Is a Bullet’s Expansion Ratio (Mushroom Size) a Valid Metric to Evaluate Wounding?

By Scott Fletcher

“Evaluating a bullet’s mushroom is like judging a woman’s derriere: size and shape accrue no technical merit, only highly subjective style points.”  -  Scott Fletcher

The answer to the question posed by the article’s title is “no”, more likely than not.

A bullet’s expansion ratio, ER, is equal to its mushroom diameter divided by its caliber. For example, a bullet with an ER of 2 has a mushroom diameter 2 times greater than its caliber. The interpreted intent of determining a bullet’s ER is to evaluate its wounding potential based on a test or actual wounding in an animal.

The wound in an animal caused by a bullet has a volume. When a bullet’s ER is used to evaluate wounding, the wound is presumably in the approximate shape of a cylinder comprised of both a length and an end-area dependent upon the diameter of the bullet’s mushroom. The inferred basis for judging wound “length” is penetration, and the inferred basis for judging wound “end area” is the circular area computed using the diameter of the bullet’s mushroom.  The implied assumption is that the mushroom diameter of the bullet retained in either the test media or the animal is representative of its diameter throughout the majority of its penetration length.  

Any determination of expansion ratio only reflects the bullet’s diameter at the end of its penetration journey through either the test media or the animal. During the penetration journey through media, a bullet can experience radical changes in its mushroom diameter based on its generic design and impact velocity. During a bullet’s journey through an animal, it can also experience radical changes in mushroom diameter not only due to its generic design and impact velocity, but also due to breaching bone of variable thickness and passaging through tissue of variable tenacity/density. Consequently, the mushroom diameter and corresponding ER of any retained bullet may not represent an assumed uniform, minimum, or maximum value throughout its penetration length.

An example of a bullet’ mushroom diameter variation during penetration through test media is described in the 2023 management hunt report. As discussed in report section 8.3.3, the 220 Sierra Pro Hunter (SPH) could not maintain its initial mushroom diameter during passage through the 20% synthetic gel. Based on report Photo P-13, the initial mushroom was apparently stripped away because its end area could not withstand the imposed drag force. Gel-cavity diameter measurements, tabulated in report section 8.3.3, substantiate this mushroom diameter decrease.

These measurements also indicate that the drag forces imposed by the gel apparently re-formed the mushroom. However, interpretation of Photo P-13 indicates the bullet continued to spall shrapnel-sized shards after this mushroom re-formation. This spalling indicates the mushroom was being adversely affected, with the result potentially being some progressive decrease in its diameter. Regardless, these mushroom-spalling, mushroom-re-forming, and mushroom-spalling cycles indicate the bullet’s mushroom diameter was not uniform and varied to an unknown degree along its penetration length.

The ER of the recovered test bullet does not reflect this diameter variation. Consequently, using the retained bullet’s ER in even a simplistic evaluation of modeled wound volume is misleading, and may or may not provide a reasonable expectation of field performance.

Some degree of mushroom spalling can be expected from all expanding hunting bullets due to breaching bone. The degree of spalling is dependent on the bullet’s impact velocity, its generic design, the strength of the materials that comprise it, the thickness of the bone, and the angle of the bullet’s impact on the bone. Regardless, breaching bone can be expected to cause mushroom spalling that is far more severe than spalling that would occur in typical test media. Consequently, the bullet obtained from an animal can have a far different ER than one obtained from media testing, even if both the media-tested bullet and the field bullet have similar impact velocities. Judgements of field wounding caused by the retained bullet based on its field ER can be far more misleading because of significantly greater spalling caused by breaching bone.

The bullet recovered from Zebra Z-1 on the referenced management hunt is an example of significant mushroom spalling during penetration through an animal.  Review of the ER data tabulated in Table 5 indicates the recovered bullet ought to have a field expansion ratio greater than 2 based upon testing in 20% synthetic gel. However, the recovered bullet had an ER of 1.29, about 41% less. As indicated in Table 5, the bullet breached multiple ribs and the spine on its way through the boiler room. The retained bullet’s low ER value is likely reflective of having a considerable portion of its mushroom stripped away during that breaching process.

Photo 1 shows the recovered gel-tested bullet on the left, and the bullet recovered from Z-1 on the right. The near-perfect shaped mushroom with a 2x+ ER value obtained from the gel-tested bullet can give the expectation that a similarly high ER value “should” be obtained from a bullet recovered from an animal. However, the mushroom of the bullet recovered from Z-1 is obviously “different”, and could be judged as “ugly” and anemic compared to the mushroom of the bullet recovered from the 20% gel.

The disparity in how these mushrooms “look” could cause some hunters to inappropriately conclude that the bullet recovered from Z-1 exhibited marginal-to-unacceptable performance, even though the bullet breached two near-side ribs, the spine, the boiler room, and a far-side rib before being retained on the far-side hide. Such a judgment would also ignore the excellent wounding produced in the recovered animal.

The data in Table 5 and Table 4 of the 2023 management hunt report exemplify the fallacy of attributing the volume of wounding in an animal to the recovered bullet’s ER. Photo 2 shows the bullet recovered from Zebra Z-7 on the left and the bullet recovered from Z-1 on the right. As shown in Table 5, the ER of the bullet recovered from Z-7 is 2.11, about 64% greater than the ER of 1.29 from Z-1, and is in keeping with the test-bullet’s ER of 2.18. Yet Table 4 shows that the wounding in Z-7, as determined by total bullet hole volume (TBHV), is about 80 % less than the TBHV determined for Z-1.

Photo 3 shows the bullet recovered from Zebra Z-8 on the left and the bullet recovered from Z-1 on the right. Similarly, the ER of the bullet recovered from Z-8 is 1.96, about 52% greater than the ER of 1.29 from Z-1. Again, Table 4 shows that the TBHV determined in Z-8 is about 60% less than the TBHV determined in Z-1.

The significant disparity in observed wounding between Z-1 and both Z-7 and Z-8 cannot be explained based simply on the retained bullets’ ERs. The technical basis for these explanations resides in the discussions and attendant conclusions presented in sections 12 and 13 of the 2023 management hunt report. Regardless, the reality of the actual wounding magnitude produced by these bullets is far more relevant than the veracity of any “technical explanation” of how the wounding was produced in each animal. Furthermore, the real-world examples just discussed demonstrate that the actual wound produced by any bullet, quantified by its penetration length and measured wound volume, should be the basis for judgements concerning its terminal performance, not the end-condition of the bullet produced by its wounding journey.