Terminal ballistics: The wound cavity

Left to right: 20gr .17 HMR, .22 Hyper, .17 HMR 17gr and .22 subsonic

Following on from his introduction to bullet construction and terminal ballistics, Byron Pace looks at the specifics of the wound cavity as well as assessing rimfire performance

Previously, I’ve explained how three different bullet constructions left three different wound channel signatures in the soft tissue represented by ballistic gel. We could see the permanent wound cavity left by the path of the bullet, and the visual fracturing of the gel after it extended past its elastic limit. This month I want to add a little more detail on wounding profiles, explaining a few more terms, and give a basic overview of the different components of how the wound channel is formed. This will be important going forward as we tie it into bullet construction. Fear not though – the technical stuff won’t take up the entire article. We will also look at some very interesting results from a side-by-side analysis of rimfire calibres through ballistic gel.

It is important to understand what happens inside a medium as a bullet travels along its path. We all saw from last time the effect of a .308 Win bullet passing through ballistic gel. We ended up with a clear wound path consisting of a destroyed area directly in front and around the bullet’s path, and a series of escalating and declining fractures resulting from the point where the gel had been stretched beyond its elastic limit. Everything that can be seen is related to the temporary cavity, which is possibly the most important concept in terminal ballistics.

As a bullet contacts any medium, it will immediately begin to accelerate the particles in a radial fashion away from its path. Although the bullet will be continually decelerating, the inertia of the medium having been forced outward will result in a hollow space behind the bullet, and initially, a vacuum. What is interesting to note is that the temporary cavity does not reach its max diameter at any given point until after the bullet has passed. The extent to which the temporary cavity expands will not only depend on the medium being shot, but also on bullet weight, velocity at impact and bullet construction. I will expand on this further at a later date, as this is not the entire story.

It’s easy to see that it’s the nose of the bullet that initiates the separation of material along its path, with the rear of the bullet having no contact with the medium. This will of course change if the bullet deviates from a straight line trajectory – yaw and tumble will be part of a later discussion.

.17 HMR bullets either fragmented or increased in diameter

It is probably obvious from watching YouTube clips of ballistic gel being shot that the temporary cavity doesn’t last for very long. In fact it’s only a few milliseconds. What we see is the process of the energy contained within the traveling projectile being transferred to the target. In our case, the temporary cavity is the first stage of a successful kill. Anyone who has done basic physics will probably remember the law of conservation of energy, which states that energy cannot be created or destroyed, only transferred and transformed. Thinking of this in terms of terminal ballistics, all the energy stored by the travelling bullet has to be completely dissipated by the time it stops. Apart from the small amount of heat generated as the bullet passes through tissue, most of the energy is converted to elastic energy.

Tissue within the body of your quarry has a certain amount of elasticity. Pinching your own skin is a demonstration of this. In the same way that the ballistic gel expands and stretches as the bullet passes through it, so too will flesh (we will tackle the effects on organs later). The extent to which a medium will stretch is defined as its elasticity, which – for those engineers among us – is determined by Young’s modulus, which is a ratio of stress over strain. Intuitively, it is easy to see that the more elastic a medium is, the larger the temporary cavity will be, as a given amount of energy transferred from the bullet will push the medium more easily away.

What’s interesting is that after the temporary cavity expands and collapses, residual energy remains. This creates a secondary temporary cavity, expanding once again before collapsing. This continues in a pulsing motion until no elastic energy is left. Just like the declining height with which a ball bounces, each created cavity will be smaller than the last. During this time, debris can be pulled into both the entry and exit wound.

With the tissue relaxed back to its stable state, we are left with a permanent wound channel of destroyed tissue, and an area around this known as the extravasation zone. Unlike the permanent wound channel, which can be defined visually from the by the residual signature left behind in the ballistic gel, the extravasation zone shows no visual destruction. Here, the stretching imposed by the temporary cavity isn’t sufficient to tear tissue, but is enough to rupture sensitive parts of the body such as capillaries. Next month, with a little bit of basic maths, I will show you just how this translates into wound cavities in living tissue.

The .243 Win 75gr SST’s bullet path

For now, though, let’s look at how the rimfire calibres compare side by side. This is a good first insight into the stark difference bullet construction can have on wound channels. Most hunters will be aware of the devastating effect with which the .17 HMR knocks down a rabbit, and most will also have concluded that the 17-grain ballistic tip does this more effectively on small, soft-bodied animals than the 20-grain HP equivalent. Here we can see visually why this is, with the ballistic gel showing just how explosive the early energy release is from the fragmenting bullet.

In terms of stored energy, there is only 4ft/lb difference between the 20-grain and 17-grain at 100 yards, and virtually nothing at the muzzle. Yet, as is evident from the pictures, the effect in the gel is vastly different. This comes down to bullet construction. Almost the same amount of energy has been transferred to the gel, but each bullet has completed this at very different rates. If we look at the total depth penetrated (which has been averaged over a number of tests), we have a difference of 21cm. That means that the 20gr hollow point had more than double the penetration of the ballistic tip. On the flipside, the initial expansion of the B-Tip is very violent, and many times the diameter of the 20-grain bullet. In fact, there isn’t much between the 20-grain HP and a .22LR bullet with regard to the wound channel left behind.

If we go on to look at the wound profile left in our ballistic soap, a similar theme is evident, with the reduced penetration of the B-Tip, and smaller overall cavity left behind. Bear in mind that everything is considered three-dimensionally, so although the difference between the two profiles is not as extreme as you might have thought, the depth of the cross-section actually amplifies small variances in straightforward diameter measurements.

As I stated in the last article, using soap as a medium allows us, to some extent, freeze the temporary cavity created. This shows an interesting result when considering bullet construction, something overlooked in other authors’ analysis of bullet construction terminal effects.

Here’s how the Geco Plus and Express performed in soap

It is clear to see from both the .17 HMR comparison and the Geco .308 Win tests that bullets designed for penetration, with high weight retention (see table), provide smooth, gradually increasing wound profiles before tapering off. On the other hand, bullets designed for more explosive expansion, such as a V-Max and, to a lesser extent, the Geco Express (which is not a fragmenting bullet), show ragged channels that are far less even. In both cases, and exaggerated in the .17 V-Max owing to its total fragmentation, the shedding of lead and copper provide secondary projectiles, each creating their own new wound channel.

It is for this reason that using a plastically deforming ballistic material (such as soap) cannot tell the whole story. It is clear from the analysis of the gel that the difference in terminal effect in the first inch of the 17-grain bullet is vastly different to that of the 20-grain bullet. Although the soap shows a more delayed expansion, the true extent of the permanent wound profile is hidden. Having said that, there is a fixed relationship between what we see in the soap and the lasting result in tissue when we are looking at non-fragmenting bullets. Indeed, as the start of the article suggested, all terminal ballistics can be tied back to the temporary cavity.

A final example of this is clearly shown in the side-by-side profiles produced from the Geco Plus and Geco Express. Given that the Plus is designed for deep penetration on large game, we expect to see a slower, more gradual expansion build-up, holding a longer, wider tail end than the Express. This is borne out in the stats.

Thanks to Defensible Ballistics: www.defensible.co.uk