Originally Posted by DocGKR
If you truly, "want a cartridge that will reach out and touch someone at 800M and do it with authority", then neither the 6.8 mm SPC nor the 6.5 mm Grendel are what you need; a .338 Lap Mag is likely the best and most combat proven choice.
Typical 10-16" barrel combat carbines with red-dot optics like Aimpoint/EOTECH, 4x ACOG's, or 1-4x optics are NOT going to be routinely effective at ID'ing, let alone hitting human targets 800 m in a combat environment. Most experienced folks are reaching for a sniper, indirect fire, or CAS at that range.
Again, as I noted above, 6.8 mm offers substantially better terminal effectiveness compared to ALL 5.56 mm loads, including Mk262, especially through intermediate barriers.
Like I always emphasize, "Only after proper foundational and ongoing repetitive refresher training, cultivating warrior mind-set, and ensuring weapon system reliability do you need to worry about ammunition selection. Most folks would be far better off practicing with what they have, rather than worrying about what is "best". As long as you know your what your weapon and ammo can realistically accomplish, it is all just a matter of training and shot placement."