What is the Guppy Model for Evaluating a Hunting Bullet’s Terminal Performance, and Why Should I Care?

“Identifying a complex problem’s relevant aspects can be hard, then figuring out how to actually solve each aspect can be harder. However, getting the combined solution to reasonably and consistently predict reality is the real bitch”.  -  Scott Fletcher

The answer to the question posed by the article’s title is the Guppy Model, in conjunction with gel testing, identifies legitimate “over-achiever” bullets that can max-out desired field terminal performance from any chambering. Specific bullets launched from “calibers” (chamberings) considered anemic because of presumed inferior muzzle velocity can produce comparable to superior field terminal performance compared to bullets launched from hotrod magnum chamberings.

The cartridges shown in Photo 1 exemplify this notion of “over-achiever” bullets. The cartridge on the left is for a 358 Winchester. It is loaded with a .358-caliber, 250-grain, flat-base cup-and-core bullet with a muzzle velocity of 2463 fps (750 mps). On the right is a cartridge for a 375 H&H. It is loaded with a .375-caliber, 300-grain boat-tail cup-and-core bullet with a muzzle velocity of 2615 fps (797 mps). At a distance of 135 yards (123 m), the Guppy Model with attendant testing introduced in this article indicates the 250-grain bullet is an “over-achiever” by exhibiting wounding only 4% less than the 300 grainer, yet penetrating 33% further. 

Passage of an expanding hunting bullet through gel typically produces a relic wound-cavity shape that resembles a fish called a guppy. The Guppy Model is based on this shape and is shown here. Shown within the schematic shape are arbitrarily selected mathematical ways, called metrics, to quantify it. The mathematical definitions of these metrics are identified here. Photo 2 shows the Guppy shape in 20% synthetic gel. eBook Chapter 12 presents the general gel-testing procedures, including how relevant Guppy Model dimensions and volumes are obtained.

The Guppy Model, hereafter simply referred to as the Guppy, is personally derived based on an engineer’s/hunter’s perspective. The model’s intent is to empirically predict a bullet’s field performance or a measured field value based on a its gel-test metric value. Such evaluations allow a strategic bullet selection based on both hunt and hunter terminal performance objectives regardless of a bullet’s weight and caliber (diameter) and the chambering used to launch it.

The previous paragraph uses the word “empirically” to describe how the Guppy is used to evaluate terminal performance. The American Heritage Dictionary 0f the English Language definition of empirical is 1. “Relying upon or derived from observation or experimentation.”; 2. “Guided by practical experience and not theory, especially in medicine.” Use of the words “empirical” or “empirically” means that the field terminal performance of any bullet predicted from any test or any combined analytical model/test is approximate, at best. There are simply too many unaccounted-for variables, such as the physiological condition of the animal at the instant of bullet impact, the number/type of bones breached, specific vital organs breached, the degree of bullet tumbling, etc. that contribute to the observed field outcome that is the basis for validating an empirical analytical model. 

The precision implied by testing in FBI ordinance gel instead of any synthetic gel, test measurements to the “fuzz”, and calculated results to the third decimal from any formula do not directly translate into a precise, predictive “answer”. Any combined test and analytical modeling process associated with evaluating a hunting bullet’s likely field terminal performance is “hand-grenade” analysis, simply intended to obtain a performance prediction that is reliably and consistently “close enough” to its observed field results.

The metrics of bullet impact energy, expansion ratio (mushroom diameter), and retained weight are all empirical. Previous articles on this website have shown that the commonly accepted/expected field outcomes “predicted” by these empiricisms are either completely false or can be badly misleading based on field and skinning-shed autopsy data obtained in the 2023 management hunt report.

A reliable method to predict a hunting bullet’s wounding, penetration, and meat damage is of primary importance for most hunters. Field and skinning-shed autopsy data from the 2023 management hunt have shown that Guppy metrics can be used to empirically predict each of these primary field-performance parameters.

Metric V(ST) empirically models (simulates) the majority of wound volume produced in the animal, including bloodshot tissue that surrounds the actual bullet hole. The bloodshot tissue can contain bullet shrapnel produced by a bullet spalling its mushroom during penetration. Although not specifically identified during the skinning-shed autopsies conducted on the management hunt, this modeled volume can be reasonably assumed to contain bone shards acting as shrapnel that have been sprayed into adjacent tissue by a bone-breaching bullet.

V(ST) is considered to be an empirical quantification of the wound cavity volume referenced by both Colonel Whelen and Colonel Fackler, as discussed in a previous article. Consequently, the magnitude of V(ST) is considered to be empirically related to its killing power, with a larger numerical value indicating a shorter time to death.

