Hello fellas. I have a 7mm wildcat on a 300 WSM case. I need help with COW fireforming, I neck down 300 WSM to .314 neck diameter in two steps. It’s a tight crush fit in my chamber. I have no issue using Winchester, Hornady or Norma 300 WSM brass. After fireforming I use a bent paper clip to check for case head separation and no issue. As soon as I use Bertram 300 WSM I get severe case head seperation. The pictures show my necked down 300 WSM brass on the left the neck 3 cases are Bertram. The next cases are Winchester, Hornady and last Norma. All these were done today using exact same die set up and loads. I have used Winchester and Norma brass the last couple years and no issue. Now I want to fireforming some more brass since the corona thing and I’m having issues. What do you guys think is going on? Any help would be appreciated!!