Ackley chambers are supposed to have a 0.004-6" crush fit on the parent round to obviate the need to jam the bullet into the lands.
If the primer was backed out, this could be because your chamber is a skosh too long, the firing pin drove the case forward, where it expanded against the case walls and stretched rearward.
Or it could be the powder... I had a 6mmAI break a couple of cases like that, but just below the shoulder when fireforming. A change of powder (to a faster & easier to light powder) and it doesn't happen anymore.
I hope I'm wrong, but it really sounds to me like your chamber is too long. Talk to your gunsmith about it.
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