From my experience, the single most likely culprit here is applying too much crimp. In this situation, you should be applying no more taper crimp than is necessary to remove the bell you put in the case mouth when you flared it prior to seating the bullet.
Some plated bullets are very prone to deforming under crimp pressure. The deformation leads to destabilization and keyholing.
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MOLON LABE.
- Leonidas of Sparta, 480 B.C.
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