When I was in the army (Canadian), before we went to the 5.56, we used to do run downs from 1000 yds. We were using peeps graduated in 100 yd increments. Nobody ever hit dirt from 1000 - 600 but after that we were on target using standard ball ammo.

My experience with Winchester ammo is that the Power Points always provided the best accuracy in the .270, 30-06 and 300 Win Mag. Whenever I got into their high end stuff, things opened up. Just my experience, your mileage may vary. Lot's of variables at 500 yds that need be taken into account.