1. Take any mobile phone with sound editing app. 2. Go to known distance away from target. Like 6 feet in my case. 3. Set mobile to record sound. 4. Fire. 5. stop recording. 6. You will see THIS: That spike is the sound of impact. But is it all that is? Zoom in. Keep zooming in until you see THIS: You will see two events. One is the sound of projectile hitting the target. And before that is the sound of the bands snapping. Write down the time when the second event just starts from background noise. And then write down the time when the bands have snapped and released your ammo. I assume it is at the highest point of the first event. Now then.... 22.33 seconds minus 22.30 seconds is 0.03 seconds. 6 feet divided by 0.03 equals 200. My projectile was traveling at 200 feet per second.