wow thats a big group at 600 yards for getting a 3/4 inch group at 200.

At 600 yards it should be more like 2 1/2 inches
The only thing I can think of is maybe the wind is bucking the smaller/lighter bullets from the .223.
So think of a Cal. that you will be able to shoot heavier bullets that will buck the wind better.
But the heavier the bullet the bigger the case needs to be to push it.