A bit late in finding this Post, though shall toss in a .50 caliber cartridge recommendation here: .50 B&M Alaskan.

Not widely known, but very versatile. Basically it's the .50-110 Starline case sized down to use .500 diameter bullets (as used in the .500 S&W cartridge) instead of using .510 bullets, which are not as plentiful as the .500 bullets on the market out there. If you're going to mainly use cast bullets, then either the .50 B&M Alaskan or the .50-110 probably doesn't make much difference.

Here's a bit of a read on the .50 B&M Alaskan:
http://www.b-mriflesandcartridges.com/50-B-M-Alaskan.html