NorfolkShooter, welcome to the NE Forums.

(Just an aside: I was based at RAF Bentwaters as an A-10 Warthog pilot 40 years ago, now THAT is a big rifle!))

I have a .500AHR which is a "modernized" version of the .500 Jefferey. My suggestion is to try before you buy, the .500 Jeffery full power loads even in a well shaped rifle of sufficient weight are quite another step up from even the .450's or .470

Not a bad thing……just have to be attuned to the performance level. .500 Jeff, .505 Gibbs, .500 NE all will be about equal for your test firing decision making.
- Mike