How an Expanding Hunting Bullet Creates a Wound Cavity, Part 2: The Effect of Generic Design.

By Scott Fletcher

“It’s about the bullet, isn’t it?” – Donnie from Africa Hunter Quest

As discussed in Part 1 and illustrated by Graph TP-1, an expanding hunting bullet’s impact velocity controls both the rate and the extent of its mushroom deformation that directly affects the resulting wound cavity volume and penetration length. The concepts presented are applicable to all expanding hunting bullets of any generic design, not just the one tested.

The concepts presented in Part 1 resulted from critical analysis of Guppy metric values obtained from personally testing 11 bullets in 20% synthetic gel. (Guppy metrics are mathematically defined in Guppy Tech.) Gel testing was performed on four of the five personally interpreted expanding hunting bullet generic designs. The evaluation of the wound cavities produced by these various generic designs demonstrated the bullet’s design features, the properties of materials used in its fabrication, and the procedures used during manufacturing either singularly or in combination controlled the Guppy-metric test values and the resulting wound-cavity configuration observed in the gel-test blocks.

This article presents the conceptual mechanics of wound cavity formation in Guppy-metric terms, using 20% synthetic gel test results from bullets of the same caliber representing two different generic designs as examples. These bullets are shown in Photo 1. All are 35-caliber, fired from a 358 Winchester into gel blocks placed at 135 yards (123 m), the average distance of 18 shots taken on animals in the Limpopo Province of South Africa. From left to right, a 225-grain Sierra Game King; a 225-grain Barnes TSX; a 225-grain Barnes TSX modified by hand-installing a “small” 0.091-inch (2.3 mm) shank-diameter poly tip; and a 225-grain Barnes TSX modified by hand-installing a “large” 0.115-inch (2.9 mm) shank-diameter poly tip.

As discussed in Part 1, gel testing at multiple impact velocities, including ones expected in realistic hunt circumstances, is required to accurately evaluate an expanding bullet’s likely wound cavity volume and penetration field performance. However, real-world limitations of personal time and finances prevented such comprehensive testing of the referenced bullets. Testing performed at the selected average test distance is considered a reasonable compromise, allowing comparison of test-metric values to assess the field performance that could be expected from each bullet for a “typical” hunt circumstance.

Table 1 catalogues each bullet’s interpreted generic design, nomenclature, and abbreviations used in this article. Table 2 presents the Guppy metrics and their test values used in the following narrative to describe the wound cavity formed by these bullets in the gel. The mathematical definitions of the Guppy metrics can be found here.

The generic design of the Sierra is interpreted as a cup-and-core. The materials used in its construction consist of both a metallurgically “soft” copper-alloy jacket and a lead-alloy core. The jacket’s material is comparatively “stronger” than the lead-alloy core it surrounds. However, the jacket is “thin” and does not appreciably contribute to the bullet’s ability to resist mushroom formation because of its thickness, and there is no bond between it and the lead-alloy core it surrounds. The result is a plane of weakness (discontinuity) at the jacket-core interface that prevents the metallurgically “stronger” copper-alloy jacket from effectively reinforcing the softer lead-alloy core. Without such reinforcement, both of these materials deform rapidly immediately upon impact.

The alloy content of the jacket and core can be adjusted to increase their hardness and resistance to deformation. However, the cup-and-core’s generic design includes no premeditated features or manufacturing steps to limit the actual diameter of the mushroom. The result is this generic design has the potential for unlimited mushroom expansion, no matter what the metallurgy of its components may be.

The potential for unlimited expansion, combined with the discontinuity between the relatively soft jacket and the core materials, can allow a relatively large mushroom to form. The large mushroom results in a large drag force that can limit penetration. Furthermore, the large drag force produces shear stress in both the jacket and the core materials that can exceed each material’s shear strength. The result is both materials can easily spall (shear) during penetration, with both jacket and core shards being individually propelled radially away from the bullet’s aim alignment. Such shards can be correctly characterized as shrapnel, and contribute to wounding as described in section 8.3.3 0f the hunt report, and in this article. Such mushroom spalling is quantified as bullet weight loss, the inverse of retained weight.

