The upper is getting replaced so it can be beefed up a bit to withstand the pressures.
The barrel and especially the barrel throat have to be able to handle 80k+ psi 5.56mm loads from the new superhot rounds.
It's fit right onto a factory M4A1 lower so it's using the standard carbine buffer length.
The new rounds should be dimensionally identical to M855/A1 or Mk 262. It's just a LOT hotter.
"Primary Attributes:
HICAR shall be chambered in 5.56x45mm NATO. (T=O)
HICAR shall be compatible and capable of fit and functioning with the M4A1 lower receiver group without permanent modifications (T); with zero modifications (O)
HICAR shall fire and function with M855A1, Mk262 as well as Government supplied M855A1+ hypervelocity ammunition loaded to 82kpsi (T); with future hypervelocity ammunition designs optimized for projectile velocity and terminal effects on target (O). "