It makes little sense for civilians to worry about fmj bullets when better options are available.
I can carry a lot more 5.56 than .308, and I like having more in the magazine. At 400 yards, anyone with some marksmanship skills will be effective. And in a civilian confrontation, I doubt many shots will exceed 200 yards. But that is pure speculation and others will have better data.
I have a .308 “sniper” rifle should the need arise. But will use the AR as the main rifle if the SHTF.
A load out of 5.56 is heavy enough for me. Would not want to carry that much in .308.