You load your gun, aim at the target and pull the trigger. If your equipment and shooting technique are in good order, the bullet will hit your intended target at the point you aimed. But what happens during that split second between ignition and impact? Ballistics is the answer.
A typical modern cartridge consists of several components: case (usually brass, aluminum or steel), primer (either centerfire or rimfire), powder charge, and projectile. When the firing pin strikes the primer, setting off the charge and forcing the bullet out of the case, everything that happens from that moment until the bullet hits the target can be explained by three phases of ballistics: interior, exterior and terminal.
Internal Ballistics
Internal ballistics describe the forces on the projectile from the moment of ignition until it exits the barrel. Powder charge, bullet weight, and barrel length all contribute to the speed of the bullet. Barrel twist rate, which is measured as the number of complete rotations of the grooves in the barrel also plays a factor. Ammunition handloaders spend a lot of time and effort on the details of internal ballistics trying to develop a perfect load, but for the average shooter using factory loaded ammunition, internal ballistics are not much of a concern beyond selecting the right load for your application.
The important thing to know about internal ballistics is that all the speed and energy generated by the projectile is produced while the bullet is still in the barrel. Once it exits the muzzle, everything literally goes downhill on the way to the target. Muzzle energy and velocity begin dropping immediately once the bullet leaves the barrel in reaction to the forces of gravity and aerodynamic drag.
External Ballistics
External ballistics describe the flight of the projectile from the muzzle to the target. After the bullet leaves the barrel, various forces act to slow it down or shift its path of travel, including gravity, wind deflection, aerodynamic drag, and in extreme cases, even the rotation of the earth. Target shooters and hunters are very interested in the trajectory of the projectile, also referred to as bullet drop, as this affects how the weapon is sighted and its maximum effective range.
Muzzle velocity is the speed, measured in feet-per-second, the projectile is traveling when it exits the barrel. Muzzle energy is the amount of kinetic energy, measured in foot-pounds of force. Both are related, as the higher the weight and faster the speed of the projectile, the greater the muzzle energy. While it might seem like more velocity and energy is always better, these forces need to be balanced with recoil (remember the concept of equal and opposite forces from physics class) as well as the intended purpose of the projectile. Especially for hunters, balancing recoil, range and energy is the name of the game.
Bullet weight, length, shape and construction all are all factors in how a projectile responds to aerodynamic forces during its flight. Generally, a longer, heavier bullet retains more of its speed and energy over any given distance than a lighter one. To understand why, let’s take a look at two key aspects of bullet construction: sectional density and ballistic coefficient.
Sectional density (SD) is the ratio of an object’s mass to its cross sectional area with respect to a given axis. It conveys how well an object’s mass is distributed to overcome resistance along that axis. Ballistic coefficient (BC) is a measure of a projectile’s ability to overcome air resistance during flight. In simple terms, the higher the BC, the better the ability of the bullet to resist aerodynamic drag and retain higher velocity.
For any given caliber, or bore diameter, a bullet with a higher SD and BC is more capable of resisting wind deflection and will retain more velocity and energy during its flight from muzzle to the target. It will also fly on a flatter trajectory, which can make it easier to make accurate hits at various distances between the shooter and the target.
Terminal Ballistics
Terminal ballistics describe the effect of the projectile when it hits the target, including the kinetic energy transferred upon impact, as well as the penetration of the projectile itself into the target. In addition to the speed and energy imparted upon impact, bullet construction plays a large role on the performance of the projectile when it hits the target. Soft point and hollow point bullets are designed to expand upon impact, creating a larger wound path without over penetrating the target. Full metal jacket projectiles may fragment upon impact, penetrating deeper with the ability to pierce heavy clothing, wood, metal and other barriers while still maintaining enough energy to be effective against the target.
This article barely skims the surface of the many facets of ballistic science, which is a deep field of study that can lead to a lifetime of study. But whatever your level of interest, understanding the basic concepts will make you a better shooter and more informed ammunition buyer, and Patriot Defense is here to help you make the right ammunition selection for your needs.