I shoot a custom 7 mag. Killed several elk out to 600 yds. Browning is a good choice. If you also want something for pronghorn, 7 mag is a great choice. Fast, flat with good BC. I shoot Berger 168 Hunting VLD. Everyone likes different rounds and what you choose will depend on what your gun likes to eat.
The only problem I see is that if you're serious about 1000 yards for elk, the remaining energy starts to be questionable/reliable to kill a 500-750lb animal unless you're one heck of a shot.
I agree with Elkeater. 0.4MOA with a .30WM @ 800 yards is a good limit. If you want to go 1000 yards, 0.25MOA is close to what you need with a smaller bullet.
IMHO