Even 125 years ago, ballisticians understood the advantages of 6.5mm bullets. Cartridges in that caliber were adopted by Italy (1891), The Netherlands (1892), Sweden (1894), Japan (1897), Greece (1903) and Portugal (1904). Even with the powders and metallurgy available at the time, the long, thin bullet of a 6.5mm cartridge offered high ballistic coefficients and sectional density that let it punch well above its weight.
Those 6.5mm military rounds faded away after World War I, but the driver there was the need for larger calibers in heavy machine guns. So you saw Italy adopt the 7.35mm, Japan the 7.7mm and Portugal the 7.92mm. Sweden retained its 6.5mm rifles, but adopted a separate 8x63mm cartridge for machine guns.
Since suitability for belt-feds is of only theoretical importance to most of us,…