I cannot say anything about the flattening, but I have also had 6.5x55 cases do this. With my ones it was a relatively 'normal' load with 42gr powder so not a hot load. With mine, it was only noticed when I extracted the fired round and only that part of the case come out of the rifle. The front bit stayed in the chamber. It flicked out easy enough with a nail we found so we suspect the brass I was using was towards the end of its usefulness. I will say, mine was not Norma- it was Sellior and bellot and had only been reloaded the once.
I suspect there is two factors at play in the above exmple. The load was hot which flattened the primer AND the case was at the end of its life with the spearation. (Hot loads would accelerate this..) If it ws like mine, the load would have made no difference and the separation would have occurred anyway even with a lighter load. I now check all my brass fo this but having loaded a number of calibres, the 6.5x55 is the only one that has ever done this...
NOTE Above was not me - unfortunately I made the error in jdugment and sold my 6.5x55 some years ago .......
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