Lothar Walter do .505 barrels. 500 Jeffery would be easier simply as barrels are .510" so much more out there.

I must admit I've been tempted by 730gr bullets in the 45-120 http://www.castbulletengineering.com.au/bullet-moulds/rifle/458-calibre?Page=3&Items=12

For now though it's 300gr and Ramshot Hunter.