Depending on your particular case you may need a false shoulder or jambing a bullet to hold the case back against the bolt face for fire forming, if your blowing a shoulder forward without holding the case back against the bolt face your actually blowing the base of the case back to the bolt face thinning the base of the case.
Thanks for that important factoid. I may have mistakenly concluded that this necessity, prior to C-O-W fireforming, had already been covered and was understood.