Why not go with both?
All you need is a different upper and bolt carrier assembly, I believe.
Also, I eschew the meme that after The Fall we will be walking around looting dead bodies of their stuff. I’d like to think this will be true, but I gotta be reasonable.
Only difference is the barrel. Same upper receiver, magazine, and BCG.
Looting Dead Bodies...?
Zombie Attacks will at least leave
A fairly clean environment.
Mags.
The beauty of the .300 BLK is you can shoot it from an AR-15 lower, and with 5.56 mags.
To go with a .308 (7.62 NATO), you need an AR-10 lower.
I am building some .300 BLK rigs for my Wife, Son and myself. Next will be an AR-10 in .308 as I have a shitload of that ammo from my youth in harness.