Chris,
Does that mean it would be reasonable to start with a Colt AR-15 LE and eventually modify it to handle the 6.5 Grendel round? Or would the Armalite AR-10 be a better place to start?
The beauty of the Grendel is that it works with off the shelf parts in the AR15 architecture. Parts are way more plentiful and alot less expensive.
In the AR-10, you can move to something really extreme, like the 6.5 based 300 I'm playing with.
The issue is overall cartridge length. The length of the Grendel makes it workable in the 15-length magazines and with the 15 bolt stroke length.
If you build one, get a competent gunsmith to work you up a throat depth based on the projectile you'll use. I like the 123gr Scenar. It has a fairly plump ogive that allows you not so deep throating and still chambers well, but will give you tight bullet/bore spacing and better accuracy.