My understanding is that small rifle primers give lower SD and ES than large rifle primers in the 6.5 Creedmoor cartridge. 6.5x47 Lapua and 6x47 Lapua, (which, alongside 6.5 Creedmoor and 6MM Creedmoor, as the top ranking cartridges in the Precision Rifle Series) have a small primer pocket, as well.