There are actually two versions of the .338 Whisper. The original .338 Whisper is based on the 7mm Remington Benchrest case, which simplifies conversion of rifles that use .308 Winchester case head dimensions (.470"). The .338 Whisper #2 is based on the .221 Fireball case. This allows easier conversion from rifles originally chambered in .223 Remington, such as the AR-15 and variants. 

This text is based on information from “Cartridges of the World”, Hodgdon reloading manual, the cartridge designer and/or own resources.