How an Expanding Hunting Bullet Creates a Wound Cavity, Part 1: The Effect of Impact Velocity.

By Scott Fletcher

"Truth is stranger than fiction, but it is because fiction is obliged to stick to possibilities; truth isn't".   -  Mark Twain 

In the early 1900’s, Colonel Townsend Whelen of the US Army Ordinance Corps articulated the basis of a bullet’s lethality: “The killing power of a bullet in flight depends upon the average size of the wound it makes in the animal, and upon nothing else. The size of the wound (in turn) depends on the size, weight, construction, and shape of the bullet, and the velocity with which it strikes, and upon no other details.”

Note the factors Colonel Whelen attributes to wound creation do not include bullet impact energy, mushroom size, or weight loss. Analysis of field and skinning-shed data obtained on a zebra management hunt and presented in the 2023 hunt report validates Colonel Whelen’s lethality assertion. Individual articles explaining why the factors of expanding hunting bullet impact energy, mushroom size, and weight loss are not relevant or misleading can be found here.

Colonel Whelen’s terminal performance factors of “the size, weight, construction, and shape of the bullet” that contribute to the formation of the wound cavity can be summarily described as the bullet’s specific or generic design features, shortened to simply generic design. Bottom Line: Colonel Whelen’s assertion concerning the basis of a bullet’s lethality can be reasonably condensed into the following statement: The killing power of a bullet depends on the average size of the wound it creates, dependent only on its generic design and impact velocity. This article conceptually discusses how a bullet’s impact velocity controls wound cavity formation with its attendant penetration, independent of its generic design.

The wound cavity created by an expanding hunting bullet unaffected by tumbling is produced by the bullet’s deformation both along its length and across its initial tip diameter. The combined deformations produce a circular-shaped mushroom, as depicted in Photo 1.

Currently accepted terminal performance evaluation practice is interpreted to use the tested bullet’s mushroom diameter as an indicator of the qualitative wound cavity volume it is capable of producing. “Large” retained-bullet mushroom diameters are believed to produce “large” wound cavities. Furthermore, the mushroom diameter obtained from one test’s impact velocity is assumed to be representative for all impact velocities. However, testing the same bullet at varying impact velocities in a medium that allows the wound cavity volume to be either directly measured or calculated has demonstrated a retained bullet’s mushroom diameter obtained from just one test does not realistically represent the wound cavity volume the bullet produces at different impact velocities.

Similarly, the retained weight of an expanding hunting bullet from a test at one impact velocity is used as a qualitative indicator of penetration length at other impact velocities.  A tested bullet’s retained weight is assumed to decrease at a higher impact velocity. The presumed effect of a decrease in retained weight is a decrease in a bullet’s penetration because the weight loss decreases its momentum. Gel-test data presented in this article indicate this presumption can be incorrect.

The relationship between an expanding hunting bullet’s impact velocity and the resulting wound cavity volume with its attendant penetration is complex, and is best illustrated graphically with test data obtained and published by Richard Mann. These data were obtained from testing in synthetic gel called Bullet Test Tube. Just like FBI ordinance gel and the current synthetic ordinance gels, the material comprising Bullet Test Tube can form a Guppy-shaped wound cavity upon bullet impact. However, unlike FBI and synthetic ordinance gels, the total modeled wound cavity volume can be directly determined in Bullet Test Tube by pouring water from a graduated cylinder calibrated in cubic centimeters (cc) into the cavity formed by the bullet. As with FBI and synthetic ordinance gels, the penetration length also can be directly measured and the test bullet recovered for evaluation.

The actual test-data compilation is presented in Richard Mann’s book “Rifle Bullets for the Hunter, a Definitive Study”, first edition, published in 2006 by Ballistic Technology in Princeton, West Virginia. Table 1 in this article is a summary of Mann’s data from pp. 54-55 of his book. Mann’s data are considered both exemplary and unique because the data set is for only one bullet tested at various impact velocities. Test data include both penetration lengths and wound cavity volumes at essentially seven impact velocities ranging from about 1800 to 3200 fps. I have found no other such published data set for just one bullet where the wound cavity volume and the penetration length have both been obtained at multiple impact velocities.

Mann presents no graph of these data in his book. Graph TP-1 is a personal plot of selected data in Table 1. This graph will be used in conjunction with the following narrative to describe a conceptual interpretation of how an expanding hunting bullet’s deformation and resultant mushroom formation at increasing impact velocities control both its wound cavity volume and attendant penetration. This conceptual narrative applies to any expanding hunting bullet, no matter what its generic design may be.

Graph TP-1 shows the mushroom expansion ratio, penetration, and wound cavity volume data from Table 1 plotted versus impact velocities ranging from about 1800 to 3200 fps (548 - 975 mps) for a 6.5-mm, 130-grain expanding hunting bullet. The mushroom expansion ratio (ER) is simply the recovered bullet’s test-measured mushroom diameter divided by its caliber of .264 inches (6.5 mm).

