The beauty of 6.5's is being able to shoot long heavy bullets. Not so with the Grendel, I think the heaviest available is 140G.
With the ARC you can utilize almost any 6mm bullet and you gain some velocity.
And if you're buying ammo I think the arc is generally cheaper. On tne other hand, there is a bigger selection of factory grendel ammo.
The hogs don't care.
