Internal ballistics (also interior ballistics), a subfield of 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 artillery. For rocket-propelled projectiles, internal ballistics covers the period during which a rocket motor is providing thrust.
General concepts Interior ballistics can be considered in three time periods:
Lock time - the time from sear release until the primer is struck Ignition time - the time from when the primer is struck until the projectile starts to move Barrel time - the time from when the projectile starts to move until it exits the barrel. The burning firearm propellant produces energy in the form of hot gases that raise the chamber pressure which applies a force on the base of the projectile, causing it to accelerate. The chamber pressure depends on the amount of propellant that has burned, the temperature of the gases, and the volume of the chamber. The burn rate of the propellant depends on the chemical make up and shape of the propellant grains. The temperature depends on the energy released and the heat loss to the sides of the barrel and chamber. As the projectile travels down the barrel, the volume the gas occupies behind the projectile increases. Some energy is lost in deforming the projectile and 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, which adds resistance to its forward motion. The breech and the barrel must resist the high-pressure gases without damage. Although the pressure initially rises to a high value, the pressure starts dropping when the projectile has traveled some distance down the barrel. Consequently, the muzzle end of the barrel does not need to be as strong as the chamber end. Mathematical models have been developed for these processes. The four general concepts which are calculated in interior ballistics are:
Energy - released by the propellant Motion - the relation between the projectile acceleration and the pressure on its base. Burning rate - a function of the propellant surface area and an empirically derived burning rate coefficient which is unique to the propellant. Form function - a burning rate modifying coefficient that includes the shape of the propellant.
History Internal ballistics was not scientifically based prior to the mid-1800s. Barrels and actions were built strong enough to survive a known overload (Proof test). Muzzle velocity was surmised from the distance the projectile traveled. In the 1800s test barrels began to be instrumented. Holes were drilled in the barrel and fitted with standardized steel pistons which exerted pressure which compressed standardized copper cylinders when the firearm discharged. The reduction in the copper cylinder length is used as an indication of peak pressure, known as "Copper Units of Pressure", or "CUP" for high pressure firearms. Similar standards were applied to firearms with lower peak pressures, typically common handguns, with test cylinder pellets made of more easily deformed lead cylinders, hence "Lead Units of Pressure", or "LUP". The measurement only indicated the maximum pressure that was reached at that point in the barrel. Piezoelectric strain gauges were introduced in the 1960's, allowing instantaneous pressures to be measured without destructive pressure ports. Instrumented projectiles were developed by the Army Research Laboratory that measures the pressure at the base of the projectile and acceleration.
Priming methods
Methods of igniting the propellant evolved over time. A small hole (a touch hole) was drilled into the breech, into which a propellant was then poured, and an external flame or spark applied (see matchlock and flintlock). Percussion caps and self-contained cartridges have primers that detonate after mechanical deformation, igniting the propellant.
Propellants
Black powder
Gunpowder (Black powder) is a finely ground, pressed and granulated mechanical pyrotechnic mixture of sulfur, charcoal, and potassium nitrate or sodium nitrate. It can be produced in a range of grain sizes. The size and shape of the grains can increase or decrease the relative surface area, and change the burning rate significantly. The burning rate of black powder is relatively insensitive to pressure, meaning it will burn quickly and predictably even without confinement, making it also suitable for use as a low explosive. It has a very slow decomposition rate, and therefore a very low brisance. It is not, in the strictest sense of the term, an explosive, but a "deflagrant", as it does not detonate but decomposes by deflagration due to its subsonic mechanism of flame-front propagation.
Nitrocellulose (single-base propellants)
Nitrocellulose or "guncotton" is formed by the action of nitric acid on cellulose fibers. It is a highly combustible fibrous material that deflagrates rapidly when heat is applied. It also burns very cleanly, burning almost entirely to gaseous components at high temperatures with little smoke or solid residue. Gelatinised nitrocellulose is a plastic, which can be formed into cylinders, tubes, balls, or flakes known as single-base propellants. The size and shape of the propellant grains can increase or decrease the relative surface area, and change the burn rate significantly. Additives and coatings can be added to the propellant to further modify the burn rate. Normally, very fast powders are used for light-bullet or low-velocity pistols and shotguns, medium-rate powders for magnum pistols and light rifle rounds, and slow powders for large-bore heavy rifle rounds.
Double-base propellants
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