Buffalo Head is right on the money from a military perspective as they are limited to FMJ bullets. If you are going to stick with FMJ then 1/12 twist with 55 grain is the way to go.
However, please keep in mind that you are not limited to FMJ ammo. A wide variety of 223/556 defensive and hunting ammo is available which provides superior performance to the M855 and M193 mil rounds (Hornady TAP would be one example).
If it were me, and I’ve been down this path, I would opt for a 1/7 or 1/8 twist shooting HP rounds in the 69-77 grain range. If you choose to go with a modern (M4 style) rifle, please look at Lewis Machine & Tool. Quality is superior to almost every other manufacturer and you don’t have the politically correct features (oversize trigger pins, half-circle bolt carriers) that you have to put up with from Colt.
Just my two cents.