What Is Wound Cavity Volume, and How Is It the Basis for Evaluating a Hunting Bullet’s Field Terminal Performance?

By Scott Fletcher

“Wounding damage produced by the bullet, not bullet damage produced by the wounding, is the basis for evaluating an expanding hunting bullet’s field terminal performance”.  -  Scott Fletcher

The articles on bullet impact energy, weight loss, expansion ratio and penetration all identify terminal performance evaluation methods that either are incorrect or can give misleading-to-incorrect expectations/representations of a bullet’s field terminal performance. This article presents the basis for consistently, scientifically, and accurately evaluating a bullet’s field terminal performance, as substantiated by both U.S. Army experts and field and skinning-shed autopsy data contained in the 2023 management hunt report.

The actual wound volume created by a bullet as it passes through tissue is the basis for evaluating its field terminal performance. This basis was first articulated by Colonel Towsend Whelen of the U.S. Army Ordinance Corp early in the 20th century. According to Colonel Whelen, “The killing power of a bullet in flight depends entirely on the average size of the wound it makes in the animal, and upon nothing else. The size of the wound in turn depends upon the size, weight, construction, and shape of the bullet, and the velocity with which it strikes, and upon no other details.” According to Colonel Whelen, such “details” as a bullet’s impact energy, weight loss, and expansion ratio are not individually considered in evaluating its field terminal performance.

Colonel Martin Fackler, a physician in the U.S. Army Medical Corps, did extensive research in the 1970’s on the killing power of typical small-arms military ammunition using FBI ordinance gel as a test medium. The results of this testing and research confirmed Colonel Whelen’s assertions. Colonel Fackler concluded that the wound cavity volume produced by a bullet was responsible for its killing power. Quite simply, the larger the wound cavity volume produced by a bullet, the greater is its ability to kill. The implication of Colonel Fackler’s conclusion is that a progressively larger wound cavity within a specific vital organ or set of vital organs produces a progressively shorter time to death.

A common response by an animal to the initial shot is to sprint to safety. That unique sprint speed is essentially uniform for each animal species. As a consequence, the distance traveled during sprinting is mathematically proportional to time to death. Because Colonel Fackler concluded that a progressively larger wound cavity volume produces a progressively shorter time to death, this proportionality indicates a progressively larger wound cavity volume in a common vital organ or set of vital organs in the same species could logically produce a progressively shorter travel distance after the shot.

Butchering a game animal for consumption includes removing its hide and all organs within its thoracic cavity. During this process, wounding to the carcass and the animal’s vital organs can be easily observed. As justified by data in the management hunt report, the wound cavity can be either the actual hole in the tissue made by the bullet or the zone of bloodshot tissue surrounding (and including) the hole. The bloodshot tissue is debilitated-to-incapacitated, and therefore is a “wound”. Direct measurements of this observed wounding can be obtained to calculate actual wound volumes of specific vital organs.

Skinning-shed observations of vital-organ wounding indicate the bullet hole can range from circular to oval, depending upon the degree of tumbling that has occurred. The attendant zone of blood-shot tissue can have various geometries as well, ranging from near-circular through various configurations of a trapezoid. Observations also indicate that the bullet hole is typically not centered within the boundary of bloodshot tissue.

Measured dimensions commensurate with the geometries of the wound shapes in the near-side lung, heart, far-side lung, and other vital organs breached by the bullet can be obtained during butchering. Direct measurements of wound length can be obtained in vital organs other than the lungs. Based on these dimensions and geometric/trigonometric formulas, wound areas with attendant volumes can be calculated.

The lungs in a recovered animal will be deflated. Based on anatomical depictions in both The Perfect Shot II by Kevin Robinson and The Perfect Shot North America by Craig Boddington, the lungs essentially fill the rib-to-rib width in an animal’s thoracic cavity. The length of bullet travel required to calculate a wound cavity volume in the lungs can be approximated by measuring the distance in the animal’s thoracic cavity between the bullet breach in the near-side rib and the bullet breach in the far-side rib. If the heart has been breached, a direct measurement of wound length can be determined, and this measurement can be subtracted from the thoracic cavity length to obtain the approximate length of bullet travel through the lungs.

