On its trajectory, the projectile makes its way through the air. This results in remarkable phenomena that deserve an analysis.

• Stability and transonic velocity

The stability of a projectile depends on the air resistance which is particularly intense in the area of the velocity of sound. In this transonic region, the stability of the projectile is strongly perturbed. In particular when its velocity, initially supersonic, decreases with distance to become subsonic and that, at the same time, its spin speed has also decreased.

• Subsonic and supersonic projectiles. Shock wave, Mach wave. Wake

When the molecules of the ambient air are locally agitated (sound source or other...), this vibration is propagated from near to near to the neighboring molecules. It propagates at a precise velocity, which depends on the characteristics of the medium, called specific velocity. It is the velocity of sound in this medium, air in our case (a # 340 m/s).
It should be noted that there is transfer of energy but not of matter.

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- Stationary source If the source S (black point) of the vibration is immobile, the waves generated will be either circular or spherical depending on whether they propagate along two or three spatial dimensions. In all cases, they will be concentric. During its flight, the tip of the projectile strikes air molecules. This disturbance will propagate, in the form of spherical waves, in all directions and in particular in the direction of the progression of the projectile.

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- Subsonic velocity The projectile moves at a velocity V lower than that of sound (subsonic velocity), the sound waves it has generated will move away from it indefinitely and in particular from its tip.

 

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- Sonic or transonic velocity The projectile moves at or near the speed of sound. The waves emitted by the shaking of the air molecules by its point remain at the level of the latter. The accumulation of these waves locally increases the density of the air and constitutes a kind of barrier that the projectile will have difficulty in crossing. The front of the projectile is accompanied by a wave front.

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- Supersonic velocity The velocity of the projectile is greater than that of sound. The waves emitted at each moment are overtaken by the tip of the projectile and are left behind. This wave is, at the tip of the projectile, supersonic. It is a shock wave. At the rear of the projectile, the sound waves propagate at the velocity of sound and interfere in places. It forms an accumulation of energy on the envelope supported on the edges of the waves. This envelope has the shape of a cone whose top coincides with the tip of the projectile. This wake is the Mach wave. It is this dry "slam" that we hear that is clearly different from the mouth wave. This « slam" becomes the "bang" for supersonic aircraft.

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• Shock wave

A shock wave is a sudden variation of certain physical parameters. An air shock wave is a sudden variation of the pressure followed by a return to normal preceded by a phase of depression.

The image below represents the ombroscopy of a 7.62 mm projectile moving at a velocity of 1.3 Mach.

• Characteristics of a shock wave

In an air environment, a pressure wave must meet a certain number of criteria to be qualified as a shock wave :

1 - The relative velocity of the wave increases with the amplitude of the shock ;

2 - The velocity of sound behind the shock wave is always greater than that of the wave ;

3 - A shock wave is always subsonic with respect to the medium behind it ;

4 - The shock generating the wave is always supersonic with respect to the medium it encounters ;

5 - A shock wave is not an acceleration wave, which travels at the velocity of sound.

• Shockwave and misconceptions in wound ballistics

Attempts to explain the injuries caused by small, high-velocity projectiles, of which the .223 Remington is an example, have led to fanciful theories such as the devastating effect of the shock wave. Even today, this theory reappears from time to time.

For a shock wave to create lesions, it must carry enough energy. But above all, it must exist. A simple analysis of the characteristics of a shock wave shows that it does not exist when a projectile, even at high velocity, travels through the body of a human being or an animal.

We will retain particularly characteristic 4 above. It tells us that for a projectile to create a shock wave, it must have a supersonic velocity in the medium in which it is moving. The velocity of sound in biological tissues is just over 1500 m/s. This is much higher than the velocity of high velocity projectiles such as the .223 Remington (5.56 x 45mm). In the field of wound ballistics, even these very fast projectiles cannot create a shock wave in body tissue. Therefore, one cannot attribute the observed lesions or part of them to this physical phenomenon.