6.5 Grendel is THE top dog hotrod caliber that will fit in the AR15 platform. The .257 TCU and 7mm TCU will also fit, but they're not hotrods. They're built and designed off the .223 case. Same goes for .300 Blackout (7.62x35).
That's not quite accurate, but I believe you had the right intention. Just miss-typed.
The 6.5 Grendel MAY BE THE "top dog hotrod" but of course, that depends on what you feel "hotrod" would normally mean.
One could argue that "hotrod" would equate to velocity, in which case the Grendel is obviously not the top.
You may have meant that "hotrod" refers to the bullet size that can be sent...and that's not true either as the 6.8, 7mm and .7.62 are all fed in some form or another..
So I assume that you're referring to either the kinetic energy deliver at a specific range or the ballistic trajectory at a specific range.
At any rate...it's not the best of anything.
I will concede that it's probably the best over all cartridge for ranges from 400-800 yds in terms of energy delivery and ballistic trajectory.
.308, 7mm-08, .260 Rem, WSSM calibers, 6.5 Creedmoor, etc...Will all have to be built on an AR10.
Nemo Arms makes a .300 WinMag AR style rifle. They're not cheap, but they are badass.
The WSSM is actually designed for the AR15 and with the likes of the 243 and 25WSSM pushing 105-117gr bullets upwards of 3000-3100+ fps. The ballistic advantages are clear.
It's the SAUM/WSM that goes into the AR10 platform.
As to the OP's question about the Grendel...it's just a name, much like 17 Hornet or 221 Fireball.
Do some reading about "Beowulf" the story and you'll probably pick up on why it was called "Grendel" and also why Bill Alexander chose them to name his babies ".50 Beowulf and 6.5 Grendel."