Re: 6mm cm long range load development.
The way I was taught to do load development by my mentor:
Find the powder and bullet which it sounds like you have, chrono it and check es, sd(single digits). Play with seating depth and see if you can do any better. Print groups as far as you can see the group develop. For me it is 300yds with the optics and range I shoot at. Don't just shoot one group of five, shoot four groups of five. You will get a better idea of what the rifle actually will average. The first time I watched my mentor do this it was only a couple hours at the range. He had a load worked up for his comp gun. A portable press is very handy for load development and matches like the NBRSA Nationals in Calif. where you can get six days of shooting.
"Amazing things can happen when preparation meet opportunity" Richard Schatz
"The will to win, compares little with the will to prepare to win" Donovan Moran