If barrel life is not a big concern of yours, and you want to shoot game to 500 yards ( if YOU can do it) and paper to 1,000 yards--.284 or 6.5/.284.
You did say custom rifle---there are a few smiths that have this cartridge down pat and can recommend actions, barrel make and twist, bullets, loads and everything else --it's a darn good cartridge---and you can be up and running with very little effort.
http://www.6mmbr.com/gunweek088.html
If you are concerned about barrel life---game to 500 yards/paper to 600 yards and beyond---.308, 7MM-08 etc.
A lot of calibers CAN take game @ 500 yards---A lot of hunters/shooters can't and shouldn't try.
Dipper