Tim,
I have a Rigby .303 double that is marked '38 gr RIFLITE'. I been battling for years to get the rifle to group with 215 gr until someone recently told me that the rifle was regulated for 150gr. I have since tried ordinary factory 150gr ammo with great results. It is apparently only Rigby that regulated some of their rifles for 150gr however with doubles there always seems to be exceptions.
Regards,
Charls