OKH is right about the Mini14 firing out of battery. Like the M14, the web (or bridge) of the receiver cams the firing pin back until the bolt is fully locked up. Once the bolt is rotated into full lockup, the firing pin is free to be driven forward by the hammer. This safety feature could be defeated if the firing pin was broken.
From what I've heard, it sounds like the ammo is at fault. Remember that all guns are proof loaded (150% above SAAMI specs) at the factory. To blow up a gun the load would have to be considerably higher.
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