So 500NE would be a good choice. Would the CZ550 magnum be a good action to use?

The CZ550 magnum is also available in a higher grade version (ie more expensive) in 500 Jeffery and 505 Gibbs. Would either of these cartridges works okay with slightly reduced loads to meet the minimum power requirement for group 3?

(I would love a double but price precludes me from one of these yet [pending lotto win], next choice would be a bolt action. Actually how does the single shot compare price wise with a bolt action?)

I use a 375H&H for group 1 and 458WM in group 2, both are CZ550s. They have heavy and have excellent recoil pads. I can shoot the 458 off a bench okay - probably all day with 400gn projectiles, but not sure about the 500gn (I have shot 2 x 5 shot groups with them off a bench - no pain or bruising but they make you feel very "un-loved").

I've watched the guy shooting the 505 Gibbs (he's probably around 90kg) and it knocks him back a fair bit. It looks much worse when standing behind him. He said that he is usually sore the next day but he does fire many rounds, lots more than the match requires. He fires many practice rounds before, during and after the match and is a very good shot.