The short/fat cartridges tend to be very efficient. I have long admired the 22 PPC and 6mm PPC. We also see the trend in the WSM family.
The 6mm AR, 6.5 Grendel, 6.8 SPC are all variations on this same theme, and all designed around the AR platform.
I've often wondered what history would have looked like if the U.S. Military had required a 6mm or 6.5mm projectile instead of the 5.56mm? But history is what it is, and I snap back into reality when my mind starts playing "what if".
I do think that at extended ranges the 6mm, 6.5mm & 7mm projectiles have far better terminal performance than the 5.56mm projectiles. I also think they are less affected by wind drift.
Several states, including mine, require calibers greater than 23 to take deer. So 6mm becomes the default floor for that legal requirement.
I think our collective quest for extreme velocity may have got a little out of hand in the late 1950's and early 1960's and it took a little while for that to work itself out.
Within the confines of an AR platform, the 6 through 7mm options are well represented.
Focusing on the 6.5 Grendel, it appears the bullet weights range between about 85 to about 130 grains. The 85 grain loads can exceed 3000 fps with the right powder and a long enough barrel….but if you need all that speed the 5.56mm can get you there.
The 130 grain bullets are down around 2400 fps and probably slower in most AR length barrels; that seems a bit counter-productive to me. There are better options in other platforms.
The 100-120 grain bullets seem to offer the best compromise. Considering the relatively small case capacity of the Grendel, the performance of the 100-120 grain bullets is impressive.