Wound Ballistics: A Bullet’s Life on the Inside, By Mike McTee

Hunters spend plenty of time arguing about whether certain rifle calibers will do the job, but the topic of wound ballistics ought to enter the conversation too. Wound ballistics is a facet of terminal ballistics that describes what a bullet does after it hits hair. Bullet manufacturers advertise the wound ballistics caused by their bullets by tossing around phrases like “knock-down power” or “devastating terminal performance.” Yet understanding how a bullet accomplishes those feats can be tricky.

When you turn to shooting experts, they often cite personal experience and manufacturer testing. In some cases, they shoot bullets into ballistic gels or similar mediums intended to replicate animal or human tissue. These tests tell the shooter how well their bullet penetrates and retains weight, but ballistic gels lack skin, bone, and organs, all of which influence a bullet’s performance. Ultimately, a ballistic gel won’t tell the shooter if their bullet will drive past bone or blow apart and fail to penetrate.

In the last decade, however, scientists have been testing the theory of wound ballistics in wildlife. Researchers measured weight retention in bullets collected from hunter-harvested moose in Norway, Sweden, and Finland. Researchers in Germany, using medical imaging techniques, measured wound diameters created by various bullets.

These studies have churned up surprising findings. Bonded lead bullets, which are designed to be tougher than lead core bullets, broke apart on bone just as often as their more inexpensive counterparts. Are bonded bullets worth the extra cost? Also, lead and copper bullets created similar sized wound channels. These results allow us to skip the personal anecdotes and zoom straight to the data.

Ballistic Theory Behind the Hair

The science of wound ballistics started with forensic and military research. Dr. Joe Caudell, a biologist at Murry State University, applied this science to wildlife. At the time, he worked for Wildlife Services, a branch of the United States Department of Agriculture that deals with conflicts between humans and wildlife. They often manage animals in tight spots, like in urban areas, where they can’t afford a stray bullet. Like hunters, they must understand bullet performance.

Caudell found that shot placement matters in his 2013 review of the wound ballistic literature in the academic journal Wildlife Society Bulletin. For big game to die instantly, the bullet must strike the central nervous system. But for most hunters, aiming for the brain or spine doesn’t give enough room for error; an ill-placed bullet or sudden movement from the animal could cause a miss or a wounding. This is why instructors in hunter education classes teach students to target the heart and lungs where an animal will die quickly from blood loss.

This illustration compared the different terminal ballistics of two bullet types, adapted from “Review of Wound Ballistics Research and Its Applicability to Wildlife Management”, by J.N. Caudell, 2013, Wildlife Society Bulletin, 37, pp. 824-831. DOI: 10.1002/wsb.311

Dr. Caudell argues that bullets kill two primary ways. First, they create a central cavity, sometimes called a permanent cavity or wound channel, as they crush and tear tissue. Bullets that have greater diameters create wider wound channels, but it’s also critical for bullets to expand, or “mushroom”, which is driven by bullet velocity and construction. Penetration depth plays an important role, too, because longer wound channels mean more damage to vital organs and greater blood loss.

The second way a bullet causes damage is by creating a temporary cavity. As a bullet penetrates elastic tissue, such as muscle and lungs, tissue separates from the wound channel as if a balloon was rapidly being blown-up from the inside. Slow-motion videos of ballistics gels being shot demonstrate the concept well. Bullets that have blunt shapes or travel at high velocities produce the largest temporary cavities, and when the size of the cavity exceeds the elastic limits of the tissue, the tissue tears and the size of the central cavity grows. This explains why small game, like prairie dogs, seemingly vaporize when shot with high-velocity rounds (e.g. .220 Swift; >4000 fps).

When considering big game, temporary cavities cause more damage to less elastic organs that can’t undergo significant stress, like a kidney or heart. In cases where a bullet fragments, the fragments could, in theory, increase the size of the wound channel by damaging tissues under strain by the temporary cavity.

Bullets may destroy tissue in other ways, such as by creating a shock wave, which is a type of sound wave also used in medicine to break up kidney stones. Shock waves travel through liquid filled tissues without displacing mass, so many researchers believe they cause too little tissue damage, if any, to be considered a main factor toward incapacitation.

Boiling Down Bullet Construction

Cross sections of a lead core bullet (left) and copper bullets (right). Photo credit: Conservation Media

Radiograph of a lead core bullet (top) and a copper bullet (bottom) shot into ballistic gelatin. Photo credit: Conservation Media

Even though hunters use dozens (if not hundreds) of different rifle cartridges in various calibers, bullet construction generally falls into several broad categories that researchers compare. They usually include lead core, bonded lead, and copper. These categories don’t encompass all bullet constructions, such as partition bullets, but they represent the majority used by hunters.

