[Encyclios]

Internal ballistics

Factors inherent in internal ballistics are primarily the barrel core, bullet weight and jacket, propulsive charge, and characteristics of the chamber and eventual case. With the exception of mortars, rocket launchers, and shotguns with split ammunition, all barrels have a rifled core.

The purpose of the rifling, which always has a helical pattern, is to give the bullet a rotary motion on the longitudinal axis, thus obtaining a gyroscopic stabilization that mitigates the effects of drift. In the case of short firearms, especially pistols, the bore is exactly cylindrical and the rifling has a constant pitch; as the power of the firing charge increases, in order to avoid maximum pressure peaks at the moment of the bullet notching in the rifling and instead obtain a thrust that is as progressive as possible until it exits the muzzle, the bore of the barrel assumes a truncated cone profile and the rifling is carried out with a progressive progression. In this way, the bullet encounters a progressively increasing resistance and the pressure is maintained at high levels until it exits the barrel.

Barrels made in this way also give better shooting accuracy and excellent ballistic performance, even with steel jacketed bullets. In large caliber artillery pieces, to avoid the enormous resistance to notching in the rifling and the rapid wear of the rifling itself, pre-cut bullets are used with helical grooves corresponding to the reliefs of the rifling. In any case, artillery projectiles, even those of medium caliber, are equipped with bronze rings, called force rings, which are designed to cut into the rifling and avoid direct contact between the barrel and the steel jacket.

In light artillery pieces and portable weapons, the ballistic behavior is also influenced by the characteristics of the case, which is generally made of brass. In these cases, in fact, it is the case that ensures the gas tightness when fired, following the expansion of the collar against the walls of the chamber.

Therefore, the degree of elasticity of the collar of the case and the amount of crimping, i.e. the tightening of the collar around the bullet, are important, as they make it possible to graduate the resistance to disengagement and consequently the value of the initial pressure. One of the most important factors in internal ballistics is the quality and quantity of the powder constituting the launch charge. Depending on the chemical composition, degree of fineness and external grain coating, powders have a higher or lower burning rate (liveliness).

Vivacious powders are suitable for shotgun and short arms cartridges, medium vivacity powders for rifles and machine guns and slower powders for artillery. For best results a powder should burn completely and progressively up to the exit of the bullet from the muzzle without giving excessive initial pressures and disruptive phenomena.

Certainly internal ballistics as a scientific discipline was born after external ballistics: if, in fact, the latter refers exclusively to the principles of mechanics, because it deals with the behavior of a mass, that of the “projectile”, in the gravitational field (in the presence of non-simple phenomena such as the resistance of the medium and the gyroscopic effect), internal ballistics focuses on the study of the combustion of the launch charge, on the consequent pressure trend inside the “barrel” of the firearm and on the induced effects that very often, beyond the prejudice regarding the result of the shot, call into question the safety of the shooter. The existence of various national firearms test benches (the Italian one is particularly famous) demonstrates the need to subject firearms to particular tests, which concern their “internal” behavior and which require, once passed, a special marking.

More than Galileo and his laws, internal ballistics refers to chemistry and thermodynamics: for this reason it is certainly a recent discipline. It should be added that the phenomena mentioned take place in transient regimes of very short duration, which greatly complicate the possibility of measurements and surveys. However, the fundamental purpose of internal ballistics is the measurement or prediction of the pressure trend inside the barrel. This is particularly important in weapons where the repetition of the shot occurs as a result of the “automatism” triggered by the previous shot. In fact, if, due to this automatism, the bolt were to retract before the residual pressure in the barrel due to the shot had dropped to acceptable values, there would be risks for the shooter’s safety, who would be hit by the high temperature pressure “dart” that would be released from the breech. The elements that contribute to the internal ballistic behavior of a system are: the type of launch charge (gunpowder), its primer and quantity, the boundary conditions of humidity and atmospheric pressure, the inertia of the projectile, the way it engages the rifling, friction, etc.

In principle, it can be said that the greater the “difficulty” of the bullet to exit the barrel, the greater and more dangerous will be the pressure inside it. In this case, the so-called “progressive” gunpowders are used, characterized by longer combustion and more gradual pressure increases, as opposed to the so-called “lively” powders. The situation is similar to what happens in the combustion chamber of a reciprocating engine: the piston is the projectile, the mixture is the launch charge, the spark plug is the ignition. The monitoring of the phenomenon is constituted by the relative pressure diagram, otherwise known as “indicated” diagram, whose relief allows to judge about the technical goodness of what happens there.

Even in the case of a spark ignition engine the combustion of the mixture must take place gradually: hence the extreme impropriety of the term “internal combustion engine”. When the mixture bursts, the engine behaves badly, because it hits the head: the release of energy is so sudden and violent that the piston can not keep up to transform it into work: so it is dissipated in the form of pressure waves of high peak that damage the engine because they break the protective veil of the lubricant and create hot spots that lead to absolute anarchy of successive combustions.

Hence, modern gunpowders are different from explosives, even though they share the same origin: they must moderate the release of energy: if the energy were released in a disruptive way, the barrel would burst. Therefore, modern gunpowders represent a compromise between the explosive action of nitroglycerine and the retarding action of cellulose: hence the diffusion of the so-called double base powders, nitroglycerine and nitrocellulose, which allow to obtain this compromise.