound volume created by the bullet. (This application of Colonel Fackler’s research conclusion was proven on the management hunt, as discussed in Part 1 and indicted by Graph 1.)

One objective of the Guppy model is to identify a wound-cavity metric whose value can empirically predict the relative magnitude of field wounding. The intent is to simply compare gel-test metric wound volumes of any cartridge-bullet combinations using the simple judgement criterion that “more test wound volume is better”.  A large test value would indicate a large field wound volume with an attendant short time to death, indicated as a short travel distance. Conversely, a small test value would indicate a small field wound volume with an attendant long time to death, indicated as a long travel distance.

Review of the Guppy model and the Guppy-metric definitions in Guppy Tech indicates six wound cavity volumes can be determined from gel testing. However, only the wound cavity volume represented by both V(ST) and V(T) is of primary interest in assessing the potential field wounding magnitude.  

V(ST) is comprised of V(S), the primary bloodshot tissue volume, and V(PS), the bullet hole volume within that bloodshot tissue. These combined volumes represent the greatest percentage of bloodshot tissue produced by the passage of a bullet. The wound cavity volume represented by V(T) is the summation of all the Guppy wound-volume components that have been produced in the gel throughout the bullet’s penetration length.

Gel testing eleven expanding hunting bullets indicates V(T) is typically about 10% more than V(ST). However, V(ST) was chosen as the metric for evaluating field wounding for two reasons. First, skinning-shed/field autopsies reveal the wounding produced through vital organs by an expanding hunting bullet is comprised of significant bloodshot tissue surrounding the bullet hole. This wounding is consistent with how V(ST) is modeled and defined.

Second, virtually all the volume difference between V(ST) and V(T) is represented by V(Pe). (Refer to the Guppy model). V(Pe) can represent a field wound-volume component that is not obtained if a “penetrator” hunting bullet completely exits the animal, particularly on a preferred broadside shot. Although complete penetration through an animal is a field-performance characteristic considered desirable by many hunters, selecting V(T) as the Guppy model’s evaluation metric would over-predict a bullet’s wounding potential if it consistently exits the animal.

A simple comparison of V(ST) values, i.e. “bigger is better”, allows a determination of which cartridge-bullet combination is “best” in terms potentially producing the largest field wound volume. However, predicting a travel distance depends on comparing the V(ST) of any cartridge-bullet combination to the V(ST) of a cartridge-bullet combination that has produced a known, consistent travel distance after a kill shot through a common set of vital organs. A cartridge-bullet combination with such an established consistent travel distance can be designated as a performance standard.

A “performance-standard” cartridge-bullet combination should be selected based on its demonstrated/documented ability to produce a reasonably consistent maximum travel distance after breaching common vital organs of the same or similar-weight animals. Gel testing should then be performed on this designated performance-standard cartridge to determine its V(ST) value at the average shot distance associated with the maximum travel distance. Testing of alternative cartridge-bullet combinations should then be performed at the same distance to assess if their V(ST) values are comparable the cartridge-bullet standard. Any candidate cartridge-bullet combination with a V(ST) value similar to the performance-standard’s V(ST) can be expected to achieve a maximum travel distance similar to the performance-standard cartridge. Any test candidate’s V(ST) value that is less than the performance standard’s can be expected to have a maximum travel distance that is greater; any test candidate’s V(ST) value that is greater than the performance standard’s can be expected to have a maximum travel distance that is less.

A “maximum” travel distance was selected because it results from the minimal field wounding produced by a bullet that does not tumble or produce significant wounding from bone-shard or weight-loss shrapnel due to breaching bone. Such minimal field wounding is reasonably simulated by the Guppy model if no bone is placed in the gel. The resulting V(ST) test value can be judged as modeling a lower-bound field wound volume that is consistent with a maximum travel distance. Furthermore, V(ST) obtained from testing without bone embedded in the gel serves as a “common denominator” value for comparatively judging the wounding capability of any cartridge-bullet combination.

A 375 H&H case, loaded with a .375-caliber, 300-grain Sierra Game King (SGK), was selected as the wounding performance-standard cartridge for the 2023management hunt.  Seven African plains-game animals weighing from about 400 to 700 pounds (182 to 318 kg) had been shot with the 300 SGK on the shoulder through both lungs and the heart. One of the animals had been a zebra stallion. 

