Internal, External and Terminal Ballistics

Your knowledge of basic ballistics can help customers be more effective in their chosen shooting sport.

Internal, External and Terminal Ballistics

When an early hunter threw a spear at an animal, he intuitively knew it would begin falling as soon as it was cast. He also knew that if he wanted to throw it farther, he had to angle the throw in an upward arc, throw it harder, or both. Repeated practice taught him how to angle it and how much force to use for different distances.  

These actions were a rudimentary form of the science we now call ballistics. Very few people are concerned these days with throwing spears. But every customer who enters your store is interested in firearms ballistics, even if they don’t think about it in those terms. They all want to know that they can accurately hit targets at various distances and that the bullets will have maximum effect. That’s where you come in. Your ability to discuss firearm ballistics and the equipment used to achieve maximum results can open many sales and profit opportunities for your business. Let’s delve into the factors that influence ammunition performance. I promise there will be no math. 

The first step is to understand that ballistics is separated into three general categories: internal, external, and terminal (or wound) ballistics. The difference between the three is determined by which part of the bullet’s path you refer to.

Internal Ballistics

Internal Ballistics deals with the forces acting upon bullets from when the primer is ignited until the bullet exits the muzzle. Those forces come from the primer, powder charge, the barrel’s condition and twist rate, how tightly the bullets are crimped, whether the bullet’s length has been optimized for a particular chamber, etc. Each factor combines with the others as the bullet travels from the cartridge case to the muzzle, thus affecting how the bullet reacts after it exits the barrel.  

Fortunately, ammunition makers constantly research how each factor affects the overall results, so most shooters don’t worry about internal ballistics. Shooters purchase ammunition suitable for their particular firearm, zero their gun, and shoot or hunt confidently, knowing it will be safe, accurate, and effective. They know that commercially produced ammunition can be relied on to perform consistently. 

External Ballistics

The science of external ballistics picks up where internal ballistics leaves off. It’s the study of the forces acting upon bullets from the time they leave the muzzle until they strike their target. When people speak of ballistics, they usually refer only to external ballistics. Bullet velocity, shape, and weight all affect the bullet’s trajectory. But there are other factors that most people don’t think about. Air pressure, temperature, humidity, elevation and shot angle are also significant. Even more obscure is that the Earth’s rotation affects long-range shots and must be added to the calculations.

If all of this sounds complicated, that’s because it is. Long-range shooters and snipers spend a lifetime developing their skills to account for these factors. These days, shooters concerned with a bullet’s flight and its ultimate impact tend to use ballistic computers, programs or apps.  Remember when I said no math? That’s how it’s possible. Winchester, Federal, Sierra, and Hornady all offer ballistic calculator programs. Some Kestrel environmental devices even have calculators built into them. Input the specifics of the ammo, conditions, and other factors, and they will produce a chart showing the projected bullet drop at different distances. There are a lot of shooters who put the info on a card inside their flip-up scope caps as a quick reference. Others print them on an index card to carry into the field. Truly dedicated shooters may even remember the data. The only thing left for them is to determine the range and adjust for wind deflection.

Terminal Ballistics

Terminal ballistics is often referred to as wound ballistics. It is the study of how bullets react once they hit their target. Target shooters aren’t concerned with this, but hunters, especially big- and dangerous-game hunters, consider it of primary importance. They not only need to shoot accurately, but they must also ensure that the bullet carries enough energy and is the right type to penetrate and expand upon impact. Velocity, the bullet’s sectional density (sometimes referred to as bullet weight), tip design, bullet composition, and jacket design affect how the bullet will react upon impact.

The discussion to this point gives a quick introduction to firearms ballistics. These factors apply no matter the ammunition used. Some shooters, looking for the ultimate accuracy and performance, customize prescribed loads to find the best combination. By tweaking each variable, they can often create better ammunition than commercially available loads.  

