6 mm Whisper and 6 mm KAC PDW

I happened to be checking some of my cartridges and noticed that the 6 mm Whisper cartridge is virtually identical to KAC’s 6 x 35 PDW round. In fact, they look interchangeable.

Is this just a coincidence or are they related? And are they in fact interchangeable? I expect that guns chambered in 6 mm Whisper would need a tighter rifling twist to stabilise the much heavier bullets normally loaded.

Tony, if the 6 x 35 PDW is a 221 Fireball necked up to 6 mm without any other change, they share the same case dimensions. Anyway I think that Whisper chamber dimensions should be quite different