At the lowest impact velocity of 1811 fps (552 mps), both the ER and the wound cavity volume (WCV) are at their lowest values, but the penetration is remarkably at its highest. This is because the relatively modest impact force causes the mushroom to form very slowly and to only a limited extent, as indicated by the low ER value of 1.40. The result is a comparatively low-drag force on the bullet’s marginally-formed, relatively small-diameter mushroom that allows this exceptional penetration.

The low drag force at 1811 fps (548 mps) produces low shear stress in the materials comprising the bullet’s mushroom, insufficient to spall (shear) them from its tip. This comparatively low shear stress is indicated by the bullet’s 100% weight retention identified in Table 1. Even though the bullet penetrates an extended length, the WCV is the lowest (10 cc) because the anemic and slow mushroom expansion only produces a small average cross-sectional area wound cavity for each incremental increase in penetration length.

A progressive increase in impact velocity logically results in a progressive increase in impact force. As implied by Graph TP-1 and Table 1, the mushroom forms quicker and increases in diameter, indicated by a progressive increase in ER. The result is a progressive decrease in penetration because of a progressive increase in drag force. However, Graph TP-1 shows the progressive increase in ER from about 1800 to 2500 fps (549 - 762 mps) is sufficient to produce a progressive increase in wound cavity cross-sectional area that produces a progressive increase in WCV, even at a progressive decrease in penetration length.

Graph TP-1 and Table 1 indicate an impact velocity of at least 2500 fps (762 mps) is required to essentially fully form the tested bullet’s mushroom. Furthermore, Graph TP-1 shows the interpreted slopes of the ER, penetration, and WCV trendlines all significantly change at this impact velocity. Beyond 2500 fps (762 mps), ER essentially peaks and begins to decrease, likely because the increased shear stress imposed by the increase in impact force spalls progressively more material from the tip. This spalling is indicated in Table 1 by the decreasing retained-weight data for impact velocities greater than about 2500 fps (762 mps).

The increase in penetration beyond 2500 fps (762 mps) can be attributed to the bullet’s momentum exceeding the maximum drag force the test medium is capable of imposing, facilitated by the gradual reduction of the tip area due to spalling. However, the reason for the WCV’s increasing formation rate (trendline with a greater upward slope) and magnitude (actual volume) with a decrease in mushroom ER beyond 2500 fps (762 mps) is unclear, speculatively attributed to a complex interaction of applied physics, material strength, and the test-medium’s fluid mechanics (topics on which I have no credentials to even comment).

Mann provided no after-test cross-section photos of each test specimen. Based on personal gel testing results, Mann’s test specimens below 2500 fps (762 mps) could potentially resemble Photo 2, and test specimens beyond 2500 fps would likely resemble Photo 3. Photo 2 is the initial gel block produced by a bullet with a marginally-formed mushroom (Phot0 4), and Photo 3 is the initial gel block produced by a fully-formed mushroom (Photo 5). Although the wound cavities in both gel blocks can be analyzed with Guppy metrics, the wound-cavity shape in Photo 2 resembles a “snake” rather than a “Guppy” as in Photo 3. The “Guppy” shape of Photo 3 obviously represents a far greater wound volume than the “snake” shape in Photo 2. I speculate the true “Guppy” shape in Photo 3 is likely attributable to the “complex interaction of applied physics, material strength, and the test-medium’s fluid mechanics” alluded to in the previous paragraph.

Both the WCV and the penetration trendlines are interpreted to slope more steeply upward from 3066 to 3190 fps, (935 - 973 mps), indicating higher rates of both wound cavity formation and penetration with each incremental increase in impact velocity beyond 3066 fps (935 mps). The reason is not known nor can be speculated because no test photos or a specific test narrative were included in Mann’s book. However, data in Table 1 show values for both ER and retained weight continue to decrease beyond 3066 fps (935 mps).

No testing was performed at impact velocities above 3190 fps (973 mps). Conceptually, the impact stress on the bullet at some impact velocity greater than 3190 fps (973 mps) would be sufficient to cause a catastrophic structural failure of the bullet, particularly if it encountered a significant bone, such as the shoulder joint. The result can be characterized as a literal “splat”, with no further penetration, no vital-area wound cavity produced, and little-to-no bullet remains sufficient to determine a mushroom.

Mann’s test data indicate the maximum wound cavity volume did not occur at the tested bullet’s maximum ER. As catalogued in Table 1, the maximum ER of 2.27 was produced at an impact velocity of 2740 fps (836 mps), with a corresponding WCV of 100 cc. The maximum WCV 0f 180 cc was produced at an impact velocity of 3190 fps (973 mps), with a lower corresponding ER of 2.12. Furthermore, Graph TP-2 illustrates the same ER of 2.12 can be produced at two different impact velocities.

