[3] There are two versions of the .338 Whisper: A subsonic cartridge is designed to fire its bullets at velocities slower than the speed of sound (1128 ft/s at 70 °F) to avoid the sonic crack caused by the bullet breaking the sound barrier, which contributes in large part to the overall noise produced when using a firearm.
Subsonic loads are often designed to avoid the turbulent transonic zone (~900ft/s - 1350ft/s or 0.8 to 1.2 mach) entirely.
To ensure terminal performance at subsonic velocities, heavy bullets for the caliber (250–300 grains (16–19 g)) are desirable.
Currently this is the 300-grain (19 g) Berger Hybrid Open Tip Match (OTM) with a BC of 0.818 and a length of 1.820 inches (46.2 mm).
A full power load with fast powder can push a 165-grain (10.7 g) Barnes bullet to a muzzle velocity of 2,520 ft/s (770 m/s).