Float Pilot, its my understanding that the grendel has a BC of .510 and the 6.8 has a BC of .35. Is that not more than "slightly better"
I did say within reasonable hunting (or combat) distances. And do not get me wrong,
I am a big 6.5mm Fan. Particularly in bolt guns where they can be loaded long, using the longer and heavier bullets.
But the shorter bullets often nessecary in an AR action are often not real high in the BC department.
For example: some 6.8mm (.277caliber) bullet B.C.s
6.8mm Bullets
90gr Speer HP= .275 BC not sure I believe that one
95gr TTSX = .296 BC
110 gr Accubond = .370 BC
110 gr Bares TTSX= .377 bc
120 grain Hornady SST= .400 BC
120 gr Barnes Banded = .438 BC Takes a 1 in 10 twist most of the time.
130 gr Nosler Bal-Tip = .433 bc
130gr SST = .460 bc
6.5mm (264 caliber) Bullets
85 gr Sierra HP = .225 BC
95 gr Hornady A-Max = .365 BC
107 gr Sierra HPBT = .430 BC
110 gr Barnes Banded Solids = .452 BC
120 grain A-Max = .465 BC
120gr Nosler Bal-Tip = .458 BC
130 gr Nosler Accubond = .488 BC
So.... If I fire a 120 grains .277 caliber Hornady SST (BC of .400) at 2,720 fps from my 24 inch 6.8mmSPC (which I do)
And then lets say we use a 24 inch barreled AR, for the 6.5 Grendel using a 120 grain A-Max (BC of .465) at the same velocity::::::::::
120 gr 6.8mm with BC of .400
100 yrds= 2,500 fps... 300 yrds=2,093 fps...600 yrds=1,563 fps..800yrds=1,283 fps
6.5mm, 120 gr with BC of .465
100 yrds= 2,530 fps...300 yrds=2,175 fps...600 yrds= 1,700 fps..800yrds=1,430 fps
But as you can see there are some 270 and 264 caliber bullets which are pretty close in BCs.
BTW I have loaded 150 grain Speer flat based spitzers up to 2250 fps from my 24 inch 6.8mm. But those were really hot loads.