The Sierra’s tip is an exposure of its underlying lead-alloy core. This large exposure of core at the tip is a premeditated design feature that also results in a rapid mushroom formation. Depending on the metallurgy of the jacket and the core materials, the net result of the previously discussed design features is a cup-and-core bullet with an exposed lead tip can be expected to deform rapidly upon impact, with typically a very quick mushroom formation.

Article Format Note: The remainder of this article is “numbers intensive”, using values from Table 2 that are based on US measurement units. For simplicity and to remove article “clutter”, no metric units are provided. Those wishing to convert to metric units of measure can use the following conversion factors: to convert feet to meters, multiply by 0.305; to convert inches to centimeters, multiply by 2.54; and to convert cubic inches to cubic centimeters, multiple by 16.4.

The first gel block for the 225 Sierra SGK is shown in Photo 2. As indicated by this photo and demonstrated with data in Table 2 (visually supported by the Guppy model), the rapid blossoming of the Sierra’s mushroom is indicated by the length to the maximum cavity diameter value, L(Dmax), of only 5 inches. (Testing of other bullets with different generic designs indicated values ranging from 4 to 7 inches). Because there are no design features or manufacturing steps to limit mushroom expansion, a comparatively large mushroom is allowed to form, as indicated by its ER value of 2.17. This large mushroom results in a comparatively large maximum cavity diameter, D(max), of 2.078 inches. (Testing of other bullets with different generic designs indicated values ranging from 1.392 to 2.334 inches). The comparatively large mushroom diameter limits penetration, as indicated by both a bloodshot cavity maximum length value, L(S) of 11 inches (test values of other bullets ranged from 9-1/2 to 13-1/2 inches) and a total penetration length value, L(T), of 19-1/2 inches (test values for other bullets ranged from 19 to 36-1/2 inches). Furthermore, the rapid and extensive blossoming of the Sierra’s mushroom is also indicated by its comparatively high violence index value, I(V), (refer to Guppy Tech) of 11.4 (test values of other bullets ranged from 3.6 to 15.1).

The rapid and extensive mushroom formation of the Sierra limited its penetration, as indicated by both its L(S) and its L(T) values. This limited penetration resulted in moderate wound cavity volume compared to other bullets tested. The total cavity volume within the modeled bloodshot cavity’s periphery, V(ST), (refer to Guppy Tech) is 17.2 cubic inches (test values of other bullets ranged from 11.1 to 26.4 cubic inches). The total wound cavity volume’s value, V(T), is 19.7-cubic inches (test values of other bullets ranged from 13.1 to 28.4 cubic inches).

All the Barnes bullets are fabricated entirely from pure copper, and its interpreted generic design is “solid copper”. If a bullet fabricated from pure copper is unaltered by design features or a manufacturing process, it is much stronger than a bullet fabricated from both copper and lead alloy.  As a consequence, it can be far more resistant than a cup-and-core bullet to the deformation force that tries to form it into a mushroom. Furthermore, the increased shear strength of a solid (pure)-copper bullet better resists spalling at the tip, resulting in less bullet weight loss.

As shown in Photo 3, the finished solid-copper bullet has a small diameter hole of limited depth at the tip to initiate expansion. Some as-manufactured solid copper bullets have an embedded poly tip that serves as a wedge, a premeditated design feature to also initiate expansion. Through manufacturing wizardry, the interior surface of the hole in the tip is scored with limited-depth incisions in the hole’s periphery, typically oriented 90 degrees apart. These incisions weaken the bullet’s tip to allow it to blossom upon impact into four equal petals of copper, as shown in Photo 4. These petals typically form into a symmetric mushroom shape. The extent of the mushroom diameter can be controlled by both the premeditated depth of the hole in the tip and by the depth and the premeditated length of the incisions in the nose. The tip can also be annealed (softened) during manufacturing to a premeditated length to control both the rate and the extent of mushroom formation.

Table 2 shows test data for a box-stock 225 Barnes TSX as well as two others that were personally modified by hand-installing both a “small” and a “large” diameter poly tip. The initial testing was performed on the stock 225 BTSX. As indicated in Table 2 and the first gel block shown in Photo 5, the stock 225 BTSX demonstrated that it would not expand to produce a substantive wound cavity at its impact velocity of 2219 fps. Such performance was both unexpected and unsatisfactory, regardless of the implied extraordinary penetration that was achieved.

