That's pretty much what I've done. Start with 6BR Norma changed case capacity to match my measured H20 volume (39.1gr), changed case length to match my fired brass blow length, and changed the weighting factor from 0.5 down to 0.4 to account for the 40 degree shoulder and more efficient shape. Works well.
Thank you, Gentlemen. Where did the change in Weighting Factor come from? I have never paid attention to that. How much did Fire Forming change your case length?
I got the same measurements/results when fireforming, cases were all very uniform..Weighting factor has to do with how much powder gets launched down the barrel as the charge is going off. A straight walled case allows the entire powder charge to move as the powder nearest to the primer ignites first. As you add more shoulder angle and bottleneck shape the powder charge is more contained within the case, contributing to a more efficient burn and lower pressure overall since the burn is just moving bullet, not powder + bullet.
I went from 1.555 to 1.550 when fireforming 6BRA.