I’m convinced that a lot of hunters don’t fully understand how big-game bullets work. This is partly because hunters can't see a bullet at work, and partly because bullet marketing hype can be misleading. Also, many hunters just shoot their deer and drag them out of the woods, with little worry or interest in exactly how the bullet made it all possible. Two decades ago, I patented a bullet-testing media, and it involved a lot of bullet evalauation and work with bullet manufacturers. That, combined with a bunch of field experience, taught me a lot about bullets, so let me try to demystify how they work.
Instead of a complicated physics lesson, let’s start with some great advice from F&S rifles editor, David E. Petzal, who once wrote, “…the more damage you do with the first shot, the better things are going to turn out, at least for you.” I don’t agree with all of Petzal’s opinions on bullets, but there's no denying that the more tissue damage your bullet inflicts (preferably on the first shot), the quicker and more humane the kill.
But you need to know how to manage the damage. Some suggest using a big ol' magnum and to just knock the hell out of whatever you’re shooting. This can work, but magnums knock the hell out of you too. Anticipation of stiff recoil makes accurate shooting difficult. What generally works better is to shoot a manageable cartridge and to hit the animal in the right spot with a bullet that you know will upset enough and penetrate sufficiently to get the job done. That's why choosing the right big-game bullet for a given situation is so important. So, let's look at the options.
Types of Big Game Bullets
One of easiest ways to understand how big-game bullets work is to divide them into categories that include cavitating, penetrating, and balanced. Cavitating bullets are what many hunters refer to as "quick-expanders," like your typical soft-point deer bullet. Penetrating bullets are what people often call "tough" bullets; they hold together and retain a lot of weight. Balanced bullets are, as you'd expect, represent a happy medium, with controlled expansion and good weight retention and therefore penetration.
There is a simple math equation for figuring out what category a bullet belongs in; you simply take a bullet recovered from a big-game animal or test media and divide the percentage of weight retained by the bullet’s expansion factor. (Expansion factor is the bullet’s recovered diameter divided by its caliber.) The result will be somewhere between 0 and 100 and will position the bullet in one of the three categories, as the chart above details.
There is an x-factor, however, and it is impact velocity. Velocity is what creates the energy that makes bullets work. Depending on impact velocity, some bullets can fall within all three categories. The goal of bullet engineers is to create a bullet that will live within in a single desired category over the widest velocity range.
For example, a Hornady SST bullet impacting slower than about 1800 fps will hardly deform at all and will penetrate very deep. One impacting at about 2800 fps and faster will create a lot of cavitation and make a nasty wound cavity but with less penetration. Somewhere in between those velocities, an SST bullet will work in a more balanced manner, with measurable tissue destruction and decent penetration. As a hunter, if you want reliable bullet penetration, you need to consider how fast your bullet will be traveling at the distance of your target.
How Energy Affects Bullet Performance
When a bullet impacts an animal, it begins to deform instantly. This deformation—commonly called expansion—happens over a very short distance. Most bullets finish shape shifting before penetrating 6 inches, and even before rotating a full turn inside the animal. The materials used to make the bullet, how it’s designed, and its impact velocity, determine how this shape shifting plays out. Many hunters like to reference kinetic energy when it comes to terminal performance, and this where that energy comes into play.
For example, a mono-metal bullet like the Barnes Triple Shock will fully deform almost instantly, and it only takes about 50% of the bullet’s energy to fuel this transformation. After upset, it will still have all or most of its weight and about half its energy left. This allows it to penetrate well, and within its ideal velocity range, it acts as a balanced bullet. Bonded and partitioned bullets act similarly, but because of their softer lead and lead-alloy cores, they’ll have a bit less weight and energy left over after deformation.
More traditional jacketed lead-core bullets take a bit longer to fully deform because the independent jacket and soft lead core remain malleable at slower velocities. These bullets shed weight and energy to deeper depths, and after deformation may have as little as 50% of their weight and only 20% of their energy remaining. These bullets won’t penetrate as deep, but the energy used creates a lot of tissue damage. These are cavitating bullets.
As for penetrating bullets, some are designed not to deform, like the solids used for dangerous game hunting. But keep in mind that bullet that is made to deform at typical impact velocities can fail to do so and essentially become a penetrating bullet when they impact at a velocity slower than necessary to cause measurable deformation. This usually occurs when the distance to the target is too far.
Exceptions to the Big-Game-Bullet Rules
As with everything, there are exceptions. Some traditional jacketed bullets for low-velocity cartridges deliver a more balanced performance within their ideal impact velocity range. This is especially true with bullets loaded for cartridges like the 360 Buckhammer or 45-70 Government. These cartridges just don’t have the velocity to push these bullets into the cavitating classification.
Then there are hybrid bullets—usually mono-metal bullets—designed to shed petals or create shrapnel right after impact. These are unique because they tend to damage a lot of tissue and penetrate well, without retaining a lot of weight or energy. Much like with conventional mono-metal bullets, the higher the impact velocity, the more work these bullets do, because they have more energy to work with. Remember, you square velocity when calculating kinetic energy.
What You Need to Know About Splash
Now we come to the least-understood aspect of how bullets work, and it’s what I call the "splash." Animals are made of blood engorged tissue, or as some like to say, 70% water. Like water, blood is uncompressible, so when a bullet impacts these tissues at high velocity, the tissues displace. This displacement, if violent enough, can damage the tissues/organs inside the animal. With little animals like prairie dogs, there’s not enough animal to contain the splash, which is why they sometimes explode. Big-game animals usually contain the bullet’s splash or tissue displacement.
To get an idea of how bullet size and velocity contribute to the intensity of this splash, drop a golf ball in a pond and observe the water’s dispersion. Now, throw the ball and notice how the splash increases. You can control the splash by how hard you throw the golf ball or by using a larger ball. Since a bullet changes shape soon after impact, its increased frontal diameter and velocity dictate the intensity of the splash or internal destruction. The more the velocity or the larger the bullet’s upset diameter, the more tissue it’ll damage.
But there’s an unseen thing that happens with lead-core bullets and those hybrid bullets that shed material during deformation. These miniature lead or larger metal pieces that separate from the bullet increase the intensity of the splash, because they have their own velocity and energy. This is why traditional lead-core bullets tend to outpace balanced bullets in tissue damage. They may not penetrate as deep, but they damage more tissue, especially on broadside shots.
So, Which Big-Game Bullet is Best?
The larger the splash, the more cavitation and damage you have. Bullets that penetrate the deepest deliver the least amount of splash. You cannot have the best of both worlds, but you can get close, which is why bullets in the balanced category are so popular. They don’t damage voluminous amounts of tissue or always exit, but they provide a practical balance of performance. However, that does not mean they’re always the best choice.
Cavitating bullets can work great on deer-size game because you rarely need extreme penetration. They can even work well with larger game, too, if you don’t take extreme quartering shots. Of course, all bullets can become penetrators if impacting slow enough, but you really only need them for dangerous game. In the end, no one bullet is best for every situation. Instead, you need to apply what you know about how big-game bullets work, and pick the right bullet for the job at hand.
