I have an old fixed 4x German Pecar with a 26mm tube on my No.1 in 7x57 and have never found it wanting.
Optics are crystal clear. I can see a 6" gong clearly at 275 yards and can hit it most times under field conditions.
Yep, the gong looks small, but it is clear.
My choice for the Rigby would be something similar, a good quality European fixed 4 power.
Best
ALEX