In a write-up of his 6XC cartridge on David Tubbs' website, Tubbs says:
“A detailed study of large and small rifle primers showed that large rifle primers worked best when the propellant charge exceeds 35 grains as is the case with the 6XC.” Which is why he apparently never consider SR primers for his cartridge, which has slightly less capacity than the 6.5 (or 6mm) Creedmoor.
I haven't run that study down (though must admit have been too busy to try very hard), but I tend to give serious consideration to whatever Tubbs says about rifle accuracy.
