I completely agree with Jerry, and a couple other guys, that it’s the barrel that is the number one source of accuracy on either of these AR platforms. A poorly cut or chambered barrel, will never shoot fantastic, no matter what.
After a barrel swap, then a precision trigger, adjustable gas block/longer gas tube, and possibly upgraded bcg/buffer system.
I bought a used Stag in Creedmoor, with only a trigger upgrade, and with handloads, it would regularly shoot around 1 MOA. Sent it to the magicians at Insite Arms, swapped out the stock barrel for a 26” bull barrel Krieger 4 groove, with a plus 2” gas tube and AGB, and it easily shoots more then twice as accurate. Not a cheap upgrade though.
I’m sure it isn’t any tougher to add this swap to the other non restricted ARs as well, just depends on your accuracy expectations vs price tolerance.