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.