Hi Mike,
It's been my experience with my 45-90 roller that a 535 grn or larger bullet at anything over 1250 fps is pretty effective to a 1000 yds. That's assuming that the places I've shot it were really 1000 ranges. If they weren't, they were close enough for government work. If you can work up a load that will come close to shooting the possible at 500 yds, there's a real good chance it'll hold at 1000. Hope to see you at Puyallup next week.
LJ