Tinker
As you know the majority of 45 cal 300 grainers are really made for 45/70 velocities so the Barnes X is a good choice for MV of 2700 fps IMO.

Thanks for all those other details on your Whitworth as they all help for sure.
It is pretty easy to tell which Whitworths actually saw British soil during assembly.
More on that at some later time.
I'm still hoping someone out there has one in 7 mm Rem Mag and can supply some details.