Re: 6.5-284 on deer sized game..
The 6,5-284 with the amax 140 should get you well past 800 yds. The Berger is a tougher bullet in my experience, so for not heavier game I would use the amax, although its BC is lower than the berger´s.
I have tried the berger in a red deer, dropped it in its tracks but it was a close shot, offhand while it was going through the bush, so not very informing for LR purposes.
I have shot lots of medium game with the amax and the berger, and the terminal ballistics of the amax are better for me, the berger seemed to be too tough for animals weighing up to 90lbs, meaning the expansion and shock was far more noticeable with the amax.
For a bigger dear such as elk, I would rather use the berger.