If you have a way to do it, do what Mach 1 advised and put up paper. See what the bullet is doing at distance. The hits on paper will give you better information about what is happening at distance. Also, if you have a spotting scope, use that to watch the strike of the round. Put your target up at 50 yard intervals and fire 5 to 10 rounds. If the bullet has gone unstable, that will show it. You'll also be able to tell if its stringing due to differences in velocity in individual shots. Wait about 10 minutes between shot groups so the barrel can cool. If its cool enough to hold for about 30 seconds without being uncomfortable, its cool enough to shoot true. Shoot a group at distance, and then go back to 400 yards and shoot another control group. If the rifle still shoots to 1 moa at 400 and does it consistently, its not the rifle. Just a question: after you shot at 650yds, did you go back to 400yds and shoot again? If you did, what was the result?