Thanks. I took some time to play with the seating depth and it proved to help a bit. . Each incremental change showed small improvements.
This is the last target. Picture isn't oriented correctly. First two shots were touching with one to the left (bottom) was the third shot.
I've loaded up 20 rounds at this length and will be testing at 300, 400, 500 & 600. Really hoping it holds at the longer ranges.