Metric L(T) empirically models the total penetration length of a bullet through the animal, with a greater numerical value indicating a greater penetration length. This penetration length is simplistically intended to account for breaching bones, either singularly or in any combination, even though no bones were embedded in the personal gel testing conducted to evaluate selected expanding hunting bullets. Typical gel testing performed by others has not included any embedded bone because there are no established standards for bone type, placement depth, orientation, width, thickness, density, etc.

Metric I(V) empirically models the severity of tissue destruction immediately adjacent to the bullet hole. This tissue destruction is qualitatively related to the degree of bloodshot meat produced, with a greater value of I(V) indicating a likely greater volume of bloodshot meat.

So, how is this modeling and test-metric analysis applied? Answer: by obtaining data from bullet gel tests to enable simple “same-or-different” metric value comparisons. Between two or multiple bullets, a greater V(ST) value is better because it empirically represents a greater lethality and a subsequently shorter time to death. A greater L(T) value is better because it empirically represents a greater penetration length. A lower value of I(V) is better because it empirically represents a lower volume of bloodshot meat.

Preferred metric-value comparisons occur between a cartridge loaded with a bullet that has known field performance and any cartridge loaded with a potential replacement (candidate) bullet. Selected metric values of a bullet with known field performance are considered the terminal performance standards to which the metric values of candidate bullet must compete. Quite simply, the metric values of candidate bullets with their attendant cartridge chambering should reasonably match or exceed metric values of bullets with known field performance.

The primary intent of the 2023 management hunt was to identify if a 30-caliber bullet launched from a 300 Winchester could replicate the previously demonstrated field performance of a .375-caliber, 300-grain bullet launched from a 375 H&H, but without the prospect of meat damage considered as excessive by an experienced African Professional Hunter (PH). Performance criteria that the candidate 30-caliber bullets had to reasonably achieve are described in detail in section 8.0 of the referenced report from pages 10 to 20, including comparisons of V(ST), L(T), I(V) values of the candidate bullets to those of the performance-standard bullets.

Photo 3 identifies the bullets with attendant cartridges that produced metric values considered to be the standards for comparing candidate 30-caliber bullets. Left: a 375 H&H with a .375-caliber, 300-grain Sierra Game King (SGK). Test metric values of V(ST) and L(T) from this bullet were considered as performance standards. Right: a 308 Winchester with a 30-caliber, 165-grain Barnes TSX (BTSX). The test metric value of I(V) from this bullet was considered as the performance standard.

Photo 4 identifies the 30-caliber candidate bullets, all fired from a 300 Winchester. Left-to-right:  a 200-grain Woodleigh Weldcore (WWC); a 240-grain Sierra Match King (SMK) with a hand-installed poly tip; a 220-grain Sierra Pro Hunter (SPH); and a 180-grain Swift A-Frame (SAF).

The 200 WWC, the 240 TSMK, and the 220 SPH were selected for evaluation on the management hunt. Detailed discussions of why these bullets were selected are presented in section 9.0 from pages 21 t0 24 of the management hunt report. The explanation of why the 180 SAF was excluded from field evaluation is presented in the Terminal Performance Timeout #2 on p. 17.

Prior to the hunt, comparison of Guppy metric V(ST) and L(T) values obtained from gel testing indicated the lighter-weight, smaller-diameter 200 WWC was a realistic wound-volume and penetration-length contender to the much heavier and larger diameter 300 SGK considered as the field-performance standard. Field and skinning-shed data contained in the 2023 management hunt report identified the 200 WWC as a legitimate “over-achiever” bullet when launched from a 300 Winchester because it produced travel distances after the kill shot and penetrations through animals from typical shot angles that were both comparable to the 300 SGK.

Prior to the hunt, no field data were available to calibrate the relationship between the magnitude of I(V) and the qualitative volume of bloodshot meat.  Skinning-shed autopsy data demonstrated that the 200 WWC produced an acceptable volume of bloodshot meat from shots on the shoulder. As a consequence, the 200 WWC’s I(V) gel-test value of 10 can be considered as a tentative empirical boundary between acceptable and unacceptable production of bloodshot meat. I(V) values progressively less than 10 can be expected to empirically indicate a progressively-less volume of bloodshot meat, while I(V) values progressively greater than 10 can be expected to empirically indicate a progressively-greater volume of bloodshot meat.

These field and skinning-shed data indicate Guppy metrics of V(ST), L(T), and I(V) are reasonable empirical indicators of a hunting bullet’s likely field terminal performance. Consequently, Guppy metric value comparisons of bullets shot from the same chambering enable selection of a bullet that is the most appropriate for a specifically identified hunting scenario. Moreover, Guppy metric value comparisons enable selection of alternative cartridges and bullets that can produce field terminal performance that is  comparable to a chambering and bullet with known field terminal performance. Such comparisons enable identification of “over-achiever” bullets that otherwise could not be determined.