Subsonic ammunition is any cartridge that propels its projectile at a velocity that is below the speed of sound, which is approximately 1100 feet per second depending on atmospheric conditions and elevation. The purpose of subsonic ammunition is to avoid the crack from the round breaking the sound barrier, which is perceptible all along its flight path. Subsonic ammunition is designed to operate at velocities below the speed of sound, which avoids the supersonic shockwave or "crack" of a supersonic bullet, particularly for suppressed firearms, influencing the loudness of the shot. Subsonic ammunition usually uses heavier bullets to retain as much energy as possible at the lower velocities. Some subsonic ammunition is used in non-suppressed firearms to gain the advantages of heavier bullet weights. Subsonic ammunition is typically extremely quiet through a suppressor, mild recoiling, fun to shoot, and useful in hunting and self-defense applications. The most common subsonic calibers include .300 Whisper / 300 AAC Blackout (7.62×35mm), .338 Whisper, 9×39mm, 12.7×55mm STs-130, and .510 Whisper.