6.8 X 43 SPC,
6.5 x 45 APC, or
6.8 X 45.
The Grendel requires new bolt and a new magazine to function, has a significantly reduced magazine capacity, and an unacceptable increase in weight (210 rounds in magazines). The 6.8 SPC suffers from the same defects.
The 6.5 x 45 and the 6.8 x 45 use the same bolt face and magazine (with minor modifications) as the 5.56 X 45 with minimum increase in weight.
I can’t watch this from work, but I think this is the Barrett 468 assault rifle video.
I have a Bushmaster M4 in 6.8. - "significantly reduced magazine capacity" - no. Going from 30 rounds to 26 is hardly significantly reduced, especially when you consider what you're getting for that 13% capacity loss.