The usual explanation from the primer companies is that the primers have been exposed to ammonia ions after leaving the manufacturer .
This etches the stressed part of the primer cup leaving a week point which fails under normal pressure.
Federal had this happen with small rifle primers years ago.
I use Rem 7.5 sr primers in 6 PPC .
Pity as the Winchester plated primers were very good.
The Russian primers do not seem to have this problem.
Glenn
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