Question: what is a 6mm creedmoor?
Answer: As noted, the 6mm Creedmoor was hatched from the 6.5 Creedmoor, and that round is the ballistic twin and nearly identical physical twin to the 260 Remington. It's essentially the 260 Rem. case with the shoulder pushed back a bit to lengthen the neck. The shoulder is sharpened from 200 to 300 as in the 30 TC cartridge.