All animals had been shot in the Limpopo Province of South Africa at an average distance of about 135 yards (123 m). Two of the seven had dropped to the shot (nyala and hartebeest bulls), with the remaining five sprinting after the shot in an attempt to flee.  Paced travel distances had ranged from about 10 yards (water buck cow) to a maximum of 90 yards (blue wildebeest cow and kudu bull). The zebra stallion had sprinted a paced 35 yards. Based on this experience, 90 yards (82 m) was preliminarily selected as the maximum field travel-distance performance criterion.

Gel testing of the 300 SGK was performed at the average shot distance of 135 yards to determine its V(ST) magnitude. Various 30-caliber bullets, most fired from a 300 Winchester magnum (300 Win), were considered as alternative candidates to the 300 SGK. Gel testing was also performed on these various 30-caliber bullets at a distance of 135 yards to obtain their V(ST) values. Table 1 from the management hunt report identifies these bullets, and Table 2 of the referenced report identifies the V(ST) test values.

Photo 12 identifies the 375 H&H cartridge loaded with a 300-grain SGK as well as the 300 Win cartridges loaded with the bullets evaluated on the management hunt. Left to right: a 375 H&H case loaded with a 300-grain, cup-and-core SGK; a 300 Win case loaded with a 200-grain bonded-lead cell Woodleigh Weldcore protected point (WWC); a 300 Win case loaded with a 240-grain Sierra Match King that has been modified with a hand-installed poly tip (TSMK); and a 300 Win case loaded with a 220-grain cup-and-core Sierra Pro Hunter (SPH).

As shown on Table 2, none of the V(ST) test values of the 30-caliber candidate bullets referenced in Photo 12 matched or exceeded the 26.4 cubic inch value of the 300 SGK. The closest value was 23.5 cubic inches obtained by the 200 WWC, about 11% less. Based simply on comparing V(ST) values, the maximum travel distance of a zebra shot at an average distance of 135 yards through both lungs and the heart with a 200 WWC was expected to be slightly greater than 90 yards.

The V(ST) calculations for both the 240 TSMK and 220 SPH did not include the volume contributed by bullet weight-loss shrapnel that had occurred from simply penetrating through the gel. (Refer to section 8.3.3 on p. 14 of the 2023 management huntreport.) These under-predictions of both the 240 TSMK’s and 220 SPH’s V(ST) values potentially indicated their maximum travel distances could be comparable to the 200 WWC’s.  Bottom line: all three 30-caliber bullets, fired from a 300 Win, were judged to potentially produce field wounding that resulted in a maximum travel distance slightly greater to reasonably comparable to the 90 yards obtained with a 375 H&H cartridge loaded with the 300-grain SGK.

The management hunt was conducted in the Free State of South Africa. Due to the more widely dispersed vegetation in the Free State compared to the Limpopo, the average shot distance was expected to be greater than the 135-yard test distance used for all bullets. As indicated by Graph TP-1 in a previous article, an increased average shot distance would decrease each 30-caliber bullet’s impact velocity, and would thus decrease the field-wound volume of each bullet. As a consequence, the maximum travel distance of 90 yards obtained at an average shot distance of 135 yards in the Limpopo would likely under-predict the maximum travel distance for a greater average shot distance in the Free State. Accordingly, the maximum travel-distance performance criterion was arbitrarily increased from 90 yards to 100 yards (91 m).

Table 3-1 is a compilation of test and field data for all zebras shot on the management hunt with 30-caliber bullets that breached both lungs and the heart (or the plumbing directly above the heart), as confirmed by skinning-shed autopsies. No zebra traveled beyond the 100-yard maximum travel distance performance criterion. This performance criterion was achieved based solely on the field wound volume produced by each bullet, regardless of its generic design, initial weight, impact energy, or retained properties such as weight.

Data for Z-7, Z-8, and Z-9, all shot with a 200 WWC, illustrate the extreme variability of field wound volume that can be produced by the same bullet. The primary reason for this radical wound-volume variation is judged to be the highly variable wounding caused by breaching bone. Breaching bone caused variable bullet tumbling, likely variable secondary wounding produced by bone shards sprayed into the nearside lung, and variable secondary wounding produced by bullet weight-loss shrapnel sprayed into both the lungs and the heart.

Note the lowest TBSTV associated with Z-7 occurred with no bullet tumbling and only a 20% bullet weight loss. Such a modest weight loss indicates modest secondary wounding from shrapnel. This lower-bound TBSTV with its attendant maximum travel distance is considered consistent with a V(ST) that has been determined from testing where no bone was placed in the gel.

