Sonic Crack , .22 LR Ammunition & the Transonic Zone: Fun Experiment

At what velocity does a .22 bullet traveling through the air start producing a miniature sonic boom?

In a recent discussion online, people who have studied the effect noted aircraft start to encounter related turbulence effects long before they reach the speed of sound. In addition, the speed of sound in air varies with the temperature of the air. Thirdly, any lot of ammunition produces a range of velocities. It is exceptional ammunition in which the range from the average to the maximum is less than 25 feet per second (fps).

Part of the theory of a transonic zone is that the production of a supersonic shock wave is not an instantaneous event, as a projectile breaks the speed of sound. Turbulence starts producing supersonic effects before a projectile exceeds the speed of sound. Turbulence creates a distribution of waves, some of which are supersonic shock waves, even though the projectile is traveling below the speed of sound.

It was suggested, to avoid the distinctive noise of a “sonic crack” while shooting a .22 fitted with a suppressor, the average speed of .22 ammunition should be less than 1,000 fps.

The experience of this correspondent has been: when a suppressor is being used, the noise of a sonic crack is easily discerned from a subsonic muzzle blast.

A simple experimental procedure was developed. Shoot 25 rounds of the ammunition through a rifle with a good suppressor.

Record the velocity & subjective properties of the sound associated with each shot, for later

This jives with data published by Alan C Paulson about 30 years ago based on tests begun in the early 90s.

From the 1992 joint suppressor project conducted in Finland:

"...The development of whiplash noise does not begin abruptly at velocity Mach 1.0, but gradually, when the velocity of a rifle caliber projectile is more than Mach 0.90....

The Finns are big on suppressors because their snipers were so effective with them in the "Winter War" against the USSR in 1939-40. They have a saying, "The silencer doesn't make the sniper silent, but it does make him invisible."

In a nutshell, it isn't about how fast the bullet is going, it's about how fast the air has to move to get out of the bullet's path. As the speed of sound approaches, "compressibility" comes into play and air behaves stiffer, less springy, and moves more abruptly when getting out of the projectile's path.

This also goes to the point that the bullet has absolutely no response to crossing the speed of sound. It responds progressively as it enters and then leaves the transonic region, but absolutely nothing of note happens at the instant the bullet slows to exactly the speed of sound.

The definition of "transonic" is that the air passing over the object is a mixture of subsonic and supersonic, neither entirely supersonic nor entirely subsonic. Going from supersonic to subsonic (or vice-vera) never happens instantaneously. In the case of any object moving through air that experiences aerodynamic drag (which only leaves out radiation, like radio and light waves, but includes bullets), the only way to get from super-or-sub-sonic to the other is through the transonic region.

Some shooters think that a suppressor is pointless if you're shooting a supersonic bullet because the "crack" of the bullet will give away your position. But in all likelihood, the crack might alert the target that it's being shot at but it is unlikely to the extreme to give any useful information regarding the shooter's position.

Here's why:

This is a shadowgraph of a bullet in supersonic flight. It shows the shockwave (the cause of the supersonic crack) propagating outward from the bullet in a conical shape.

Because the shockwave is sound, it travels at ... guess what? ... the speed of sound. Because the bullet is traveling faster than the speed of sound (and you can tell from this image that this particular bullet was flying at more than the speed of sound because the angle of the shock cone is less than 90°), relative to the bullet's trajectory, except for that tiny bit where it's surrounding the bullet, the shockwave is entirely behind it.

That means it's impossible to hear the bullet until after it's gone past you. So if you were standing where the red oval is (not drawn to scale) and facing directly toward the shooter, you couldn't yet hear the bullet's crack even though it already has gone past you. And you won't be able to hear it until it has traveled far enough downrange for it's shock cone to pass over you.

Which means, as I've drawn this, when you finally do hear the crack of the bullet, it will sound as is it's coming from behind you and over your left shoulder. And presuming that the shooter was at least a good enough of a shot to get somewhat close, your sense of hearing will tell you that the shot came from behind you.

Watch videos of hunting feral hogs at night with NVGs and suppressed rifles and you can see this in effect. Whatever hogs are still alive will tend to ruin from the direction they heard the 'crack' coming from, which sometimes puts them running in the general direction of the hunters.

The silencer doesn't make the hunter silent, but it does make him (or at least his location) invisible.

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