Brad,
The Blaser is a "modern" design with a safety/cocking feature which I do not like also it has a cover or shroud over the breech when opened would slow down a fast reload also the calibre 500/416 is a newer cartridge that duplicates the 416Rigby and I presume the same pressure which you do not want in a double!.

I would go for a Heym in 450/400 a much better rifle.

steve