Can't deny the 6.8SPC is better than the 762x39, but I'm not sure 6.8 is anywhere near "twice as powerful" as 556. Just googled some load data, and 556 is in the 1100 ftlb neighborhood, while 6.8 SPC is around 1600 ftlbs (16" barrels). That's substantial, but not double.
The ~115 gr class of 0.277 bullets doesn't even really beat the ~69 gr class of 0.224 bullets in BC either. Roughly twice as big, but quite a bit slower and similar BC, so they don't really ever start "passing" the 556 at distance.
I tossed some JBM calculations into my comparison spreadsheet. Here's a breakdown of my rough calculations. To keep it even, I used Sierra Matchkings for all calibers, and kept bullet weights in the "suitable for mil-spec use" category of "middle of the road". I estimated 16" bbl velocities using the great and powerful google...
JBM Inputs below (standard atmosphere box checked, same conditions, and 2000 ft elevation). Only thing I changed between modeling runs was bullet and velocity.
1) 556, 69gr SMK, 2700 fps (from my own rifles' hand load data)
2) 6.5 Grendel Beefy!, 107 SMK, 2600 fps (added ~100 fps to some interweb reports from 16" bbl Grendels)
3) 6.8 SPC, 115 SMK, 2500 fps (interweb reports of 16" bbl velocities)
4) Real 6.5 Grendel, 107 SMK, 2500 fps (interweb reports of 16" bbl velocities)
Velocity in FPS
View attachment 124517
Drop in inches
View attachment 124518
Energy in ftlbs
View attachment 124519
Wind drift (10 mph, 90°) in inches
View attachment 124520
Based on my calculations, it seems that if they could make a "beefy" 6.5 Grendel, that might be the real ticket. If you add about 100 fps to published 6.5G speeds, it's equal to the 6.8 SPC in power at the muzzle, and the gap only widens as it goes down range.
Even the existing Grendel catches the 6.8 around 120 yds out, and exceeds it from there on. Perhaps there's reliability issues in the fat bottomed, skinny necked, Grendel that preclude it from being suitable for military use in dirty, disgusting, poorly maintained, machine guns...?
I'd think the higher sectional density in the 6.5 would also be well suited for armor penetration, and/or the use of fancy military rounds with weird stuff inside (tracers, incendiary, AP, etc).