Long range rifles may seem to be capable of delivering accurate shoots at every possible distance, but they have a limit: the transonic region.
In the previous articles about static stability and dynamic stability, we have seen what it takes to properly stabilize a rifle bullet to keep it accurate to the farthest distance possible. However, at one point during the bullet flight, the stability may be compromised, exactly as it travels through the transonic region, so let’s see what that is and what happens to the bullet when that happens.
When the bullet exits the muzzle of a high powered/ long range rifle, it generally travels at a rate of two or more times the speed of sound (the speed of sound is approximately 343m/s, or 1125fps, in standard atmospheric conditions), and so bullet speed (in these cases) is therefore considered supersonic. When the bullet flies supersonic, it compresses the air in front of itself, generating a series of shockwaves that origin from the bullet tip and propagate behind the bullet as a cone. You can observe this on the image to the left, which is a shadowgraph photo of a supersonic bullet in flight at Mach 2.66 (that is, 2.66 times the speed of sound).
These shockwaves generate the crack that you can hear if you are alongside the trajectory of a bullet that is travelling at supersonic speeds. It is the same explosion, the so-called sonic boom (or sonic bang), that you hear when a jet fighter flies supersonic above your head.
When the bullet flies supersonic, the center of pressure (of which we talked in the article about bullet shape) is located somewhere between the bullet tip and the center of gravity (it is possible, but rare, for the center of pressure to be behind the center of gravity).
Going downrange, the bullet loses its initial speed, due to drag, and reaches the “transonic region” when its speed reaches Mach 1.2. Going farther, it crosses the sound barrier at Mach 1, and then it exits from the transonic region when its speed falls below Mach 0.8.
When a bullet flies through the transonic region, the aerodynamics dramatically change. As we can see from the set of shadowgraph pictures to the right, the
shockwaves shift from the tip of the bullet backward to the tail as the bullet approaches and then crosses the sound barrier at Mach 1.
What happens in terms of ballistics and accuracy, is that the center of pressure shifts forward toward the tip of the bullet. The shifting of the center of pressure lengthen the lever between it and the center of gravity, amplifying static and dynamic instability. The result is that the bullet angle of attack and yaw might dramatically change, invalidating all the trajectory predictions made by ballistic software, and making it impossible for us to compensate correctly for drop and drift. It might also produce an increase in wobble, which can lead to an accuracy decay and can make the bullet tumble. That’s the reason why shooting beyond the transonic range (the distance at which the residual speed reaches Mach 1.2) results in “key holes”, that is, holes made on the target by tumbling bullets that impacted on their side, instead of at the tip.
I said might because a bullet might eventually pass through the transonic region without any trouble. The ability of a bullet to pass through the transonic region is hard to predict because too many factors come in play—many of those factors are not measurable without specific equipment.
Generally, the best thing to do is to try to avoid the transonic region and set the distance at which our bullets go transonic as the maximum effective range of our weapon system.
This article closes the series on external ballistics. In the next article I’ll talk about MOA an Mil, explaining what they are and trying to clarify some doubts that shooters often have about them. After that, we’ll start to put all this theory we’ve learned up to this point into practice, starting with ballistic tables (learning how go read them and how to create one), and on to talking about long range shooting techniques. Stay tuned!
Featured image, courtesy of National Geographic, represents a fighter jet crossing the sound barrier. It is an example, on an observable scale, of the passage of a flying object through the sound barrier.
