Couple other things that really helped with my accuracy was to switch to Large Pistol primers, to Anneal the cases, and to only neck size a little ...I watched my groups go from 10"+/- at 300yds to less then 3" with just this simple change.
I have found that by useing unsized, fire formed 45-70 cases and raising the powder charge to 74gr 2f KIK compressed .3, behind the 535Postel, that the load holds together better at longer 800yrd+ ranges. I am sure that the velocity increase has played a large role in keeping things stable past 500yds.
Maker of Horse Tack and Cowboy Gear.
Mister, why do you carry a 45? "Cause Sam Colt don't make a 46."