The field of ballistics is commonly divided into three areas of study: internal ballistics, external ballistics, and terminal ballistics. While each discipline focuses on a different phase of a projectile’s life, there is significant overlap between them. Developing an effective and well-balanced load requires an understanding of all three. The following sections provide an overview of each area.

Internal Ballistics

Internal ballistics describes everything that occurs from the moment the primer ignites until the bullet exits the muzzle. Powder type, bullet design, brass, and primers are all critical variables that influence cartridge performance and can have downstream effects on other ballistic domains. Firearm characteristics are also part of internal ballistics. Chamber dimensions, rifling geometry, twist rate, suppressor use, and barrel harmonics all fall within this field of study and directly influence the bullet’s external ballistic performance.

While there are many variables that affect internal ballistics, there are typically two primary outputs of interest: the maximum internal pressure generated (P) and the muzzle velocity (V) at which the bullet leaves the barrel.

Additional variables may also be important depending on the firearm and application. In gas-operated systems, gas port pressure is a critical factor. Ammunition temperature sensitivity is another consideration, as pressure and velocity can change significantly when cartridges are exposed to extreme heat or cold, such as ammunition left on a dashboard in a hot Arizona summer or stored in a vehicle during an Alaskan winter.

When someone becomes a reloader or handloader, they take on the responsibility of understanding and managing these internal ballistic variables. This is the area of ballistics that this website is primarily focused on studying and explaining.

External Ballistics

External ballistics is the study of a projectile in flight, whether that projectile is a bullet, a sabot, or a missile. Once the projectile leaves the barrel, it is primarily influenced by two forces: gravity and aerodynamic drag. Internal ballistics determines the projectile’s initial velocity and spin, but after muzzle exit, those imparted characteristics govern the projectile’s trajectory, which is the primary output of external ballistics.

There are several variables of primary concern to the external ballistician. These include projectile drag, commonly expressed as coefficient of drag (Cd) or ballistic coefficient (BC), atmospheric conditions, velocity, wind, projectile spin, and projectile mass, among others. Each of these variables influences how the projectile behaves in flight and how accurately its trajectory can be predicted.

Anyone who takes hunting or shooting seriously studies external ballistics, whether formally or informally. In the field, hunters attempt to predict bullet trajectory before taking a shot. Doing so correctly can mean the difference between cleanly harvesting a prized mule deer and coming home empty-handed. To aid in these predictions, ammunition manufacturers publish ballistic coefficients and trajectory data for their projectiles.

As engagement distances increase, the number of significant variables also increases. Effects such as spin drift and the Coriolis force, which are largely negligible at distances under roughly 600 yards, become increasingly relevant at longer ranges. For long-range shooters, a solid understanding of external ballistics is not optional; it is mandatory.

Modern tools such as Doppler radar have significantly advanced the study of external ballistics. These systems track the bullet in real time as it travels downrange, measuring velocity as a function of distance. Changes in velocity over flight allow ballisticians to calculate an accurate coefficient of drag, which can then be translated into a ballistic coefficient. This technology enables engineers to make incremental design changes to projectiles and directly observe their effects on downrange performance.

Terminal Ballistics

Terminal ballistics describes what happens when a projectile reaches the end of its flight and interacts with its target. That target may be an earthen berm, steel, or living tissue. At this stage, the projectile has a job to do, and terminal ballistics is the field concerned with how effectively energy is transferred from the projectile to the target and what effects that energy transfer produces.

Over the past several decades, there have been significant advances in defensive handgun and rifle ammunition. Much of this progress can be traced to the 1986 FBI Miami Shootout. In the aftermath of that incident, the FBI conducted an extensive review of the factors that contributed to the deaths of two agents and the wounding of five others. One of the shortcomings identified was inadequate terminal performance of the ammunition used.

As a result, greater emphasis was placed on improving bullet design to provide reliable penetration through intermediate barriers while maintaining effective terminal performance. In pursuit of these goals, and driven in part by law enforcement and military procurement requirements, manufacturers began developing projectiles designed to perform well under the FBI Ammunition Test Protocol. These tests involve firing bullets into calibrated ballistic gelatin through various barriers, including automotive glass and heavy clothing, to evaluate penetration, expansion, and consistency.

Initially, the FBI adopted the 10mm cartridge in an effort to improve terminal effectiveness. However, continued advances in bullet design eventually allowed smaller calibers to deliver comparable terminal performance. As a result, many law enforcement agencies, including the FBI, returned to the 9x19mm cartridge due to its lower recoil, higher magazine capacity, and modern terminal performance that rivals larger handgun calibers.

Advancements in terminal ballistics have not been limited to handgun ammunition. Rifle bullet design has also improved substantially. Modern hunting and tactical projectiles feature bonded or monolithic construction, improved penetration, and reliable expansion, even after passing through bone or dense tissue.

When developing a load, the intended end use should drive bullet selection. For target shooting, full metal jacket or ball ammunition is typically appropriate. If limited penetration is required, or if the goal is an ethical and effective harvest of game, understanding terminal ballistics becomes essential. Bullet construction, penetration depth, and expansion characteristics all matter, and studying terminal ballistics allows the shooter to make informed choices rather than relying on assumptions.

-Jay-

Revised 2/7/2026