Mark, this rifle is marked, "6.5 M/M 256 RIMLESS" I see in the Ross Rifle Story, they list this model rifle as having been chambered in .256 Mannlicher. As it is a rimless case, I am sure is is chambered in the 6.5 x 54 Mannlicher-Schoenauer. That cartridge was designed in 1900. This rifle was made in 1902, I believe. As Sir Charles was very future-minded, it would make sense he would choose a new and recently developed cartridge.