How Gel-Test Guppy Metrics Can Be Used to Evaluate an Expanding Hunting Bullet’s Likely Field Performance, Part 1: Defining the Tissue Wound.

By Scott Fletcher

“I don’t know (if they were men or women running naked across the field). They had bags over their heads.”  -  Yogi Berra

The two-part articleon how an expanding hunting bullet produces a wound cavity identified gel testing’s importance in determining a modeled wound cavity volume at any given impact velocity to realistically characterize its likely field terminal performance. The unspoken assumption was the wound cavity modeled (simulated) by such testing reasonably represented the wound cavity produced in tissue by an expanding hunting bullet. This article presents a physiological basis for defining a tissue wound that can be empirically modeled by gel testing, justified by both field skinning-shed autopsy and field performance data obtained on the 2023 zebra management hunt.  

A wound in tissue must first be defined to have a reasonable way to model/simulate it. A broad medical definition of a wound is a physical injury that damages the body’s integrity. Photo 1 shows a black wildebeest’s near-side lung whose integrity has been damaged by passage of an expanding hunting bullet. The damage consists of an actual hole surrounded by an irregular, approximate oval shape of discolored dark-red-to-purple-to-black lung tissue. The discolored area is bloodshot tissue, and the wound can be interpreted as both the hole and the bloodshot tissue that surrounds it.

The hole obviously represents a physical injury that has damaged the lung’s integrity because the bullet has displaced and crushed the lung’s cells. The oval-shaped, bloodshot tissue area of about 2 inches by 4-1/2 inches (5 cm by 11.5 cm) surrounding the hole consists predominantly of cells that have been ruptured/destroyed by hydraulic fracturing from passage of the bullet. An engineer’s interpretation of the hydraulic fracturing mechanics within the capillaries that produce bloodshot tissue is described in section 11.18.1 on page 38 of the 2023 hunt report and in an article found here. The hydraulic fracturing of the lung tissue is also a physical injury that has damaged the lung’s integrity because the destroyed capillaries prevent the lungs’ proper metabolic/physiologic function.

Bloodshot tissue’s inability to properly function is exemplified by Photo 2 and Photo 3. Photo 2 shows a large area of bloodshot muscle tissue on a zebra’s shoulder below the hole made by the bullet. This muscle tissue has been disrupted by hydraulic fracturing to the extent that it has no integrated connectivity to enable it to function as a “muscle”. This tissue was easily scraped (rather than cut) away using a knife, and its total volume and putty-like consistency are indicated by Photo 3 (boots for scale).

A significant percentage of shrapnel produced by bullet weight loss can typically be identified within the periphery of bloodshot tissue, as indicated in Photo 4. Photo 4 is of a zebra’s far-side lung. The dowel is pointing at one of at least six holes identified within the lung’s bloodshot periphery attributed to bullet weight-loss shrapnel. Shrapnel launched radially away from the primary bullet hole can create high blood-flow tributaries within the bloodshot tissue. As with the drainage tributaries of a river, the tributaries created by the passage of shrapnel can rapidly drain blood back to the main bullet hole. The result can be rapid, high-flow bleed out as indicated by the free-blood flow from the bullet exit hole shown in Photo 5. This physiological evidence justifies defining a wound as all the damaged cells within the peripheral extent of bloodshot tissue that surrounds the bullet hole.

Medical professionals typically only consider actual bullet-hole volume as a wound. A larger bullet hole obviously represents a larger wound, and the result of a larger wound is a shorter time to death. If the total volume of damaged cells within the periphery of bloodshot tissue is an actual wound, a larger volume of bloodshot tissue should also result in a shorter time to death.

The legitimacy and reality of this reasoning were confirmed with data obtained on the 2023 zebra management hunt. All but one zebra on the hunt sprinted in response to the kill shot in an attempt to flee. Of the zebras that attempted to flee, there was no obvious variation in the maximum sprint speed observed. Because of this apparent uniform sprint speed, the time to death can reasonably be assumed as directly related to the distance traveled after the shot, with a shorter travel distance after the shot representing a shorter time to death. As a consequence, a shorter travel distance should have resulted from a larger wound volume, however determined.

Five zebras on the management hunt, identified as Z-3, Z-5. Z-7, Z-8, and Z-9, had a common wound that included breaching both lungs and the heart by the kill shot. Skinning-shed autopsies were conducted to determine total wound volume attributable to bloodshot tissue surrounding the bullet hole. These volumes are identified as TBSTV and are catalogued in Table 4 from the hunt report. TBSTV values for the referenced five zebras were plotted versus travel distance, resulting in Graph 1. The trendline for this graph was mathematically determined by linear regression analysis. This graph shows both a linear and a logical relationship between wound volume and travel distance, as the travel distance decreases linearly with an increase in wound volume.

The correlation coefficient (R) for Graph 1 is -0.92. This correlation coefficient can be conservatively considered “very good”. (A statistically “perfect” relationship between wound volume and travel distance would be indicated by a value of -1.0.) The coefficient of variation (CV) for this graph is 23%. This coefficient can be considered “good to very good”. (A statistically “perfect” CV would be 0%, indicating no variation of the data from the trendline.)

The total bullet hole volumes were also determined for the referenced zebras. These volumes are designated as TBHV and are also catalogued in Table 4. The TBHV values were plotted versus travel distance, resulting in Graph 2.  The trendline for this graph was also mathematically determined by linear regression analysis. Like Graph 1, Graph 2 also shows the same linear and logical relationship between wounding and travel distance, as the travel distance also decreases linearly with an increase in wound volume.

The correlation coefficient (R) for Graph 2 is -0.93 and can also be considered very good. However, the coefficient of variation for this graph is 63%, indicating a significant variation of the data (scatter) from the trendline.

The correlation coefficient of the trendline in Graph 1, plotted using the total volume of bloodshot tissue encompassing the actual bullet hole, is essentially identical to the correlation coefficient of Graph 2, plotted using only the total bullet hole volume. These essentially identical correlation coefficients demonstrate that the total volume of bloodshot tissue surrounding (and including) the bullet hole can justifiably be called a wound.  Its volume, like total bullet hole volume, is directly and linearly related to time to death.

Although data used to construct both graphs are limited, the significantly better coefficient of variation value associated with Graph 1 indicates that there is significantly less scatter in the data points used to construct the graph. Based on the statistical mathematics of these two graphs, the results of Graph 1 indicate total bloodshot tissue volume is a potentially better way to make field judgements about a bullet’s wounding performance, as this volume would also typically include the previously described wounding benefits of shrapnel associated with bullet weight loss.

Theses graphs indicate the total bloodshot tissue volume created by passage of an expanding hunting bullet can satisfactorily describe and quantify a wound. Classifying total bloodshot tissue volume as a wound volume is fundamentally important because it allows a bullet’s field performance to be empirically determined from gel-test modeling. The basis for an evaluation model using simulated bloodshot tissue determined from a gel test is discussed in Part 2, found here.