On the Applied Ballistics’ Facebook Page, you can find a series of fascinating posts that show bullet traces at different speeds from Mach 0.86 up to Mach 3.0. At the slowest speed (Mach 0.86), i.e. There is turbulence, but no shockwave, at 962 FPS. At the highest speed, Mach 3.0 (3375 FPS, sea level, 68deg F), a dramatic double-nose and tail wave is formed.
To learn more, visit TheScienceofAccuracy.com. You can also subscribe to the member-only podcasts and watch exclusive video content on that site. You can also purchase Applied Ballistics on the Science of Accuracy website.
Mach 3.00 Bullet Flight Image
At Mach 3 (335 fps), this bullet has a well-established shock wave at the tip and base. The aerodynamics, or shock waves, are not changed in any way compared to the transition through Mach 1.0. This means that the aerodynamics, and stability, are continuous, predictable, and easy to model. As you accelerate, the shockwaves create a shallower arc because the bullet moves forward 3X faster than it is moving away. The shock wave has a rise/run of 1/3.
If a bullet were to fly within 10 feet and travel at this speed, it would sound about the same as a 22-caliber. You would need hearing protection, as the energy in a shock wave of Mach 3 is high. How high …? In 10 yards, the bullet slows down from 3355 fps (the speed at which it travels) to 3334 fps (the speed at which it travels). This happens in a time frame of 0.0090 second. The 55 ft lb of Kinetic Energy that is lost in this 10 yard distance is due to aerodynamic drag, which is composed of wave, base and skin friction drag components, with the majority of drag being due shock wave formation. The 55 ft.lb. of energy that is lost in 0.0090 second requires a power output 6,111 ft.lb/sec, which is 11.1 horsepower. Most of the energy goes into creating a shock wave. Remember that it is a 3-D cone which travels a great distance and gets its energy by stealing the velocity of your bullet!
Mach 1.00 Bullet Flight Image
The image of the transonic structure shockwave was captured by firing many shots at Mach 1.00. Local airflow in some areas of the bullet is now greater than Mach 1.0, as the bullet is now traveling at the speed sound. When something moves through the air faster than air can move out of the path, you get a “shockwave”. This is what you can see in the image. The areas where air density changes quickly (in the compressed wave) are visible near vertical lines, and a detached front bow wave. As the bullet moves through transonic speeds, a shockwave structure is formed. This structure has a strong effect on the drag of the bullet (wind sensitivity), and its stability.
The exact development of shockwaves, and the effects they produce, are unique to each bullet geometry. It is also very difficult to predict the effects when the bullet transitions from subsonic flow (incompressible without shock waves) into supersonic flow (compressible with shock waves). This is because each bullet geometry behaves differently, which is why it can be difficult to determine the transonic stability criteria of bullets with different shapes.
Mach 0.86 Bullet Flight Image
Here’s an example of a bullet traveling at Mach 0.86 (which is 86% the speed at which sound travels, or 962 FPS in 61deg F). As you can tell, the bullet is not moving fast enough to create waves. However, you can still see turbulence (right side of photo) in its wake. On the bullet tip and at the boat tail/bearing surface junction, you can see the beginnings of small shockwaves. The airflow surrounding this bullet is primarily subsonic. This bullet would not produce a supersonic crack as it passes the observer.
