The only SAME, RELIABLE source that I know of shows the 6.5 Grendel superior to the 6.8 even with a 1.5" SHORTER barrel!!!
I used to own a Grendel (I won't explain why I sold it), but I was getting 2600+ FPS with an 18" barrel and 120gr AMAX's.
Qualified Rifleman/Marksmanship Instructor/Lic. Medic
"There is therefore now no condemnation to those who are in Christ Jesus, who do not walk according to the flesh, but according to the Spirit" Romans 8:1