I agree with working on the 38.5. Try adjusting the seating depth. Also, when you're reaching out to longer ranges, neck tension and neck length will really show an effect.
I've use the OCW for years and it works for me. Since I've gotten into long range shooting, I do the ladder tests at 600 yds. and look for vertical dispersion as you do. One quickly learns that a tight group at 100 yds doesn't always translate to one at 1000 yds.
