What round does the Russian Mosin rifle fire?

The M1891, 91/30, M38, M91/56, M44, and all the Finnish variants all fire the 7.62 x 54mm Rimmed cartridge. Usually known as 7.62x54R, or 7.62 Russian.

The only Mosin Nagant rifles which fire a different cartridge are the M1891 rifles captured by the Austro-Hungarian Empire during the First World War. These were converted to fire the 7.92x57 cartridge, and are extremely rare.