Terminal ballistics is in many cases, the least considered aspect of ballistics when determining the kind of bullet to use for a specific purpose. Yet, it's an important process to understand if you want to achieve optimal results. Terminal ballistics is the study of how a projectile behaves when it hits its target and transfers its kinetic energy to the target. The bullet’s design, as well as its impact velocity, plays a huge role in how the energy is transferred.
Also known as wound ballistics, terminal ballistics is important to hunters because it illustrates how a particular bullet will transfer its potiental energy when it strikes the target. Ethical hunters want as quick and humane a kill as possible, and death is ultimately caused in one of two ways:
- Severely interrupting or stopping the flow of oxygenated blood to the brain by damaging a major blood-bearing organ or by causing significant damage to the vascular system; or
- Causing severe damage to the brain and/or cerebellum.
Bullet design plays a very siginificant role in what kind of wound cavity the bullet will make in soft tissue. The type of wound cavity is critical to the quick and humane kill that hunters owe to the animals they hunt.
Permanent and Temporary Cavitation
The primary way a bullet causes damage to an animal is through the permanent cavity it leaves - the hole that is created as the bullet passes through skin, bone or flesh. This wound channel is the same diameter as the bullet/bullet fragments and is a function of bullet penetration and expansion.
A secondary way that a bullet causes damage is by the temporary cavity it causes. When a bullet hits soft tissue, the tissue acts more like a fluid than a solid as it gives way and tries to absorb the bullet’s energy. The bullet does not immediately penetrate the tissue; instead it makes an impact crater that stretches in until the bullet penetrates the tissue.
As the bullet continues its path, it violently pushes the tissue ahead of it both directly and indirectly in such a way that the tissue is stretched beyond its elasticity and is cut and torn as it quickly tries to return to its original position and beyond.
In essence, a bullet going through soft tissue has the same effect as dropping a stone into a pail of water - if the stone (bullet) enters the water slowly, the water (tissue) displacement is so gradual that is has little effect on the surrounding molecules. If the stone (bullet) enters the water (tissue) with a lot of momentum, however, the surrounding molecules have to act a lot more quickly and violently, resulting in a splash (temporary cavity). Temporary cavitation is important because it can be a tremendous wounding mechanism.
Both permanent and temporary cavities are greatly affected by a bullet’s design, sectional density and velocity at the time of impact.
How Bullet Design Affects Cavitation
Bullets designed with heavier jackets tend to stay together better and penetrate deeper, while lighter jacketed bullets tend to fragment and expand more rapidly, creating a wider wound channel and increasing temporary cavitation, but generally don’t achieve as much penetration.
Expansion can be controlled by the bullet’s tip design and by the jacket construction. Rapidly expanding bullets create a wider wound channel, displacing even more tissue and increasing temporary cavitation, but they also increase drag, thus requiring more energy and momentum to drive through tissue, and in general don’t penetrate as deeply as bullets designed to expand more slowly. Expansion is beneficial, but penetration is essential in order to ensure that the bullet reaches the animal’s vitals.
A bullet’s sectional density also affects the amount of damage it can cause. Sectional density (a bullet’s weight in pounds divided by its diameter squared) describes a bullet’s length for its diameter: The higher the number, the longer the bullet. Generally speaking, the larger a bullet’s sectional density, the deeper it will penetrate.
The third thing mentioned, velocity, also can have a large effect on a bullet’s performance. Velocity is often the culprit behind premature bullet destruction because as velocity increases, so does drag, and the tissue being entered gets “harder,” in effect, because it cannot get out of the way fast enough. A simple analogy would be the difference between stepping into a swimming pool versus diving into it from a tall diving platform.
Choosing the Proper Hunting Bullet
The target determines the ideal behavior that you want from a bullet. On smaller, lighter game such as varmints, you normally want the rapid destruction of the bullet on impact for the instant transfer of energy, since penetration is rarely an issue.
For common big-game animals such as deer and elk, you want the controlled release of energy at impact to increase bullet penetration. By using a moderately fast-expanding bullet, more projectile energy is retained to transfer to the animal’s internal organs.
For even larger, more dangerous, big-game animals, you want bullets designed for even slower energy expenditure in order to achieve the deep penetration necessary to create permanent and temporary cavitation in the vital areas of large animals even after the bullet has broken bones along its path.