In contrast, the highest TBSTV occurred in Z-9 where bullet tumbling occurred, likely supplemented by considerable secondary wounding from bullet weight-loss shrapnel. Although no bullet was recovered from Z-9, Photo 13 of Z-9’s heart shows tissue wounding that can be characterized as “shredded”, consistent passage of significant weight-loss shrapnel.

Table 3-1’s data also indicate the adrenalin level in the animal at the time of the shot can also significantly affect travel distance regardless of wound volume. Zebra Z-8 traveled only 2 yards less than Z-7 in spite of the significant wound volume increase likely caused by bullet tumbling and weight-loss shrapnel. The reason is attributed to Z-8’s adrenalin level being considerably elevated (amped) just prior to the shot. Z-8 was part of a herd that had been driven to an ambush point by a truck. On previous hunts, just seeing the truck had caused the herd to flee. Being chased by the truck had likely caused a significant adrenalin dump in Z-8. The increased adrenalin level is interpreted to have enhanced Z-8’s stamina/tenacity for life that allowed it to travel a greater distance than would otherwise be expected simply based on wound volume.

A representative lower-bound TBSTV with an attendant maximum travel distance was not determined for either the 240 TSMK or 220 SPH. Insufficient zebras were shot with these bullets to allow for such determinations. Zebra Z-3 was the only one shot through both lungs and the heart with a 240 TSMK, and Zebra Z-5 was the only one not spined by a 220 SPH that had breached both lungs and the heart. The TBSTV’s of both Z-3 (437) and Z-5 (440) are comparable to the TBSTV of Z-9 (450). Such wounding also produced comparable travel distances to Z-9, with Z-3 traveling 51 yards (46 m) and Z-5 traveling 41 yards (37 m).

The reasons these bullets produced similar field wound volumes and travel distances as the 200 WWC used for Z-9 are that both tumbled and likely contributed considerable secondary wounding due to bullet weight-loss shrapnel. Both bullets have a cup-and-core generic design, and elevated weight-loss magnitudes can occur. Although no bullets were recovered from either Z-3 or Z-5 for confirmatory weight measurements, skinning-shed autopsies identified vital organ damage consistent with such weight-loss shrapnel.  At least six shrapnel shards are visible in the farside lung of Z-3, as indicated by Photo 14. Like Z-9’s heart in Photo 13, Photo 15of Z-3’s heart and Photo 16 of Z-5’s heart indicate both were “shredded” by weight-loss shrapnel.

Z-3’s and Z-5’s TBSTV magnitudes and the interpreted mechanisms for how those volumes were produced are remarkably similar/consistent with Z-9’s. Prior to the hunt, such field results were considered possible but not expected simply based on gel-test V(ST) comparisons. As indicated on Table 2, the V(ST) magnitudes of both the 240 TSMK (20.5) and 220 SPH (19.3) are considerably less than the V(ST) 0f the 200 WWC (23.5).

These V(ST) test value comparisons logically indicated the TBSTV achieved by both the 240 TSMK and 220 SPH should also have been considerably less than the TBSTV achieved by the 200 WWC in Z-9. These under-predictions of actual TBSTV are likely due to not including the additional V(S) volume produced in the gel by the weight-loss shrapnel from both bullets in their V(ST) calculation.

The previous interpretive discussion of Table 3-1’s data demonstrates how the complex interaction of field, wounding, and animal variability factors can affect the actual travel distance. The random interaction of these variables indicates there are likely no reasonable/simple relationships for directly calculating or scaling likely field TBSTV based on a V(ST) test value.

However, the testing and management hunt data for the 200 WWC indicate the V(ST) obtained from gel testing with no embedded bone can be considered as a “common-denominator” value that can empirically model field wounding represented by a lower-bound TBSTV. This modeling enables a simple “same-and-different” comparison of gel-test V(ST) volumes between a candidate cartridge and performance-standard cartridge that provides a reasonable empirical expectation of maximum field travel-distance performance in qualitative terms of comparable, longer, or shorter.  

Hunters may not have maximum travel distance or average shot distance data to conduct testing on a desired performance-standard cartridge. However, the results just discussed indicate that the field wounding potential of any cartridge-bullet combination of interest can be empirically assessed using gel testing at a common test distance. The cartridge-bullet combination with the highest V(ST) test value can be expected to typically produce the greatest field wound volume, likely resulting in the shortest maximum travel distance.

Complete bullet penetration through vital organs housed within an animal’s thoracic cavity is required to max out the available field wounding implied by V(ST). Modeling such penetration with the Guppy-metric L(T) is discussed in Part 4 of this article, found here.