Wikipedia is a bit confusing when you look up the .300 Whisper and .300 AKC Blackout. Where it lists the parent case they both say .221 Fireball/.223 Remington and the parent case for both of those is listed as the .222 Remington. It sounds to me like you should be able to form cases for either out of the .221, .222, and .223, but I don't know if that's right. Supposedly Hornady makes their .300 Whisper ammo within .300 BLK specs so any firearm chambered for the .300 BLK can shoot their .300 Whisper ammunition too.
One thing's for sure, you don't want to accidentally fire either one in a .223/5.56mm. A couple of guys have blown up their guns doing that.