It's "showing" on the primer as low pressure I'd venture a guess...unless you're using .300BLK trim length and a .300 Whisper chamber. The chamber is shorter on the Whisper and using BLK spec loads can run the mouth of the case into the front of the chamber crimping the brass into the bullet and not allowing it to release.
What exactly are you seeing on the primer? Unless you're loading a super fast powder, pressure isn't much of an issue on the BLK.
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