The subfields of Ballistics
There are four different subfields of ballistics : internal ballistics, transition ballistics, external ballistics, and terminal ballistics. Each one is connected to the other, but they are also very unique.
6 KAPOW / May 2014
Internal Ballistics
Internal ballistics is the study of the propulsion of a projectile. In guns, internal ballistics covers the time from the propellant’s ignition until the projectile exits the gun barrel. The study of internal ballistics is important to designers and users of firearms of all types, from small-bore rifles and pistols, to high artillery. The duration of interior ballistics is separated into three parts: lock time, ignition time, and barrel time. Lock time is the time from sear release until the primer is struck. Ignition time is the time from when the primer is struck until the projectile stands to move. Barrel time is the time from when the projectile starts to move until it exits the barrel. Some energy is lost in deforming the projectile, causing it to spin. There are also frictional losses between the projectile and the barrel. The projectile, as it travels down the barrel, compresses the air in front of it. These processes affect the gun design. The breech and the barrel must resist the high-pressure gases without damage. Although the pressure initially rises to a high value, the pressure begins to drop when the projectile has traveled some distance down the barrel. Therefore, the muzzle end of the barrel does not need to be as strong as the chamber end. There are five general equations used in interior ballistics: the equation of state of the propellant, the equation of energy, the equation of motion, the burning state equation, and the equation of the form of function.
Transition Ballistics
Transition ballistics, also known as intermediate ballistics, is the study of a projectile’s behavior from the time it leaves the muzzle until the pressure behind the projectile is equalized, so it lies between internal and external ballistics. When the bullet reaches the muzzle of the barrel, the escaping gases are still, in many cases, at hundreds of atmospheres of pressure. Once the bullet exits the barrel, breaking the seal, the gases are free to move past the bullet and expand in all directions. This expansion is what gives gunfire its explosive sound, and is often accompanied by a bright flash as the gases combine with oxygen in the air and finish combusting. The propellant gases continue to exert force on the bullet and firearm for a short while after the bullet leaves the barrel. One of the essential elements of making a firearm accurate is to ensure that this force does not disrupt the bullet from its path. The worst case is a muzzle that is cut at a non-square angle, so that one side of the bullet leaves the barrel early; this will cause the gas to escape in an asymmetric pattern, and will push the bullet away from that side, causing shots to form a “string,” where the shots cluster along a line rather than forming a normal pattern.