The “small” shank-diameter poly tip was subsequently hand-installed to determine if it could force the bullet’s tip to expand at a comparable impact velocity. The intent of the installation was to weaken the tip area by drilling a significantly larger hole required to insert the tip. The drilling process would physically remove a significant volume of copper that contributed to the bullet’s ability to resist deformation. Furthermore, the impact force would cause the poly tip to serve as a wedge to positively initiate tip expansion upon impact.

The first gel block for the 225 BSTTSX is shown in Photo 6. As indicated by this photo and demonstrated by the data in Table 2, the tip installation successfully initiated expansion that produced a substantive wound cavity. The 225 BSTTSX’s impact velocity was only 39 fps greater than the impact velocity of the stock 225 TSX. The relatively modest increase in impact velocity indicated that the tip installation was likely primarily responsible for the wound cavity that was produced.

The 225 BSTTSX’s test results were compared to the 225 SGK’s test results that had been previously obtained. (Refer to Table 2.) Installing the poly tip had resulted in a rapid initial mushroom formation that was comparable to the SGK’s, as indicated by the L(Dmax) values of 5-1/2 and 5 inches, respectively. However, the 225 BSTTSX’s metallurgy and manufacturing steps limited its ER to only 1.60 compared to the 225 SGK’s ER of 2.17. The 225 BSTTSX’s significantly smaller ER resulted in a significantly smaller D(max) than the 225 SGK’s (1.743 inches compared to 2.078 inches), but both a longer bloodshot wound cavity length, L(S) (13-1/2 inches compared to 11 inches) and a longer total penetration length, L(T) (36-1/2 inches compared to 19-1/2 inches). However, the bloodshot wound cavity volume, V(ST), of the 225 SGK’s was greater (17.2 cubic inches compared to 16.0 cubic inches) as was the total wound cavity volume, V(T), (19.7 cubic inches compared to 19.1 cubic inches) attributed to its significantly larger mushroom diameter. Although the L(Dmax) values were comparable, the much larger mushroom diameter of the 225 SGK’s resulted in in a much larger violence index, I(V) (11.4 compared to 3.6).

Because the 225 BSTTSX’s wound cavity volumes were less than the 225 SGK’s, a “large” poly tip with a larger shank diameter was hand-installed. The premeditated intent was to weaken the tip even further to initiate faster mushroom formation and potentially increase the resulting ER. In doing so, the length to the maximum bloodshot cavity diameter, L(Dmax), was expected to be be reduced; the maximum wound cavity diameter, D(max), was expected to increase; the bloodshot cavity maximum length, L(S), was expected to decrease; and the total penetration length, L(T), was expected to decrease. However, both the bloodshot wound cavity volume, V(ST), and the total wound cavity volume, V(T), were expected to increase due to an expected   larger mushroom diameter. Because of the more rapid formation of the total bloodshot wound cavity volume and a larger ER, the violence index, I(V), was expected to increase.

The first gel block for the 225 BLTTSX is shown in Photo 7. As indicated by this photo and demonstrated by the data in Table 2, all the postulated Guppy-metric value changes occurred. Compared the 225 BSTTSX’s metric values, the 225 BLTTSX’s ER increased from 1.60 to 1.68; its L(Dmax) decreased from 5-1/2 inches to 5 inches; its D(max) increased from 1.743 inches to 1.817 inches; its L(S) decreased from 13-1/2 inches to 13 inches; its L(T) decreased from 36-1/2 inches to 33-3/4 inches; its V(ST) increased from 16.0 cubic inches to 18.0 cubic inches; its V(T) increased from 19.1 cubic inches to 20.9 cubic inches; and its I(V) increased from 3.6 to 8.6.

The previous comparison-and-contrast discussion of the Guppy-metric values for the cup-and-core Sierra and all of the solid-copper Barnes demonstrates the significant effect that a bullet’s generic design has on its terminal performance characteristics.  A bullet’s specific design features, the properties of the materials used in its construction, and the effect of manufacturing processes during its fabrication all contribute to the magnitude of Guppy metric values identified in Table 2. Furthermore, the effect of a specific design feature on the wound cavity produced is clearly shown by the Guppy-metric values of the Barnes bullets. Simply installing a poly tip in the basic solid copper design to positively initiate expansion was necessary to produced wound cavities comparable to the Sierra’s but with at least 1.7 times greater total penetration, as indicated by L(T) values.

