6mm PPC --> 6.5 Grendel --> 6mm ARC - Why?

[Lemonaid]

Getting back to the O.P.'s question. New bullets in 6mm that required faster twist barrels made the 6ppc (with a fast twist) and 6 Arc better at longer range than 6.5 Grendel. An example you can plug into a ballistic calculator is a Barnes 112 match burner (b.c.=.624) going at 2650 fps. This I got from my 24 inch barrel 1-7 twist 6 ppc bolt action. Even with the same barrel lengths in an AR platform, with the higher b.c. bullet in 6mm you get some extra yards before subsonic.
Now add in that Hornady can put out a new shiny product and make more money and here we are.