Smallbore,
I think you are on the right track !! How about a .303 ???? I have doubles in most of the other ones you are thinking about (450/400, 9.3X74R, 375FL. , 450 NE, etc.) , but a nice "English" .303 oh my !!!!!! The small bores are harder to find than the "big bores" and when you do find .303's the bores are "shot out" . Good luck with the "project"
Robert