How Gel-Test Guppy Metrics Can Be Used to Evaluate an Expanding Hunting Bullet’s Likely Field Performance, Part 2: Empirically Modeling the Field Wound Based on Gel Test-Block Fractures.

By Scott Fletcher

“The man with a new idea is a crank until the idea succeeds.”  -  Mark Twain

Part 1 presented field evidence indicating the volume of disrupted/destroyed cells within the peripheral limits of bloodshot tissue that included the bullet hole can legitimately be called a wound. This article describes how gel testing can be evaluated using the Guppy model to calculate a simulated wound volume based on this definition.

Measurable fractures (cracks) are produced in the interior of gel blocks from passage of an expanding hunting bullet. The configuration and extent of the cracks in gel model (simulate) the configuration and extent of bloodshot tissue surrounding a bullet hole. This crack configuration is the basis for the Guppy analytical model.

Crack measurements can be obtained to determine Guppy-metric values. Field and skinning-shed data obtained on the 2023 zebra management hunt indicate specific Guppy-metric values can be used to empirically predict a tested bullet’s relative lower-bound field wound volume as well as its likely penetration length and the qualitative degree of meat damage it produces during that penetration.

Photo 6 is what can be observed in the first gel-test block of clear 20% synthetic gel after passage of an expanding hunting bullet. Visible are radial fractures (cracks) in the gel produced by the bullet’s passage. As indicated by the photo, the configuration/shape of these cracks generally conforms to the Guppy shape. 

The block shown is comprised of 2o% synthetic gel. The consistency/tenacity of this material allows it to be sectioned into ½-inch (13 mm) or 1-inch (25 mm) thick “wafers” without substantively affecting the crack lengths to enable direct observation and measurement. Photo 7 shows the radial cracks in an individual ½-inch thick wafer. Photo 8 shows the base of a boat-tail bullet embedded in a reduced-size wafer illustrating the radial crack array perpendicular to the bullet’s path through the gel. Photo 9 shows a gel block that has been scored with an ice pick into ½-inch (13 mm) thick segments prior to being manually sliced with a large butcher knife. Photo 10 shows both 1/2-inch and 1-inch (25 mm) thick gel wafers that have been sliced from blocks.

Individual wafers can be examined (Photo 11), and the extent of the cracks can be measured with a dial caliper. Guppy metric values, as identified and defined in Guppy Tech, are either direct measurements of these cracks (e.g. Dmax), the length-wise extent of these cracks (e.g. L(T)), or calculated values based on direct measurements of theses cracks (e.g. V(ST)).

Note in Photo 7 and Photo 11 there is no realistically measurable hole caused by an expanding bullet’s mushroom. The primary reason is because the 20% synthetic gel precisely replicates the hydraulic fracturing of blood-filled capillaries and the crushing of cells in tissue that occur from passage of a bullet. As a consequence, no simulated bullet-hole volume can be determined.

However, the bloodshot tissue volume simulated by both the radial and the length-wise extent of the gel cracks can be calculated with the “average end-area method”, commonly used by civil engineers and grading contractors for computing earthwork cut-and-fill volumes. Maximum and minimum crack-diameter measurements of each wafer allow an average crack diameter to be calculated, simulating the average bloodshot-tissue diameter that surrounds the bullet hole in an actual field wound.

The area represented by each wafer’s average diameter can then be calculated. The modeled wound volume between any two adjacent wafers is simply the average of two adjacent wafer’s areas multiplied by the thickness of the wafer. The adjacent-wafer volumes within a Guppy-metric length of interest, such as L(S), are then summed to obtain the Guppy-metric volume V(ST), Likewise, all adjacent-wafer volumes within the total length of bullet penetration, L(T), are summed to obtain the Guppy-metric V(T).

As discussed in the 2023 management hunt report, gel-test Guppy-metric values of V(ST), L(T), and I(V) obtained from testing cartridges of different calibers (diameters), loaded with bullets of different weights and generic designs, can be evaluated to empirically estimate the degree of field wounding, penetration length, and meat damage, respectively. These empirical estimates can be used to assess the likelihood of any cartridge-bullet combination achieving either hunter-preferred or hunt-specific terminal performance objectives.

Part 3 of this article discusses the Guppy wound-volume metric V(ST); Part 4 discusses the Guppy penetration-length metric L(T); and Part 5 discusses the Guppy meat-damage metric I(V), all referenced to the field and skinning-shed autopsy data obtained on the 2023 management hunt. Part 6 presents summary conclusions about these metrics and the context concerning their pragmatic effectiveness in predicting actual field performance.