What's the difference between the 6mm Grendel and the 6mm ARC?

[LR1955]

Maybe a tweak to prevent chambering a 6.5 Grendel?

Already covered. You won't shove a 6.5mm neck into a chamber cut for a 6mm and finally, a 6.5mm bullet isn't going into a 6mm bore.

I use my Grendel magazines with the 6 AR with no problems so I doubt a new follower is needed for the 6 ARC.

LR55