Another thing that can cause it is scratching the primer pocket when decrimping or cleaning I had that happen with a 2nd hand lot of Lapa 308 brass picked up.
5 blew out of 30 but none of my mixed lot ppu, winchester or federals did all primed with the same cci primers. Like you I didn't notice till later. Didn't worry about the bolt face though not after having seen some nra target rifles.
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