For the quietest suppressed shooting experience, we all know that subsonic ammunition is a must. Preventing the projectile from breaking the speed of sound prevents the projectile from making a sonic boom. Yes, just like a fighter jet, a bullet breaking the sound barrier generates a loud crack!!! as it flies downrange at supersonic speed.
When shooting unsuppressed, the difference in nearly all cases is difficult or impossible to hear. Subsonic or supersonic, the gunshot sound itself is so darn loud that it drowns out any sonic boom made by the bullet exceeding the local speed of sound. Throw a silencer on, however, and now the gunshot sound is suppressed so heavily that the difference between subsonic and supersonic ammo is drastic. Much larger than most of us would suspect.
Now, with many cartridges there’s also a difference in how much gunpowder is used. For example, one cannot load a heavy enough projectile into a .223 Remington to use effectively the same powder charge but cut the velocity by so much that the projectile is subsonic.
However, the velocity of some cartridges is so close to the speed of sound already that simply moving up or down in bullet weight is all that’s necessary to switch from supersonic to subsonic, or vice versa, without any change in the powder charge.
In the video embedded above (or click HERE to view it directly on Rumble), we chose one of those cartridges — your standard 9mm pistol cartridge — to demonstrate the sound difference between subsonic and supersonic ammunition, because bullet weight is the only variable that’s changing. There’s no difference in the actual noise of the gunshot, since it’s the same amount of gunpowder and same amount of pressure, etc., behind the bullet.
With a heavy, 147 grain bullet the “oomph” contained in the gunpowder isn’t sufficient to fire the bullet fast enough to exceed the speed of sound. With a lightweight 115 grain bullet, it is enough “oomph” to fire the bullet at supersonic speeds. And that supersonic crack or lack of it, my friends, is the sound difference you hear in the video.
Note: it is significantly more pronounced in real life. While the video provides an idea of what it sounds like, the microphone and the camera and your speakers or headphones simply cannot reproduce it realistically. In part because the supersonic gunshot is louder than your speakers and, in all likelihood, louder than the microphone or camera will allow. In many cases the subsonic shot is, too, but either way what this means is that the difference between the two is being compressed.
Nerd note: air temperature is by far the biggest independent driving factor in your local speed of sound. Use THIS CALCULATOR to figure out the speed of sound where you are based on the temperature outside.
Personal nerd note: with some 9mm and .22 LR loads, the velocity straddles the speed of sound so closely that I’ve been out on the range shooting suppressed where every shot was supersonic in the morning in 30- to 40-degree temperatures (speed of sound ~1,090 FPS at 35 degrees) and every shot was subsonic in the afternoon in 60- to 70-ish-degree temperatures (speed of sound ~1,128 FPS at 70 degrees).