Photo 1 shows the rib-to-rib thoracic cavity of a zebra carcass suspended in the skinning shed. The wooden dowel is aligned with the direction of bullet travel, with the entrance hole on the right.

Photo 2 shows the total wounding in a zebra’s near-side lung (bottom), heart (middle), and far-side lung (top). The direction of bullet travel is from bottom to top. The bullet has traversed through the top of the heart. The dark red (black) area surrounding the actual bullet hole is bloodshot tissue. Note that the bloodshot tissue is irregularly shaped and does not symmetrically surround the bullet hole.

Photo 3 shows a zebra’s near-side lung wounding. The dowel is pointing to the hole, and the direction of bullet travel is from the slab toward the camera. Note again that the bloodshot tissue is irregular-shaped, and is not centered on the bullet hole.

Photo 4 shows a zebra’s heart wounding. The direction of bullet travel is from left to right. Upon handling, the heart essentially broke open to expose the bullet’s path and the tissue disruption it caused. 

Photo 5 is of a zebra’s far-side lung and identifies the benefit of using bloodshot tissue to determine wound cavity volume. The dowel is pointing at copper-jacket bullet shards (mushroom-spalled shrapnel) embedded in the lung tissue. At least six can be readily identified. These shards are all within the bloodshot tissue’s periphery. As discussed in the referenced report, these shards have both destroyed tissue and created high-flow bleed-out tributaries that extend back to the bullet hole.

Wound volumes in the lungs and in the heart were determined for both the actual bullet hole and the bloodshot tissue surrounding the bullet hole for the animals taken on the management hunt. Both the bullet hole and bloodshot-tissue volumes are chronicled in Table 4 of the referenced report.

Data in Table 4 show that zebras Z-7, Z-8, Z-9, Z-3, and Z-5 all had common, cumulative wounding to both lungs and the heart.  All sprinted to flee in response to the kill shot, and continued to sprint until they collapsed. No animal was observed to struggle after collapsing, indicating death had occurred essentially at the distance of collapse. As a consequence, all travel distances are mathematically proportional to time to death.

As with all animals on the management hunt, the travel distances after the shot for these five zebras were determined by a Professional Hunter using a laser range finder. Graphs of wound cavity volumes versus travel distance after the kill shot for these five animals were drafted. The intent was to identify if there was a relationship between wound cavity volume determined in the skinning shed and travel distance after the kill shot that would substantiate Colonel Fackler’s conclusion that a greater wound cavity volume resulted in a shorter time to death.

Graph 1 from the referenced report is a linear regression plot of travel distance (TD) after the kill-shot bullet impact vs total bloodshot tissue volume (TBSTV), and Graph 2 from the referenced report is a linear regression plot of TD vs total bullet hole volume (TBHV).  The computed correlation coefficient of the trend line of Graph 1 is -0.92, and the computed correlation coefficient of the trend line of Graph 2 is -0.93, virtually identical. Both correlation coefficients indicate a very-good-to-excellent (statistically significant) correlation between travel distance and wound cavity volume, however computed. (A correlation coefficient of -1.0 would indicate a mathematically direct correlation.)

The trend lines of both graphs logically indicate an increase in wound cavity volume results in a decrease in travel distance. The trend lines of these graphs substantiate Colonel Fackler’s conclusion that a larger wound cavity volume results in a shorter time to death. 

As indicated in the referenced report, three different-weight bullets with three different shapes, three different impact velocities, and two different generic designs were responsible for producing the wounding identified on the management hunt. These bullet specifics and hunt circumstances underscore the validity of Colonel Whelen’s assertion that the cumulative wounding contributed by such bullet variables is the basis for the wound volume created rather than any individual wounding contributed from a specific bullet variable. Consequently, both Graph 1 and Graph 2 validate Colonel Whelen’s assertion that the total wound cavity volume in a vital organ or set of vital organs produced by a bullet is the sole basis for consistently, scientifically, and accurately judging its field terminal performance.