New action needs a cartridge

[Rapier]

Years ago I ordered a Pro Hunter action, a square bridge 98 Mauser/M-70 hybrid action intended for heavy game cartridges on a pre production, pre paid basis. Well Montana Rifle Co is getting close to actually producing the actions after what has turned into many years. The magazine will be 4.043 inches long with several bolt face diameters available, including a .640

I am concerned at this point with selecting a versatile cartridge. The action/magazine will handle a 505 Gibbs or a 500 A Square with ease. I have a 458 Lott at present and really do not have a use for the PH action right this very minute. However, if called to fill the order, including the bolt face diameter, I was wondering what would be the experienced hunters thought on the subject. In the past the good folks at MRC set the feed rails and bolt face for a cartridge based on the order for the preproduction actions. I would just build the rifle later when the wild hare got hold of me.

At present I am leaning towards the 500 A Square (460 Wetherby necked up and blown out) simply because in the US it is a very doable deal for bullets, cases and dies while some of the other exotic stuff is hard to put together for ammo. Plus I wanted a bolt gun to be in a rimless case without a rebated rim if at all possible. Remember, if the COL is less than 4.00 it should work in the PH action.

I hope I stated this clearly enough to make sense. I am trying to cover a base I might not have thought of.
Ed