Nominally, 300 Blackout and 300 Whisper are the same but 300 Blackout has been accepted by SAAMI and has standard dimensions. 300 Whisper is a proprietary design and folks wanting to get around the royalties officially call it something different and change the dimensions a little.
300 Blackout is shorter than 223 Remington. While you could expand the 22 caliber neck of the 223 Remington case, it works better/easier to cut the neck off at the shoulder and then begin the case forming/trimming process. Or, run the 223 cases in a trim die which will for the 30 caliber neck. Then cut the case off at the top of the trim die.
The brass in the 22 caliber neck of the 223 Remington is not needed for 300 Blackout. No sense trying to make it 30 caliber.
In theroy, you could make 300 Blackout cases out of 221 Remington Fireball cases. then you would have to expand the neck to 30 caliber. It would be best to do this in two or three steps and not all at once. Annealing the necks might also be desirable. Still, at the cost of 221 Rem Fireball cases, the 223 Remington option is more cost efficient.
