I currently hunt with a 16" 300 Blackout from LWRC.  Been fun and only 8lbs w/ scope and ammo. Deer and Hogs drop fast with 110 Barnes Factory Load out to 150 yards.  I have a 308 suppressor (Ultra 7) on the way and intend to hunt with it switching between supers for deer/hog or sub-sonic at close-in for hog.  I limit myself to an accurate 200 yard shot with the Barnes 300 Blackout 110 gr.  About to submit my F1 to SBR a lower.

Looking to have a another (already have a 556) semi-auto that I can use if there is a high likelihood of a 200-400 yard shot...

I was going to "chop" the 300 blackout but I'm wondering if the 6.5 SBR can just be used as my up to 400yard hunting rifle and sell the 300?    What am I "losing" if I only have a SBR 6.5? Can I sub-sonic the 6.5 SBR assuming I get a fast twist? I've read the dozens of 6.5 vs 6.8 and its driven me to the 6.5 (instead of 6.8).  I don't reload, so factory load from internet purchase only.

I like to walk, stalk, and hunt on the move hence I want to have a lightweight SBR option for out to 400 yards however honestly 90% shot opportunity is likely within 200 yards...

I'm in the same boat looking to jump on the Grendel wagon. I can't hunt suppressed so I'll likely be dumping my 10" 300BLK upper soon. Coyotes are about all I see where I'm at.

At your 300-400 yard possibility shot range you will not see any advantage in 6.5 over 6.8. The edge (very, very slight) would actually go to the 6.8 at the short ranges.

I use a 12.5" 6.8 for deer hunting and have no problem at the ranges your asking about.