6ar turbo 40 fire forming issue

[Headquarterz]

I’m having a problem with fire forming cases in my 6ar turbo 40. I fireformed 50 hornady cases with zero issues using the false shoulder method to keep the base against the bolt face. I then loaded 100 virgin lapua cases using the same method. Shot 15 and had split shoulders on half of them. I have several wild cats and a few of those are ackley or other forms of improved cases and have never had anything close to a 50% failure. Any ideas on what might be causing this?

I have the gas block shut off so it is acting as a bolt action during fire forming. I will say the cases are difficult to extract but no signs of overpressure.

 

[johnjohn301]

Since you're not seeing overpressure and the extraction is difficult, it might be a combination of case hardness and shoulder positioning, so you start with annealing the Lapua cases and carefully recheck your false shoulder placement, it should help reduce the incidence of split shoulders.