Certainly there are differences between the 6 and 6.5, but basically they're the same case and will have "close enough" loads to indicate at least a useful comparison of powders.
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The takeaway from this: in a nearly identical case to the 6mm LBC/Predator/Grendel, 8208XBR gave me the highest velocities while remaining within my tolerance for pressure signs.
Your assumption is flawed, and so is your takeaway.
Powder burn rate requirements for a light bullet 6.5 load are NOT the same as for heavy bullet 6mm loads. Heavier bullet (for caliber) - slower burn rate. Smaller bore - slower burn rate.
You cannot assume that the best powder in 95gr 6.5 Grendel loads is also the best powder for 105gr 6mm loads.
8208 is a great choice for light to mid weight Grendel loads. It is not the best choice for 95-105gr 6mm Grendel loads; it works but burn rate is a little fast.
Conversely, Lever is a bit slow burning for 95gr 6.5 Grendel. It is a good choice for 129/130gr 6.5 Grendel though.
