They make ARs in .308. That’s what I’d think they’d want if they want to have more firepower than most of the bad guys. You get more knockdown power and added range.
The .308 has more knockdown but surprisingly, the .223 with the long, heavy for caliber 68/69 grain bullets actually out range it. Probably have to have a 20 inch, or longer barrel for the .223 tho.