FPS or Feet Per Second is the most commonly used measurement of speed in the shooting sports world. This is a measurement of the distance a projectile or bullet travels over time. Specifically, the number of feet traveled in a second.
If a bullet moves faster then 1,126 feet per second it is then moving faster than the speed of sound, and it will create a supersonic “crack” that is very loud. Many ammunition producers create special “subsonic” ammo that travels slower than 1,126 feet per second, staying slower than the speed of sound. This means there is no supersonic crack when the bullet is moving through the air.