My theory: when the expander pulls back up through the neck, it pulls the shoulder up with it. Still fits in my L.E. Wilson case gauge just fine. But will not chamber and fire. Functional cases actually sit down in the case gauge, even after firing. Some of the brass has thicker walls than others. For cases formed without the expander die in it, the shoulder is in the proper place.
Some of my reading indicates that 300 Whisper and 300 Blackout differ in the expected parent case. Whisper is from 221 Fireball cases necked up, while Blackout is from 5.56 cases cut down. Guess what kind of die set I have? Hornady 300 Blackout/300 Whisper. I don't know if that makes a difference. Something tells me it just might. Or I am just a moron. That possibility always exists.
I've thought about trying the cream of wheat fire forming method. Not real sure about doing it in a semi auto.
Added: Went on a case killing spree. Decimated (every tenth case got