[Encyclios]

External ballistics

External ballistics concerns the behavior of the bullet from the exit of the barrel to the point of impact. Even in the so-called “straight shot” weapons, the trajectory is never a straight line, but a paraboloid curve that intersects in two points the line passing through the axis of the barrel. The accuracy of the shot is inversely proportional to the area of the scattering rectangle, that is the surface that includes the impact points of the shots, and is exclusively determined by the ballistic performance of the weapon and the ammunition.

The closer the center of the scatter rectangle is to the target, the more accurate the shot. Adjusting the shot is done by elevation and, when possible, by varying the launch charge. The most important factors in external ballistics are bullet shape and weather conditions, especially wind. The shape of the projectile must be as aerodynamic as possible and such that its center of gravity is approximately coincident with the geometric center; generally a tapered shape and tapering at the tail is chosen.

Regarding the influence of the wind, the most important phenomenon is the Magnus effect, from the name of the German physicist H. Magnus (1802-1870), who discovered that, when an air current strikes laterally a projectile rotating on its axis, cavitational phenomena occur that generate a thrust acting orthogonally both to the air current and to the direction of the projectile.

In order to understand the subject of external ballistics and in a certain sense its scientific necessity, it is convenient to start from the elementary problem: the behavior of a mass “launched” (in greek “ballo” means to launch, whence ballistics) with a certain initial velocity, independently from the way this launch is obtained: with a catapult, with a crossbow or the deflagration of a charge in the barrel of a firearm.

We know that if there were no gravity and other forces, the mass would continue to travel straight and at constant speed the trajectory impressed by the launch, according to what is stated by the first principle of dynamics. If we neglect all other forces and we admit that there is only gravity, the problem can be approached very simply considering that the initial velocity is decomposed in two components, one of which is constant horizontal and the other is uniformly decelerated vertical due to gravity. At the highest point of the trajectory the vertical component is zero and the corresponding part of initial kinetic energy is all transformed into gravitational potential energy. The trajectory is a mass independent parabola.

Then we consider the air resistance. This is a force that, due to the irregularity of the projectile shape, does not pass exactly through its barycenter where the force of gravity is applied. The result is the birth of a torque that tends to rotate the bullet, with obvious inaccuracy of the shot. To avoid this problem, the bullet is given a rotary motion around its main axis by the rifling of the barrel. Due to the principle of the gyroscopic effect, the bullet no longer rotates in the vertical plane, but tends to drift, i.e. to deviate from the plane of the theoretical shooting parabola, with an error called drift error. It is possible to correct this error with aiming and sighting devices.