Graph TP-2 shows an ER of 2.12 can result from an impact velocity of either 2400 or 3190 fps (732 or 973 mps). Although the ERs are the same, a bullet tested at an impact velocity of 2400 fps (732 mps) would likely produce a WCV of about 54 cc and a penetration of about 13.6 inches (34.5 cm). In contrast, the 3190 fps (973 mps) test impact velocity resulted in a WCV of 180 cc (about 3.3 times more), and a penetration of 15.3 inches (38.9 cm) (about 1.1 times more). Both Table 1 and Graph TP-2 demonstrate an expanding hunting bullet’s ER obtained from one test’s impact velocity cannot be relied upon to confidently evaluate its field wounding at any other impact velocity. As a consequence, gel testing should be performed on an expanding hunting bullet using a likely range of operational field impact velocities to realistically assess its field wounding capability.

Both Table 1 and Graph TP-2 also illustrate the tested bullet’s penetration can vary considerably, with its maximum penetration occurring at the lowest test impact velocity. Furthermore, these data show a bullet’s penetration does not decrease with increasing weight loss. As with evaluation of the WCV data, the test penetration data demonstrate gel testing should be performed on an expanding hunting bullet using a likely range of field impact velocities to realistically assess its field penetration capability.

Although Mann’s data demonstrate the tested bullet’s penetration at low impact velocities can be exceptional and can exceed the bullet’s penetration at high impact velocities, the WCVs produced at low impact velocities are also low. These WCV test data imply there is a lower-bound impact velocity where the tested bullet’s field wounding is so low that animal recovery is problematic, regardless of exceptional penetration. Furthermore, the reality of an unknown impact velocity beyond 3200 fps (976 mps) producing a zero-penetration, bullet structural-failure “splat” indicates the tested bullet also has an upper-bound impact velocity where a bullet’s failure to penetrate could also make animal recovery problematic.  

As just discussed, Graph TP-2 implies the tested bullet has an operational range of impact velocities where satisfactory field WCV and penetration can be expected. Such an impact velocity range has been personally interpreted and is shown on Graph TP-3. This graph identifies what is considered as a realistic and reasonable “sweet-spot” impact velocity range between 2500 and 3200 fps (762-976 mps). Test data indicate sufficient field WCV and penetration will likely be obtained within this impact velocity range, with a low-to-moderate risk expectation of the bullet’s catastrophic structural failure on bone that prevents penetration into a vital organ. Furthermore, Graph TP-3 logically indicates every expanding hunting bullet has a sweet-spot impact velocity range where satisfactory field performance can be expected, and that gel testing is required to properly assess its field wounding and penetration at representative impact velocities.

Graph TP-3 also shows both the WCV and the penetration of an expanding hunting bullet increase with increasing impact velocity within its “sweet-spot” impact velocity range. Both the increase in WCV and penetration with an increase in impact velocity have a basis in field-performance reality.

Graph 6 from the 2023 hunt report illustrates an increase in measured field wound volume occurred with an increase in impact velocity from 2605 to 2660 fps (795 – 811 mps) for a 200-grain Woodleigh Weldcore. Woodleigh has a published recommended impact velocity range of 2000 to 2900 fps (610 – 885 mps) for this bullet.  The 200 grainer’s impact velocities and field wound volumes identified on Graph 6 indicate it was producing an increase in wound cavity volume with increasing impact velocity within the manufacturer’s recommended “sweet-spot” impact velocity range.

The field penetration data for the 200 grainer, as shown in Table 5 of the 2023 hunt report, also indicate a reasonable correlation of a penetration increase with an increase in impact velocity. For the same zebras Z-7, Z-8, and Z-9 identified on Graph 6, the measured penetration lengths also increased with increasing impact velocity.  

All the referenced 200 Woodleighs penetrated the near-side shoulder bone interpreted to be the scapula, then penetrated from one to three near-side ribs. At the lowest impact velocity of 2605 fps (Z-8), the bullet penetrated 17-1/2 inches and was retained by the far-side hide. The intermediate impact velocity of 2615 fps (Z-7) resulted in a penetration of 20-1/2 inches before being retained by the far-side hide. The highest impact velocity of 2660 fps (Z-9) resulted in a penetration greater than 21-inches because the bullet completely exited the zebra.

Graph TP-1 illustrates a unique relationship between impact velocity and WCV with its attendant penetration for a specific bullet of a particular generic design. The implication is specific bullets with different generic designs can be expected to produce far different gel-tested wound cavity volumes and penetration lengths for any given impact velocity. As a consequence, specific bullets of different generic designs can be expected to have their own “sweet-spot” impact velocity ranges.   How a bullet’s generic design affects its wound cavity volume, penetration, and resulting “sweet-spot” impact velocity range is presented in a companion article found here.