AR-10 long range caliber choice?

[Pilsner]

So I have to build an AR-10 for a gunsmith class that I attend. What caliber do I want for long range plinking and hunting deer sized critters? Will be shooting suppressed 100% of the time.
6 creed?
6.5 creed?
Any other good options that I should look at?
I just got rid of all of my 308 brass and reloading dies so that is a no-go. About anything else is open.
-Doc jaynes

 

[180ls1]

25 creed and use the Sig Hybrid brass.

 

[Brontosaurus]

22 creed