Of these three main bullet constructions, lead core bullets are currently the most common and inexpensive. The lead core often extends past a copper or copper alloy jacket mantel (aka jacket), although some bullets have a polymer tip instead. A downside of lead core bullets is that they can have issues with post-impact weight retention that may limit penetration.

The components of bonded lead bullets resemble those of lead core bullets, but the bonded variety have their jacket chemically bonded to the core. This construction makes a bullet that should be tougher and retains weight better than a lead core bullet, but it often comes at a steeper price.

Copper bullets, also called monolithic bullets, are harder than lead, so they retain nearly all their mass upon impact. Hunters can expect long wound channels even with lighter bullets. The tips of the bullets often have a hollow cavity or polymer tip that aids in expansion. Traditionally, these bullets priced similar to premium bonded bullets, but presently, their cost and availability has vastly improved.

Weight Retention, Penetration, and Expansion on Scandinavian Moose

Bullets recovered by moose hunters in Norway, Sweden, and Finland. Photo credit: Lasse Botten

In a 2017 study published in Wildlife Society Bulletin, Dr. Sigbjørn Stokke, from the Norwegian Institute for Nature Research, and his colleagues examined the weight retention of bullets used on moose in Norway, Sweden, and Finland. Over 5000 hunters sent the researchers information about the rifle caliber and ammunition they used, as well as other details about their hunt. If hunters recovered a bullet from the moose, they mailed it to the researchers to be weighed.

When caliber classes were combined, weight retention by each bullet category varied drastically. Copper bullets retained more weight (85-100%) than lead core (74-82%) or bonded lead bullets (75-90%).

Lead bullets, in general, retained less weight when they struck bone (~71%) compared to soft tissue (~77%). Bonded bullets retained more weight on soft tissue than lead core bullets, but surprisingly, when they hit bone, their jackets separated from their core as often as lead core bullets. This finding upends the whole point of using pricier bonded bullets that supposedly come with a performance advantage. A glaring question remains. Does bullet fragmentation limit penetration?

The researchers answered that question in a 2019 study published in Ambio. Even though lead bullets lost more mass, they passed through moose just as often as copper bullets. The researchers also found that bullet construction played no role in the distance that moose and other big game fled after being shot.

So, does bullet type even matter? When researchers measured bullet expansion, copper bullets tended to be broader than lead bullets. In theory, a wider bullet would rip through more tissue to create a larger wound channel, but does that theory hold up under field testing?

Comparing Wound Diameters

The photo shows damaged organs recovered from a roe deer carcass. Researchers used computed tomography and direct observation to measure wound diameters created by different bullets. Photo credit: Anna Trinogga

In a study published in Science of the Total Environment in 2013, Anna Trinogga and her colleagues at the Leibniz Institute for Zoo and Wildlife Research in Berlin, Germany, measured wound diameters in hunted animals shot with a variety of lead or copper bullets.

They examined a smorgasbord of animals, including wild boar, roe deer, red deer, fallow deer and chamois. The researchers used computed tomography–which is basically X-ray imagery taken from different angles–to produce a cross-sectional image of the bullet trauma in each carcass. They estimated and compared wound diameters created by different bullets based on the imagery and direct observations.

Remarkably, the diameters of wound channels didn’t differ between lead core and copper bullets. If copper bullets had wider expansion, as discussed previously, it didn’t result in wider wound channels. Also, even though bullet fragments could, in theory, create new channels that further destroy tissue that’s under strain by the temporary cavity, the study found no evidence that fragments increased the size of the central wound channel.

The researchers gleaned other useful findings too. The sizes of entry holes were drastically smaller than the diameter of internal damage. For instance, the average diameter of entry holes were often less than half an inch, yet the estimated means of internal wound diameters ranged from 1.5–2.5 inches. Exit holes were often about 7–8 square inches and didn’t differ based on whether they were created by a lead or copper bullet.

Shooting Straight About the Science

Conducting wound ballistic research in wildlife presents challenges. One obstacle is generating a sample size large enough to see statistical differences. This is tough when each shot might have a different distance, angle, and path through the animal. It’s important to note that many of the shot distances in these studies were less than 150 yards. Longer shots might yield different velocities and impact energies, depending on bullet type, cartridge, and caliber, which would affect bullet performance. Another potential limitation of the studies is that within each bullet category (e.g. lead core, copper), researchers included many brands of bullets that may have performed differently. Even though grouping various bullets into categories might slightly limit a study’s applicability, these results still provide the best evidence yet of how hunting bullets perform on big game.

Maybe this won’t help you choose the next bullet you chamber, but it might help you interpret the mechanics of how that bullet performs on your next hunt. And if most bullets perform similarly upon impact, perhaps we should focus on shot placement, because in the end, a fancy bullet won’t save bad aim.

Posted by Nolan Osborne