I don’t know if this is useful to you but FM sells spare barrel nuts pretty cheap. There’s a thread somewhere that shows the barrel profile. Might have been on Reddit.
Haha, yeah that's me. I've just heard the 300blk uses a .750 gb journal vs the 5.56 using a .625. I was curious if they changed the barrel nut in a way that would allow using standard barrels without modifications.
From the archived thread, you'd need one with the right taper to not interfere, and turn the stop behind the journal down to allow the nut to pass over it.