Graph TP-2 in Part 1 of this article illustrates the fallacy of relying on the tested bullet’s ER, rather than WCV data directly obtained from a gel test at specific impact velocities, to assess wounding potential. Likewise, the Guppy-metric values of V(ST) and V(T) shown in Table 2 also underscore the fallacy of relying on an expanding hunting bullet’s ER to comparatively assess the wounding potential of same-caliber hunting bullets of different generic designs.

As an example, Photo 8 shows the recovered 225 SGK’s mushroom on the left and the recovered 225 BLTTSX on the right. Without benefit of gel-test wound-volume metric values, the 225 SGK’s obviously larger mushroom ER of 2.17 implies a test wound volume superior to the 225 BLTTSX with its ER of 1.68. However, the actual V(ST) and V(T) cavity volumes of the 225 BLTTSX obtained from gel testing are both greater than those of the 225 SGK. The reasons are the L(Dmax) value of the 225 BLTTSX indicates it expanded quicker than the 225 SGK (5 inches compared to 5-1/2 inches). The more rapid mushroom expansion of the 225 BLTTSX, coupled with its smaller mushroom ER allowed it to achieve a superior L(S) of 13 inches compared to the 225 SGK’s of 11 inches. This 2-inch increase in L(S) allowed the 225 BLTTSX to achieve a V(ST) volume of 18 cubic inches, 0.8 cubic inches greater than the 225 SGK’s. The reduced mushroom ER of the 225 BLTTSX also allowed it to penetrate farther than the 225 SGK (33-3/4 inches compared to 19-1/2 inches), with a resulting increase in V(T) (20.9 cubic inches compared to 19.7 cubic inches). As demonstrated with data catalogued in Table 2, obtaining gel-test Guppy metric values that include wound cavity volumes at a typical hunt-specific shot distance is obviously superior to using mushroom ERs for comparatively assessing each bullet’s likely field wounding capability.   

The Guppy-metric values in Table 2 also underscore the dependency of each generic design’s impact velocity to produce satisfactory field terminal performance. As indicated by discussion in Part 1 and illustrated by Graph TP-3 , an impact velocity outside of a bullet’s “sweet-spot” impact velocity range may not produce desired or even satisfactory field terminal performance. As an example, the failure of the stock 225 BTSX to expand at 2219 fps demonstrates it was obviously not operating within its “sweet-spot” impact velocity range. As a consequence, the 225 BTSX would likely be a poor bullet choice for use with a 358 Winchester chambering. A lung shot on a North American whitetail deer at 135 yards using this cartridge-bullet combination could potentially result in an unrecovered animal. The 225 BTSX’s reluctance to expand at the expected impact velocity of about 2220 fps could result in an unrecovered animal because of anemic/insufficient wounding that potentially allows an extended travel distance after the shot or a travel distance into terrain/vegetation incompatible with tracking. The bottom-line implication of the wound cavity volumes identified during the gel testing is a bullet of any generic design may not work satisfactorily in all cartridges and hunt-specific applications because the actual field impact velocity may not be within its “sweet-spot” impact velocity range.

In contrast, Guppy metric gel-test values obtained for the Sierra and both tip-modified Barnes indicate all are likely within their “sweet-spot” impact velocity range for a shot out to at least 135 yards when launched from a 358 Winchester. Guppy-metric values shown in Table 2 indicate any of these bullets launched from this cartridge would be far preferrable to the stock 225 BTSX for use in the previous example, likely producing wounding and penetration that would result in an easily recovered animal.

Colonel Whelen had no benefit of testing bullets in ordinance gel to evaluate an expanding hunting bullet’s terminal performance, as ordinance gels had not yet been developed. His criteria of both impact velocity and generic design controlling an expanding hunting bullet’s lethality are reality based, the result of evaluating actual field wounding in numerous animals produced by various cartridge-bullet combinations. As demonstrated in this two-part article, applying the Guppy model and Guppy-metric values to a wound cavity formed in gel by an expanding hunting bullet reasonably provides the conceptual and scientific basis for the validity of Colonel Whelen’s observations.