The ".315" designation would come from the nominal bore diameter I'd say, not the bullet diameter, which is pretty standard practice for the Poms. Hence .315 Mauser (8x57JS), .315 Mannlicher Schoenauer (8x56) and the .315 (Austrian) Mannlicher (8x50R) chambered in the Lee Speeds by BSA and others. BTW Taylor mentions the latter in passing in his "African Rifles and Cartridges".