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I only shoot maybe 150rounds/year. Think your still bit unlucky having primer do that at std pressure.

Primer pockets stretch with repeated loadings can be a factor. You can feel when they start...

Primer doesn't look flat or cratered.
Is it a Winchester primer? They're more known to get unlucky with. Torched up the bolt face?

I use 760 in 270 with 130gr. 2209 would be temp stable and generally a lower extreme spread than 760.
I'm not a good enough shot to benefit though I don't leave the ammo on the bench in the summer...