Considerations on the Evolution of War

[stanc]

Could an ~11" carbine not be issued to the billets [LRRPF52] mentioned?

Of course it could.

Maybe a debate we need to have is between the Grendelx39mm and a longer version Grendelx45mm...

That's a reasonable evaluation to make. I'm told that some military entities have done just that, and concluded that the basic 6.5 Grendel is lacking in some aspects.

However, the question here is do you think we need a new LMG in a larger caliber?

LRRPF52 has stated in a number of threads he thinks exactly that.

If so, can we obtain that without chambering the carbines in the same caliber?

Possibly, although it may be more likely that the military would want carbine and LMG to be the same caliber.