precision fire arms Neptune III?

Also how accurate is the 6.5 grendel at 800 yards? Is it hard to group that far with that caliber?

It depends on the barrel and load like any other rifle. I like the 123gr A-MAX factory load, as well as Mark's (Precision Firearms) 123gr Lapua Scenar loads. My little 16" Grendel usually groups somewhere around 1 MOA, but I have had my best results with those loads. .79" 4rds at 100yds with the PF 123gr Lapua Scenar in Lapua brass, and 1.69" 4rds at 200yds with the Hornady 123gr A-MAX factory load. They were 4rd groups, not 5 with an excluded "flyer".

I took my little AA 16" out to 1200yds this past weekend at 4400ft ASL and 81 F, and had POA/POI repeatedly within a small natural target in the terrain, which couldn't have been larger than 3ftx4ft. There was no wind, which makes a big difference of course.

Everyone I know of with a Precision Firearms Grendel has reported superb accuracy, since Mark only uses high-end pipes in his Grendel builds, like Lilja, Bartlein, Les Baer, Hart, etc. When I say superb, I mean 1/2 MOA or better. A bad group would be 3/4" at 100yds.