The new "smooth" barrel nut is tricky to get off as in different wrench or cutting it off, many people replace it with the old style.

It is a small shank.

I would try the 223 follower before I changed it.

To change the rifle from 223 to a 308 you will need a different mag box and follower a different, bolt head, barrel and possible modify or replace the front baffle.

The 4.4 screw spacing and the 4.27 screw spacing is connected to year manufactured and not caliber.