Let’s look at a few more terms related to ballistics and why they are essential. The two most common terms used to describe bullets are sectional density (SD), the ratio of a bullet’s weight to its diameter, and ballistic coefficient (BC), a comparative term for a bullet’s shape and how well it will fly through the air. The industry uses two standardized bullet designs for comparison. All rifle bullets are measured against these standards for consistency, and the BC numbers reflect the results. The first standard compares a bullet to a flat-based Spitzer bullet known as G1. The second is a comparison to a sleeker boat-tailed bullet known as G7. Let’s explore these terms.

Sectional Density

Bullet diameter is always the same in any caliber. For example, a 30-caliber bullet's outside diameter is consistent at .30 inches, meaning the sectional density varies directly with bullet weight, as shown in the chart below.  

110 grain, 30 caliber bullet SD = .166

165 grain, 30 caliber bullet SD = .248

220 grain, 30 caliber bullet SD = .331

As you can see, doubling the bullet weight doubles the sectional density. Since the diameter remains the same, the bullet length is doubled. Hunters frequently look for the highest sectional density under the misconception that a heavier bullet always penetrates deeper without considering the bullet’s construction. Some bullets are designed to expand on impact. Others are more concerned with penetration. Still others fragment into many pieces. One that expands meets more resistance than one that doesn’t; thus, it will lose its energy faster and not penetrate as deep.  One that fragments upon impact loses the pre-impact mass and will not penetrate deeply either. Bullet design is more critical than sectional density. The ultimate goal is to find a combination that penetrates deeply and expands. This will transfer the bullet’s energy efficiently and create a wound violent enough to cause incapacitation. Even with the perfect bullet design, other factors, such as velocity, shape, spin, etc., play a critical role in how the bullet functions on impact. 

Ballistic Coefficient

We mentioned that the ballistic coefficient measures bullet sleekness and how well it resists outside influences. Higher numbers represent sleeker bullets less likely to be influenced by air friction and winds. A good example would be to compare bullets to boats. An elegant racing shell (a long, sleek rowing boat used in team rowing competitions) easily slices through the water due to its narrow, pointed design, which equates to a high bullet coefficient. Compare that to a broad, flat-nosed john boat. The latter will be harder to row because it has a large surface area that pushes against the water, equating to a lower ballistic coefficient. This is why long-range shooters use longer, more streamlined bullets with boat tail designs. The aerodynamics allow the bullets to fly faster and with less variation than a low BC bullet. 

Hopefully, you can see how critical bullet design is. Choices are based on their intended use. A round-nosed bullet typically used in a lever-action .30-30 is perfect for shorter-range shots where bullet expansion in large game animals is essential. A long, pointed bullet, like that used in the 6.5 Creedmoor, is ideal for long-range competitions where exact accuracy is critical and expansion is irrelevant.

Reloading Equipment

I have friends who purchased a Lee Pro 1000 and cranked out thousands of rounds of .223 Remington for their AR. They wanted large volumes of ammunition, and accuracy wasn’t an issue. Other friends take great care in producing custom loads for precision shooters. This is ammunition of the highest quality and is usually designed for only one rifle. Bulk ammo is easy and relatively cheap to produce. Making quality ammunition, however, requires high-quality equipment and components. It also requires working in smaller lot sizes and spending much more time per round. Those requirements translate to more sales opportunities for those willing to learn what’s needed for ballistic efficiency.  Here’s a list of some of the things they’ll desire:

Brass Annealer

Premium Brass and Bullets

Headspace and Projectile Comparators


Precision Dies

Powder Throwers/Tricklers


Handloader Logs

Labels/Labeling Tools

Describing each of these items and how they’re used is beyond the scope of this article, but you have the best sources of information at your fingertips in the form of brochures and catalogs directly from the manufacturers. Familiarizing yourself with the equipment and components will permit intelligent conversations, increase sales, and help develop loyal customers who appreciate your expertise. It might inspire others to take up the hobby to maximize their ammo’s performance.


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