Breaking the sound barrier requires exceeding the speed of sound, which is approximately 761 miles per hour at seal level when the air temperature is 59 degrees Fahrenheit. As the temperature decreases, the speed of sound also decreases.
When bullets are fired from a machine gun, they travel at a speed greater than the speed of sound. The shape of high-velocity bullets played a role in the design of the Bell X-1, the first airplane to successfully fly at a supersonic speed. When a plane accelerates through the speed of sound, a loud sonic boom occurs that can be heard from the ground.