Why do you need a false shoulder for an Ackley improved chamber?

If properly headspaced, you should have the .004" crush fit against the regular 260 shoulder. No need for false shoulder.

FWIW, I have done some of my early load development while fire forming. You can't get a final load that way, but it does let me test some different bullets and powders to try and find the ones that the barrel likes. JMHO.