Trigger Control, Breathing, sight alignment.
400 yards is best with the .223, 6 inch steel target. Shooting stix is more practical, prone is dead-on. I even go to 500 and 600 yards prone. If you can consecutively hit at those ranges, closer stuff is cake. I've infected some friends too.
Practice helps, I hit my deer this year, in the neck, as it was looking back at me at 526 yards, as I was kneeling. I loaded one round in the chamber for that shot.