Have a look at how the Brits defined the "Magnum" monicker in the 1920s. Major G. Burrard, DSO, RA, Editor of "Game and Gun", in "Cartridges for Sporting Rifles"(Quote):
Small Bore: A rifle the calibre of which is less than .400 in.
Magnum Small Bore: A small bore rifle which has a muzzle velocity of 2500 fs or more.
So most of the hunting cartridges in use today qualify as "Magnum Small Bores" as the pre-WW2 Brits defined them.