Actually the 300 whisper and 300 BLK both use a 221 fireball necked up to 30 cailber or a 223 remington cut of at the shoulder and sized in the whisper or blk die to form the cartridge. Uncut 223 brass necked up to 30 calaiber would be more like the TCU wildcats used in the TC contender pistols.
Notice that the 300 whisper is loaded to nearly the same length as the 223 remington. When a 240 gain bullet is used, this leads to binding in unmodified ar-15 magazines when trying to load more than 5 rounds in a 20 or 30 round magazine. I milled a slot in the rib to eliminate this problem.
The 300 BLK deals with this problem by loading to a shorter overall length so unmodified magazines can be used can be used. As far as I know, the same dies can be used to load either cartridge and the 300 BLK will chamber just fine in the 300 whisper chamber. But a 300 whisper carridge loaded to 2.25" will probably not chamber in the 300 BLK.