Greetings @Padox,
Magnum primers have negligible effect on the cause of head separations. In a chamber that is too long for the cartridge (or a cartridge that is too short for the chamber) the case starts to stretch at a pressure of less than 40,000 PSI. The front half of the case is gripping the chamber walls and the solid head of the case is forced back against the bolt face. This results in a groove as shown in the OP. Most cartridges operate at a maximum pressure of between 55,000 and 65,000 PSI so the small amount of additional pressure from a magnum primer has no effect on head separation. 40,000 PSI is around or below start loads for most high pressure cartridges.
Regards Grandpamac.
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